1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! See [rustc guide] for more info on how this works.
13 //! [rustc guide]: https://rust-lang-nursery.github.io/rustc-guide/trait-resolution.html#selection
15 use self::SelectionCandidate
::*;
16 use self::EvaluationResult
::*;
18 use super::coherence
::{self, Conflict}
;
19 use super::DerivedObligationCause
;
20 use super::IntercrateMode
;
22 use super::project
::{normalize_with_depth, Normalized, ProjectionCacheKey}
;
23 use super::{PredicateObligation, TraitObligation, ObligationCause}
;
24 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation}
;
25 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch}
;
26 use super::{ObjectCastObligation, Obligation}
;
27 use super::TraitNotObjectSafe
;
29 use super::SelectionResult
;
30 use super::{VtableBuiltin
, VtableImpl
, VtableParam
, VtableClosure
, VtableGenerator
,
31 VtableFnPointer
, VtableObject
, VtableAutoImpl
};
32 use super::{VtableImplData
, VtableObjectData
, VtableBuiltinData
, VtableGeneratorData
,
33 VtableClosureData
, VtableAutoImplData
, VtableFnPointerData
};
36 use dep_graph
::{DepNodeIndex, DepKind}
;
37 use hir
::def_id
::DefId
;
39 use infer
::{InferCtxt, InferOk, TypeFreshener}
;
40 use ty
::subst
::{Kind, Subst, Substs}
;
41 use ty
::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable}
;
43 use ty
::relate
::TypeRelation
;
44 use middle
::lang_items
;
45 use mir
::interpret
::{GlobalId}
;
47 use rustc_data_structures
::bitvec
::BitVector
;
49 use std
::cell
::RefCell
;
56 use util
::nodemap
::{FxHashMap, FxHashSet}
;
59 pub struct SelectionContext
<'cx
, 'gcx
: 'cx
+'tcx
, 'tcx
: 'cx
> {
60 infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>,
62 /// Freshener used specifically for skolemizing entries on the
63 /// obligation stack. This ensures that all entries on the stack
64 /// at one time will have the same set of skolemized entries,
65 /// which is important for checking for trait bounds that
66 /// recursively require themselves.
67 freshener
: TypeFreshener
<'cx
, 'gcx
, 'tcx
>,
69 /// If true, indicates that the evaluation should be conservative
70 /// and consider the possibility of types outside this crate.
71 /// This comes up primarily when resolving ambiguity. Imagine
72 /// there is some trait reference `$0 : Bar` where `$0` is an
73 /// inference variable. If `intercrate` is true, then we can never
74 /// say for sure that this reference is not implemented, even if
75 /// there are *no impls at all for `Bar`*, because `$0` could be
76 /// bound to some type that in a downstream crate that implements
77 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
78 /// though, we set this to false, because we are only interested
79 /// in types that the user could actually have written --- in
80 /// other words, we consider `$0 : Bar` to be unimplemented if
81 /// there is no type that the user could *actually name* that
82 /// would satisfy it. This avoids crippling inference, basically.
83 intercrate
: Option
<IntercrateMode
>,
85 intercrate_ambiguity_causes
: Option
<Vec
<IntercrateAmbiguityCause
>>,
87 /// Controls whether or not to filter out negative impls when selecting.
88 /// This is used in librustdoc to distinguish between the lack of an impl
89 /// and a negative impl
90 allow_negative_impls
: bool
93 #[derive(Clone, Debug)]
94 pub enum IntercrateAmbiguityCause
{
97 self_desc
: Option
<String
>,
101 self_desc
: Option
<String
>,
105 impl IntercrateAmbiguityCause
{
106 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
107 /// See #23980 for details.
108 pub fn add_intercrate_ambiguity_hint
<'a
, 'tcx
>(&self,
109 err
: &mut ::errors
::DiagnosticBuilder
) {
110 err
.note(&self.intercrate_ambiguity_hint());
113 pub fn intercrate_ambiguity_hint(&self) -> String
{
115 &IntercrateAmbiguityCause
::DownstreamCrate { ref trait_desc, ref self_desc }
=> {
116 let self_desc
= if let &Some(ref ty
) = self_desc
{
117 format
!(" for type `{}`", ty
)
118 } else { "".to_string() }
;
119 format
!("downstream crates may implement trait `{}`{}", trait_desc
, self_desc
)
121 &IntercrateAmbiguityCause
::UpstreamCrateUpdate { ref trait_desc, ref self_desc }
=> {
122 let self_desc
= if let &Some(ref ty
) = self_desc
{
123 format
!(" for type `{}`", ty
)
124 } else { "".to_string() }
;
125 format
!("upstream crates may add new impl of trait `{}`{} \
127 trait_desc
, self_desc
)
133 // A stack that walks back up the stack frame.
134 struct TraitObligationStack
<'prev
, 'tcx
: 'prev
> {
135 obligation
: &'prev TraitObligation
<'tcx
>,
137 /// Trait ref from `obligation` but skolemized with the
138 /// selection-context's freshener. Used to check for recursion.
139 fresh_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
141 previous
: TraitObligationStackList
<'prev
, 'tcx
>,
145 pub struct SelectionCache
<'tcx
> {
146 hashmap
: RefCell
<FxHashMap
<ty
::TraitRef
<'tcx
>,
147 WithDepNode
<SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>>>>,
150 /// The selection process begins by considering all impls, where
151 /// clauses, and so forth that might resolve an obligation. Sometimes
152 /// we'll be able to say definitively that (e.g.) an impl does not
153 /// apply to the obligation: perhaps it is defined for `usize` but the
154 /// obligation is for `int`. In that case, we drop the impl out of the
155 /// list. But the other cases are considered *candidates*.
157 /// For selection to succeed, there must be exactly one matching
158 /// candidate. If the obligation is fully known, this is guaranteed
159 /// by coherence. However, if the obligation contains type parameters
160 /// or variables, there may be multiple such impls.
162 /// It is not a real problem if multiple matching impls exist because
163 /// of type variables - it just means the obligation isn't sufficiently
164 /// elaborated. In that case we report an ambiguity, and the caller can
165 /// try again after more type information has been gathered or report a
166 /// "type annotations required" error.
168 /// However, with type parameters, this can be a real problem - type
169 /// parameters don't unify with regular types, but they *can* unify
170 /// with variables from blanket impls, and (unless we know its bounds
171 /// will always be satisfied) picking the blanket impl will be wrong
172 /// for at least *some* substitutions. To make this concrete, if we have
174 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
175 /// impl<T: fmt::Debug> AsDebug for T {
177 /// fn debug(self) -> fmt::Debug { self }
179 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
181 /// we can't just use the impl to resolve the <T as AsDebug> obligation
182 /// - a type from another crate (that doesn't implement fmt::Debug) could
183 /// implement AsDebug.
185 /// Because where-clauses match the type exactly, multiple clauses can
186 /// only match if there are unresolved variables, and we can mostly just
187 /// report this ambiguity in that case. This is still a problem - we can't
188 /// *do anything* with ambiguities that involve only regions. This is issue
191 /// If a single where-clause matches and there are no inference
192 /// variables left, then it definitely matches and we can just select
195 /// In fact, we even select the where-clause when the obligation contains
196 /// inference variables. The can lead to inference making "leaps of logic",
197 /// for example in this situation:
199 /// pub trait Foo<T> { fn foo(&self) -> T; }
200 /// impl<T> Foo<()> for T { fn foo(&self) { } }
201 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
203 /// pub fn foo<T>(t: T) where T: Foo<bool> {
204 /// println!("{:?}", <T as Foo<_>>::foo(&t));
206 /// fn main() { foo(false); }
208 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
209 /// impl and the where-clause. We select the where-clause and unify $0=bool,
210 /// so the program prints "false". However, if the where-clause is omitted,
211 /// the blanket impl is selected, we unify $0=(), and the program prints
214 /// Exactly the same issues apply to projection and object candidates, except
215 /// that we can have both a projection candidate and a where-clause candidate
216 /// for the same obligation. In that case either would do (except that
217 /// different "leaps of logic" would occur if inference variables are
218 /// present), and we just pick the where-clause. This is, for example,
219 /// required for associated types to work in default impls, as the bounds
220 /// are visible both as projection bounds and as where-clauses from the
221 /// parameter environment.
222 #[derive(PartialEq,Eq,Debug,Clone)]
223 enum SelectionCandidate
<'tcx
> {
224 BuiltinCandidate { has_nested: bool }
,
225 ParamCandidate(ty
::PolyTraitRef
<'tcx
>),
226 ImplCandidate(DefId
),
227 AutoImplCandidate(DefId
),
229 /// This is a trait matching with a projected type as `Self`, and
230 /// we found an applicable bound in the trait definition.
233 /// Implementation of a `Fn`-family trait by one of the anonymous types
234 /// generated for a `||` expression.
237 /// Implementation of a `Generator` trait by one of the anonymous types
238 /// generated for a generator.
241 /// Implementation of a `Fn`-family trait by one of the anonymous
242 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
247 BuiltinObjectCandidate
,
249 BuiltinUnsizeCandidate
,
252 impl<'a
, 'tcx
> ty
::Lift
<'tcx
> for SelectionCandidate
<'a
> {
253 type Lifted
= SelectionCandidate
<'tcx
>;
254 fn lift_to_tcx
<'b
, 'gcx
>(&self, tcx
: TyCtxt
<'b
, 'gcx
, 'tcx
>) -> Option
<Self::Lifted
> {
256 BuiltinCandidate { has_nested }
=> {
261 ImplCandidate(def_id
) => ImplCandidate(def_id
),
262 AutoImplCandidate(def_id
) => AutoImplCandidate(def_id
),
263 ProjectionCandidate
=> ProjectionCandidate
,
264 FnPointerCandidate
=> FnPointerCandidate
,
265 ObjectCandidate
=> ObjectCandidate
,
266 BuiltinObjectCandidate
=> BuiltinObjectCandidate
,
267 BuiltinUnsizeCandidate
=> BuiltinUnsizeCandidate
,
268 ClosureCandidate
=> ClosureCandidate
,
269 GeneratorCandidate
=> GeneratorCandidate
,
271 ParamCandidate(ref trait_ref
) => {
272 return tcx
.lift(trait_ref
).map(ParamCandidate
);
278 struct SelectionCandidateSet
<'tcx
> {
279 // a list of candidates that definitely apply to the current
280 // obligation (meaning: types unify).
281 vec
: Vec
<SelectionCandidate
<'tcx
>>,
283 // if this is true, then there were candidates that might or might
284 // not have applied, but we couldn't tell. This occurs when some
285 // of the input types are type variables, in which case there are
286 // various "builtin" rules that might or might not trigger.
290 #[derive(PartialEq,Eq,Debug,Clone)]
291 struct EvaluatedCandidate
<'tcx
> {
292 candidate
: SelectionCandidate
<'tcx
>,
293 evaluation
: EvaluationResult
,
296 /// When does the builtin impl for `T: Trait` apply?
297 enum BuiltinImplConditions
<'tcx
> {
298 /// The impl is conditional on T1,T2,.. : Trait
299 Where(ty
::Binder
<Vec
<Ty
<'tcx
>>>),
300 /// There is no built-in impl. There may be some other
301 /// candidate (a where-clause or user-defined impl).
303 /// There is *no* impl for this, builtin or not. Ignore
304 /// all where-clauses.
306 /// It is unknown whether there is an impl.
310 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
311 /// The result of trait evaluation. The order is important
312 /// here as the evaluation of a list is the maximum of the
315 /// The evaluation results are ordered:
316 /// - `EvaluatedToOk` implies `EvaluatedToAmbig` implies `EvaluatedToUnknown`
317 /// - `EvaluatedToErr` implies `EvaluatedToRecur`
318 /// - the "union" of evaluation results is equal to their maximum -
319 /// all the "potential success" candidates can potentially succeed,
320 /// so they are no-ops when unioned with a definite error, and within
321 /// the categories it's easy to see that the unions are correct.
322 enum EvaluationResult
{
323 /// Evaluation successful
325 /// Evaluation is known to be ambiguous - it *might* hold for some
326 /// assignment of inference variables, but it might not.
328 /// While this has the same meaning as `EvaluatedToUnknown` - we can't
329 /// know whether this obligation holds or not - it is the result we
330 /// would get with an empty stack, and therefore is cacheable.
332 /// Evaluation failed because of recursion involving inference
333 /// variables. We are somewhat imprecise there, so we don't actually
334 /// know the real result.
336 /// This can't be trivially cached for the same reason as `EvaluatedToRecur`.
338 /// Evaluation failed because we encountered an obligation we are already
339 /// trying to prove on this branch.
341 /// We know this branch can't be a part of a minimal proof-tree for
342 /// the "root" of our cycle, because then we could cut out the recursion
343 /// and maintain a valid proof tree. However, this does not mean
344 /// that all the obligations on this branch do not hold - it's possible
345 /// that we entered this branch "speculatively", and that there
346 /// might be some other way to prove this obligation that does not
347 /// go through this cycle - so we can't cache this as a failure.
349 /// For example, suppose we have this:
351 /// ```rust,ignore (pseudo-Rust)
352 /// pub trait Trait { fn xyz(); }
353 /// // This impl is "useless", but we can still have
354 /// // an `impl Trait for SomeUnsizedType` somewhere.
355 /// impl<T: Trait + Sized> Trait for T { fn xyz() {} }
357 /// pub fn foo<T: Trait + ?Sized>() {
358 /// <T as Trait>::xyz();
362 /// When checking `foo`, we have to prove `T: Trait`. This basically
363 /// translates into this:
365 /// (T: Trait + Sized →_\impl T: Trait), T: Trait ⊢ T: Trait
367 /// When we try to prove it, we first go the first option, which
368 /// recurses. This shows us that the impl is "useless" - it won't
369 /// tell us that `T: Trait` unless it already implemented `Trait`
370 /// by some other means. However, that does not prevent `T: Trait`
371 /// does not hold, because of the bound (which can indeed be satisfied
372 /// by `SomeUnsizedType` from another crate).
374 /// FIXME: when an `EvaluatedToRecur` goes past its parent root, we
375 /// ought to convert it to an `EvaluatedToErr`, because we know
376 /// there definitely isn't a proof tree for that obligation. Not
377 /// doing so is still sound - there isn't any proof tree, so the
378 /// branch still can't be a part of a minimal one - but does not
379 /// re-enable caching.
381 /// Evaluation failed
385 impl EvaluationResult
{
386 fn may_apply(self) -> bool
{
390 EvaluatedToUnknown
=> true,
393 EvaluatedToRecur
=> false
397 fn is_stack_dependent(self) -> bool
{
400 EvaluatedToRecur
=> true,
404 EvaluatedToErr
=> false,
410 pub struct EvaluationCache
<'tcx
> {
411 hashmap
: RefCell
<FxHashMap
<ty
::PolyTraitRef
<'tcx
>, WithDepNode
<EvaluationResult
>>>
414 impl<'cx
, 'gcx
, 'tcx
> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
415 pub fn new(infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>) -> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
418 freshener
: infcx
.freshener(),
420 intercrate_ambiguity_causes
: None
,
421 allow_negative_impls
: false,
425 pub fn intercrate(infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>,
426 mode
: IntercrateMode
) -> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
427 debug
!("intercrate({:?})", mode
);
430 freshener
: infcx
.freshener(),
431 intercrate
: Some(mode
),
432 intercrate_ambiguity_causes
: None
,
433 allow_negative_impls
: false,
437 pub fn with_negative(infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>,
438 allow_negative_impls
: bool
) -> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
439 debug
!("with_negative({:?})", allow_negative_impls
);
442 freshener
: infcx
.freshener(),
444 intercrate_ambiguity_causes
: None
,
445 allow_negative_impls
,
449 /// Enables tracking of intercrate ambiguity causes. These are
450 /// used in coherence to give improved diagnostics. We don't do
451 /// this until we detect a coherence error because it can lead to
452 /// false overflow results (#47139) and because it costs
453 /// computation time.
454 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
455 assert
!(self.intercrate
.is_some());
456 assert
!(self.intercrate_ambiguity_causes
.is_none());
457 self.intercrate_ambiguity_causes
= Some(vec
![]);
458 debug
!("selcx: enable_tracking_intercrate_ambiguity_causes");
461 /// Gets the intercrate ambiguity causes collected since tracking
462 /// was enabled and disables tracking at the same time. If
463 /// tracking is not enabled, just returns an empty vector.
464 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec
<IntercrateAmbiguityCause
> {
465 assert
!(self.intercrate
.is_some());
466 self.intercrate_ambiguity_causes
.take().unwrap_or(vec
![])
469 pub fn infcx(&self) -> &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
> {
473 pub fn tcx(&self) -> TyCtxt
<'cx
, 'gcx
, 'tcx
> {
477 pub fn closure_typer(&self) -> &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
> {
481 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
483 fn in_snapshot
<R
, F
>(&mut self, f
: F
) -> R
484 where F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
<'cx
, 'tcx
>) -> R
486 self.infcx
.in_snapshot(|snapshot
| f(self, snapshot
))
489 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
491 fn probe
<R
, F
>(&mut self, f
: F
) -> R
492 where F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
<'cx
, 'tcx
>) -> R
494 self.infcx
.probe(|snapshot
| f(self, snapshot
))
497 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
498 /// the transaction fails and s.t. old obligations are retained.
499 fn commit_if_ok
<T
, E
, F
>(&mut self, f
: F
) -> Result
<T
, E
> where
500 F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
) -> Result
<T
, E
>
502 self.infcx
.commit_if_ok(|snapshot
| f(self, snapshot
))
506 ///////////////////////////////////////////////////////////////////////////
509 // The selection phase tries to identify *how* an obligation will
510 // be resolved. For example, it will identify which impl or
511 // parameter bound is to be used. The process can be inconclusive
512 // if the self type in the obligation is not fully inferred. Selection
513 // can result in an error in one of two ways:
515 // 1. If no applicable impl or parameter bound can be found.
516 // 2. If the output type parameters in the obligation do not match
517 // those specified by the impl/bound. For example, if the obligation
518 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
519 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
521 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
522 /// type environment by performing unification.
523 pub fn select(&mut self, obligation
: &TraitObligation
<'tcx
>)
524 -> SelectionResult
<'tcx
, Selection
<'tcx
>> {
525 debug
!("select({:?})", obligation
);
526 assert
!(!obligation
.predicate
.has_escaping_regions());
528 let stack
= self.push_stack(TraitObligationStackList
::empty(), obligation
);
529 let ret
= match self.candidate_from_obligation(&stack
)?
{
531 Some(candidate
) => Some(self.confirm_candidate(obligation
, candidate
)?
)
537 ///////////////////////////////////////////////////////////////////////////
540 // Tests whether an obligation can be selected or whether an impl
541 // can be applied to particular types. It skips the "confirmation"
542 // step and hence completely ignores output type parameters.
544 // The result is "true" if the obligation *may* hold and "false" if
545 // we can be sure it does not.
547 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
548 pub fn evaluate_obligation(&mut self,
549 obligation
: &PredicateObligation
<'tcx
>)
552 debug
!("evaluate_obligation({:?})",
555 self.probe(|this
, _
| {
556 this
.evaluate_predicate_recursively(TraitObligationStackList
::empty(), obligation
)
561 /// Evaluates whether the obligation `obligation` can be satisfied,
562 /// and returns `false` if not certain. However, this is not entirely
563 /// accurate if inference variables are involved.
564 pub fn evaluate_obligation_conservatively(&mut self,
565 obligation
: &PredicateObligation
<'tcx
>)
568 debug
!("evaluate_obligation_conservatively({:?})",
571 self.probe(|this
, _
| {
572 this
.evaluate_predicate_recursively(TraitObligationStackList
::empty(), obligation
)
577 /// Evaluates the predicates in `predicates` recursively. Note that
578 /// this applies projections in the predicates, and therefore
579 /// is run within an inference probe.
580 fn evaluate_predicates_recursively
<'a
,'o
,I
>(&mut self,
581 stack
: TraitObligationStackList
<'o
, 'tcx
>,
584 where I
: IntoIterator
<Item
=&'a PredicateObligation
<'tcx
>>, 'tcx
:'a
586 let mut result
= EvaluatedToOk
;
587 for obligation
in predicates
{
588 let eval
= self.evaluate_predicate_recursively(stack
, obligation
);
589 debug
!("evaluate_predicate_recursively({:?}) = {:?}",
591 if let EvaluatedToErr
= eval
{
592 // fast-path - EvaluatedToErr is the top of the lattice,
593 // so we don't need to look on the other predicates.
594 return EvaluatedToErr
;
596 result
= cmp
::max(result
, eval
);
602 fn evaluate_predicate_recursively
<'o
>(&mut self,
603 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
604 obligation
: &PredicateObligation
<'tcx
>)
607 debug
!("evaluate_predicate_recursively({:?})",
610 match obligation
.predicate
{
611 ty
::Predicate
::Trait(ref t
) => {
612 assert
!(!t
.has_escaping_regions());
613 let obligation
= obligation
.with(t
.clone());
614 self.evaluate_trait_predicate_recursively(previous_stack
, obligation
)
617 ty
::Predicate
::Subtype(ref p
) => {
618 // does this code ever run?
619 match self.infcx
.subtype_predicate(&obligation
.cause
, obligation
.param_env
, p
) {
620 Some(Ok(InferOk { obligations, .. }
)) => {
621 self.evaluate_predicates_recursively(previous_stack
, &obligations
);
624 Some(Err(_
)) => EvaluatedToErr
,
625 None
=> EvaluatedToAmbig
,
629 ty
::Predicate
::WellFormed(ty
) => {
630 match ty
::wf
::obligations(self.infcx
,
631 obligation
.param_env
,
632 obligation
.cause
.body_id
,
633 ty
, obligation
.cause
.span
) {
635 self.evaluate_predicates_recursively(previous_stack
, obligations
.iter()),
641 ty
::Predicate
::TypeOutlives(..) | ty
::Predicate
::RegionOutlives(..) => {
642 // we do not consider region relationships when
643 // evaluating trait matches
647 ty
::Predicate
::ObjectSafe(trait_def_id
) => {
648 if self.tcx().is_object_safe(trait_def_id
) {
655 ty
::Predicate
::Projection(ref data
) => {
656 let project_obligation
= obligation
.with(data
.clone());
657 match project
::poly_project_and_unify_type(self, &project_obligation
) {
658 Ok(Some(subobligations
)) => {
659 let result
= self.evaluate_predicates_recursively(previous_stack
,
660 subobligations
.iter());
662 ProjectionCacheKey
::from_poly_projection_predicate(self, data
)
664 self.infcx
.projection_cache
.borrow_mut().complete(key
);
677 ty
::Predicate
::ClosureKind(closure_def_id
, closure_substs
, kind
) => {
678 match self.infcx
.closure_kind(closure_def_id
, closure_substs
) {
679 Some(closure_kind
) => {
680 if closure_kind
.extends(kind
) {
692 ty
::Predicate
::ConstEvaluatable(def_id
, substs
) => {
693 let tcx
= self.tcx();
694 match tcx
.lift_to_global(&(obligation
.param_env
, substs
)) {
695 Some((param_env
, substs
)) => {
696 let instance
= ty
::Instance
::resolve(
702 if let Some(instance
) = instance
{
707 match self.tcx().const_eval(param_env
.and(cid
)) {
708 Ok(_
) => EvaluatedToOk
,
709 Err(_
) => EvaluatedToErr
716 // Inference variables still left in param_env or substs.
724 fn evaluate_trait_predicate_recursively
<'o
>(&mut self,
725 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
726 mut obligation
: TraitObligation
<'tcx
>)
729 debug
!("evaluate_trait_predicate_recursively({:?})",
732 if !self.intercrate
.is_some() && obligation
.is_global() {
733 // If a param env is consistent, global obligations do not depend on its particular
734 // value in order to work, so we can clear out the param env and get better
735 // caching. (If the current param env is inconsistent, we don't care what happens).
736 debug
!("evaluate_trait_predicate_recursively({:?}) - in global", obligation
);
737 obligation
.param_env
= obligation
.param_env
.without_caller_bounds();
740 let stack
= self.push_stack(previous_stack
, &obligation
);
741 let fresh_trait_ref
= stack
.fresh_trait_ref
;
742 if let Some(result
) = self.check_evaluation_cache(obligation
.param_env
, fresh_trait_ref
) {
743 debug
!("CACHE HIT: EVAL({:?})={:?}",
749 let (result
, dep_node
) = self.in_task(|this
| this
.evaluate_stack(&stack
));
751 debug
!("CACHE MISS: EVAL({:?})={:?}",
754 self.insert_evaluation_cache(obligation
.param_env
, fresh_trait_ref
, dep_node
, result
);
759 fn evaluate_stack
<'o
>(&mut self,
760 stack
: &TraitObligationStack
<'o
, 'tcx
>)
763 // In intercrate mode, whenever any of the types are unbound,
764 // there can always be an impl. Even if there are no impls in
765 // this crate, perhaps the type would be unified with
766 // something from another crate that does provide an impl.
768 // In intra mode, we must still be conservative. The reason is
769 // that we want to avoid cycles. Imagine an impl like:
771 // impl<T:Eq> Eq for Vec<T>
773 // and a trait reference like `$0 : Eq` where `$0` is an
774 // unbound variable. When we evaluate this trait-reference, we
775 // will unify `$0` with `Vec<$1>` (for some fresh variable
776 // `$1`), on the condition that `$1 : Eq`. We will then wind
777 // up with many candidates (since that are other `Eq` impls
778 // that apply) and try to winnow things down. This results in
779 // a recursive evaluation that `$1 : Eq` -- as you can
780 // imagine, this is just where we started. To avoid that, we
781 // check for unbound variables and return an ambiguous (hence possible)
782 // match if we've seen this trait before.
784 // This suffices to allow chains like `FnMut` implemented in
785 // terms of `Fn` etc, but we could probably make this more
787 let unbound_input_types
= stack
.fresh_trait_ref
.input_types().any(|ty
| ty
.is_fresh());
788 // this check was an imperfect workaround for a bug n the old
789 // intercrate mode, it should be removed when that goes away.
790 if unbound_input_types
&&
791 self.intercrate
== Some(IntercrateMode
::Issue43355
)
793 debug
!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
794 stack
.fresh_trait_ref
);
795 // Heuristics: show the diagnostics when there are no candidates in crate.
796 if self.intercrate_ambiguity_causes
.is_some() {
797 debug
!("evaluate_stack: intercrate_ambiguity_causes is some");
798 if let Ok(candidate_set
) = self.assemble_candidates(stack
) {
799 if !candidate_set
.ambiguous
&& candidate_set
.vec
.is_empty() {
800 let trait_ref
= stack
.obligation
.predicate
.skip_binder().trait_ref
;
801 let self_ty
= trait_ref
.self_ty();
802 let cause
= IntercrateAmbiguityCause
::DownstreamCrate
{
803 trait_desc
: trait_ref
.to_string(),
804 self_desc
: if self_ty
.has_concrete_skeleton() {
805 Some(self_ty
.to_string())
810 debug
!("evaluate_stack: pushing cause = {:?}", cause
);
811 self.intercrate_ambiguity_causes
.as_mut().unwrap().push(cause
);
815 return EvaluatedToAmbig
;
817 if unbound_input_types
&&
818 stack
.iter().skip(1).any(
819 |prev
| stack
.obligation
.param_env
== prev
.obligation
.param_env
&&
820 self.match_fresh_trait_refs(&stack
.fresh_trait_ref
,
821 &prev
.fresh_trait_ref
))
823 debug
!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
824 stack
.fresh_trait_ref
);
825 return EvaluatedToUnknown
;
828 // If there is any previous entry on the stack that precisely
829 // matches this obligation, then we can assume that the
830 // obligation is satisfied for now (still all other conditions
831 // must be met of course). One obvious case this comes up is
832 // marker traits like `Send`. Think of a linked list:
834 // struct List<T> { data: T, next: Option<Box<List<T>>> {
836 // `Box<List<T>>` will be `Send` if `T` is `Send` and
837 // `Option<Box<List<T>>>` is `Send`, and in turn
838 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
841 // Note that we do this comparison using the `fresh_trait_ref`
842 // fields. Because these have all been skolemized using
843 // `self.freshener`, we can be sure that (a) this will not
844 // affect the inferencer state and (b) that if we see two
845 // skolemized types with the same index, they refer to the
846 // same unbound type variable.
847 if let Some(rec_index
) =
849 .skip(1) // skip top-most frame
850 .position(|prev
| stack
.obligation
.param_env
== prev
.obligation
.param_env
&&
851 stack
.fresh_trait_ref
== prev
.fresh_trait_ref
)
853 debug
!("evaluate_stack({:?}) --> recursive",
854 stack
.fresh_trait_ref
);
855 let cycle
= stack
.iter().skip(1).take(rec_index
+1);
856 let cycle
= cycle
.map(|stack
| ty
::Predicate
::Trait(stack
.obligation
.predicate
));
857 if self.coinductive_match(cycle
) {
858 debug
!("evaluate_stack({:?}) --> recursive, coinductive",
859 stack
.fresh_trait_ref
);
860 return EvaluatedToOk
;
862 debug
!("evaluate_stack({:?}) --> recursive, inductive",
863 stack
.fresh_trait_ref
);
864 return EvaluatedToRecur
;
868 match self.candidate_from_obligation(stack
) {
869 Ok(Some(c
)) => self.evaluate_candidate(stack
, &c
),
870 Ok(None
) => EvaluatedToAmbig
,
871 Err(..) => EvaluatedToErr
875 /// For defaulted traits, we use a co-inductive strategy to solve, so
876 /// that recursion is ok. This routine returns true if the top of the
877 /// stack (`cycle[0]`):
879 /// - is a defaulted trait, and
880 /// - it also appears in the backtrace at some position `X`; and,
881 /// - all the predicates at positions `X..` between `X` an the top are
882 /// also defaulted traits.
883 pub fn coinductive_match
<I
>(&mut self, cycle
: I
) -> bool
884 where I
: Iterator
<Item
=ty
::Predicate
<'tcx
>>
886 let mut cycle
= cycle
;
887 cycle
.all(|predicate
| self.coinductive_predicate(predicate
))
890 fn coinductive_predicate(&self, predicate
: ty
::Predicate
<'tcx
>) -> bool
{
891 let result
= match predicate
{
892 ty
::Predicate
::Trait(ref data
) => {
893 self.tcx().trait_is_auto(data
.def_id())
899 debug
!("coinductive_predicate({:?}) = {:?}", predicate
, result
);
903 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
904 /// obligations are met. Returns true if `candidate` remains viable after this further
906 fn evaluate_candidate
<'o
>(&mut self,
907 stack
: &TraitObligationStack
<'o
, 'tcx
>,
908 candidate
: &SelectionCandidate
<'tcx
>)
911 debug
!("evaluate_candidate: depth={} candidate={:?}",
912 stack
.obligation
.recursion_depth
, candidate
);
913 let result
= self.probe(|this
, _
| {
914 let candidate
= (*candidate
).clone();
915 match this
.confirm_candidate(stack
.obligation
, candidate
) {
917 this
.evaluate_predicates_recursively(
919 selection
.nested_obligations().iter())
921 Err(..) => EvaluatedToErr
924 debug
!("evaluate_candidate: depth={} result={:?}",
925 stack
.obligation
.recursion_depth
, result
);
929 fn check_evaluation_cache(&self,
930 param_env
: ty
::ParamEnv
<'tcx
>,
931 trait_ref
: ty
::PolyTraitRef
<'tcx
>)
932 -> Option
<EvaluationResult
>
934 let tcx
= self.tcx();
935 if self.can_use_global_caches(param_env
) {
936 let cache
= tcx
.evaluation_cache
.hashmap
.borrow();
937 if let Some(cached
) = cache
.get(&trait_ref
) {
938 return Some(cached
.get(tcx
));
941 self.infcx
.evaluation_cache
.hashmap
947 fn insert_evaluation_cache(&mut self,
948 param_env
: ty
::ParamEnv
<'tcx
>,
949 trait_ref
: ty
::PolyTraitRef
<'tcx
>,
950 dep_node
: DepNodeIndex
,
951 result
: EvaluationResult
)
953 // Avoid caching results that depend on more than just the trait-ref
954 // - the stack can create recursion.
955 if result
.is_stack_dependent() {
959 if self.can_use_global_caches(param_env
) {
960 let mut cache
= self.tcx().evaluation_cache
.hashmap
.borrow_mut();
961 if let Some(trait_ref
) = self.tcx().lift_to_global(&trait_ref
) {
963 "insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global",
967 cache
.insert(trait_ref
, WithDepNode
::new(dep_node
, result
));
973 "insert_evaluation_cache(trait_ref={:?}, candidate={:?})",
977 self.infcx
.evaluation_cache
.hashmap
979 .insert(trait_ref
, WithDepNode
::new(dep_node
, result
));
982 ///////////////////////////////////////////////////////////////////////////
983 // CANDIDATE ASSEMBLY
985 // The selection process begins by examining all in-scope impls,
986 // caller obligations, and so forth and assembling a list of
987 // candidates. See [rustc guide] for more details.
990 // https://rust-lang-nursery.github.io/rustc-guide/trait-resolution.html#candidate-assembly
992 fn candidate_from_obligation
<'o
>(&mut self,
993 stack
: &TraitObligationStack
<'o
, 'tcx
>)
994 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>
996 // Watch out for overflow. This intentionally bypasses (and does
997 // not update) the cache.
998 let recursion_limit
= self.infcx
.tcx
.sess
.recursion_limit
.get();
999 if stack
.obligation
.recursion_depth
>= recursion_limit
{
1000 self.infcx().report_overflow_error(&stack
.obligation
, true);
1003 // Check the cache. Note that we skolemize the trait-ref
1004 // separately rather than using `stack.fresh_trait_ref` -- this
1005 // is because we want the unbound variables to be replaced
1006 // with fresh skolemized types starting from index 0.
1007 let cache_fresh_trait_pred
=
1008 self.infcx
.freshen(stack
.obligation
.predicate
.clone());
1009 debug
!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
1010 cache_fresh_trait_pred
,
1012 assert
!(!stack
.obligation
.predicate
.has_escaping_regions());
1014 if let Some(c
) = self.check_candidate_cache(stack
.obligation
.param_env
,
1015 &cache_fresh_trait_pred
) {
1016 debug
!("CACHE HIT: SELECT({:?})={:?}",
1017 cache_fresh_trait_pred
,
1022 // If no match, compute result and insert into cache.
1023 let (candidate
, dep_node
) = self.in_task(|this
| {
1024 this
.candidate_from_obligation_no_cache(stack
)
1027 debug
!("CACHE MISS: SELECT({:?})={:?}",
1028 cache_fresh_trait_pred
, candidate
);
1029 self.insert_candidate_cache(stack
.obligation
.param_env
,
1030 cache_fresh_trait_pred
,
1036 fn in_task
<OP
, R
>(&mut self, op
: OP
) -> (R
, DepNodeIndex
)
1037 where OP
: FnOnce(&mut Self) -> R
1039 let (result
, dep_node
) = self.tcx().dep_graph
.with_anon_task(DepKind
::TraitSelect
, || {
1042 self.tcx().dep_graph
.read_index(dep_node
);
1046 // Treat negative impls as unimplemented
1047 fn filter_negative_impls(&self, candidate
: SelectionCandidate
<'tcx
>)
1048 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>> {
1049 if let ImplCandidate(def_id
) = candidate
{
1050 if !self.allow_negative_impls
&&
1051 self.tcx().impl_polarity(def_id
) == hir
::ImplPolarity
::Negative
{
1052 return Err(Unimplemented
)
1058 fn candidate_from_obligation_no_cache
<'o
>(&mut self,
1059 stack
: &TraitObligationStack
<'o
, 'tcx
>)
1060 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>
1062 if stack
.obligation
.predicate
.references_error() {
1063 // If we encounter a `TyError`, we generally prefer the
1064 // most "optimistic" result in response -- that is, the
1065 // one least likely to report downstream errors. But
1066 // because this routine is shared by coherence and by
1067 // trait selection, there isn't an obvious "right" choice
1068 // here in that respect, so we opt to just return
1069 // ambiguity and let the upstream clients sort it out.
1073 match self.is_knowable(stack
) {
1076 debug
!("coherence stage: not knowable");
1077 if self.intercrate_ambiguity_causes
.is_some() {
1078 debug
!("evaluate_stack: intercrate_ambiguity_causes is some");
1079 // Heuristics: show the diagnostics when there are no candidates in crate.
1080 if let Ok(candidate_set
) = self.assemble_candidates(stack
) {
1081 if !candidate_set
.ambiguous
&& candidate_set
.vec
.iter().all(|c
| {
1082 !self.evaluate_candidate(stack
, &c
).may_apply()
1084 let trait_ref
= stack
.obligation
.predicate
.skip_binder().trait_ref
;
1085 let self_ty
= trait_ref
.self_ty();
1086 let trait_desc
= trait_ref
.to_string();
1087 let self_desc
= if self_ty
.has_concrete_skeleton() {
1088 Some(self_ty
.to_string())
1092 let cause
= if let Conflict
::Upstream
= conflict
{
1093 IntercrateAmbiguityCause
::UpstreamCrateUpdate
{
1098 IntercrateAmbiguityCause
::DownstreamCrate { trait_desc, self_desc }
1100 debug
!("evaluate_stack: pushing cause = {:?}", cause
);
1101 self.intercrate_ambiguity_causes
.as_mut().unwrap().push(cause
);
1109 let candidate_set
= self.assemble_candidates(stack
)?
;
1111 if candidate_set
.ambiguous
{
1112 debug
!("candidate set contains ambig");
1116 let mut candidates
= candidate_set
.vec
;
1118 debug
!("assembled {} candidates for {:?}: {:?}",
1123 // At this point, we know that each of the entries in the
1124 // candidate set is *individually* applicable. Now we have to
1125 // figure out if they contain mutual incompatibilities. This
1126 // frequently arises if we have an unconstrained input type --
1127 // for example, we are looking for $0:Eq where $0 is some
1128 // unconstrained type variable. In that case, we'll get a
1129 // candidate which assumes $0 == int, one that assumes $0 ==
1130 // usize, etc. This spells an ambiguity.
1132 // If there is more than one candidate, first winnow them down
1133 // by considering extra conditions (nested obligations and so
1134 // forth). We don't winnow if there is exactly one
1135 // candidate. This is a relatively minor distinction but it
1136 // can lead to better inference and error-reporting. An
1137 // example would be if there was an impl:
1139 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
1141 // and we were to see some code `foo.push_clone()` where `boo`
1142 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
1143 // we were to winnow, we'd wind up with zero candidates.
1144 // Instead, we select the right impl now but report `Bar does
1145 // not implement Clone`.
1146 if candidates
.len() == 1 {
1147 return self.filter_negative_impls(candidates
.pop().unwrap());
1150 // Winnow, but record the exact outcome of evaluation, which
1151 // is needed for specialization.
1152 let mut candidates
: Vec
<_
> = candidates
.into_iter().filter_map(|c
| {
1153 let eval
= self.evaluate_candidate(stack
, &c
);
1154 if eval
.may_apply() {
1155 Some(EvaluatedCandidate
{
1164 // If there are STILL multiple candidate, we can further
1165 // reduce the list by dropping duplicates -- including
1166 // resolving specializations.
1167 if candidates
.len() > 1 {
1169 while i
< candidates
.len() {
1171 (0..candidates
.len())
1172 .filter(|&j
| i
!= j
)
1173 .any(|j
| self.candidate_should_be_dropped_in_favor_of(&candidates
[i
],
1176 debug
!("Dropping candidate #{}/{}: {:?}",
1177 i
, candidates
.len(), candidates
[i
]);
1178 candidates
.swap_remove(i
);
1180 debug
!("Retaining candidate #{}/{}: {:?}",
1181 i
, candidates
.len(), candidates
[i
]);
1184 // If there are *STILL* multiple candidates, give up
1185 // and report ambiguity.
1187 debug
!("multiple matches, ambig");
1194 // If there are *NO* candidates, then there are no impls --
1195 // that we know of, anyway. Note that in the case where there
1196 // are unbound type variables within the obligation, it might
1197 // be the case that you could still satisfy the obligation
1198 // from another crate by instantiating the type variables with
1199 // a type from another crate that does have an impl. This case
1200 // is checked for in `evaluate_stack` (and hence users
1201 // who might care about this case, like coherence, should use
1203 if candidates
.is_empty() {
1204 return Err(Unimplemented
);
1207 // Just one candidate left.
1208 self.filter_negative_impls(candidates
.pop().unwrap().candidate
)
1211 fn is_knowable
<'o
>(&mut self,
1212 stack
: &TraitObligationStack
<'o
, 'tcx
>)
1215 debug
!("is_knowable(intercrate={:?})", self.intercrate
);
1217 if !self.intercrate
.is_some() {
1221 let obligation
= &stack
.obligation
;
1222 let predicate
= self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
);
1224 // ok to skip binder because of the nature of the
1225 // trait-ref-is-knowable check, which does not care about
1227 let trait_ref
= predicate
.skip_binder().trait_ref
;
1229 let result
= coherence
::trait_ref_is_knowable(self.tcx(), trait_ref
);
1230 if let (Some(Conflict
::Downstream { used_to_be_broken: true }
),
1231 Some(IntercrateMode
::Issue43355
)) = (result
, self.intercrate
) {
1232 debug
!("is_knowable: IGNORING conflict to be bug-compatible with #43355");
1239 /// Returns true if the global caches can be used.
1240 /// Do note that if the type itself is not in the
1241 /// global tcx, the local caches will be used.
1242 fn can_use_global_caches(&self, param_env
: ty
::ParamEnv
<'tcx
>) -> bool
{
1243 // If there are any where-clauses in scope, then we always use
1244 // a cache local to this particular scope. Otherwise, we
1245 // switch to a global cache. We used to try and draw
1246 // finer-grained distinctions, but that led to a serious of
1247 // annoying and weird bugs like #22019 and #18290. This simple
1248 // rule seems to be pretty clearly safe and also still retains
1249 // a very high hit rate (~95% when compiling rustc).
1250 if !param_env
.caller_bounds
.is_empty() {
1254 // Avoid using the master cache during coherence and just rely
1255 // on the local cache. This effectively disables caching
1256 // during coherence. It is really just a simplification to
1257 // avoid us having to fear that coherence results "pollute"
1258 // the master cache. Since coherence executes pretty quickly,
1259 // it's not worth going to more trouble to increase the
1260 // hit-rate I don't think.
1261 if self.intercrate
.is_some() {
1265 // Otherwise, we can use the global cache.
1269 fn check_candidate_cache(&mut self,
1270 param_env
: ty
::ParamEnv
<'tcx
>,
1271 cache_fresh_trait_pred
: &ty
::PolyTraitPredicate
<'tcx
>)
1272 -> Option
<SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>>
1274 let tcx
= self.tcx();
1275 let trait_ref
= &cache_fresh_trait_pred
.0.trait_ref
;
1276 if self.can_use_global_caches(param_env
) {
1277 let cache
= tcx
.selection_cache
.hashmap
.borrow();
1278 if let Some(cached
) = cache
.get(&trait_ref
) {
1279 return Some(cached
.get(tcx
));
1282 self.infcx
.selection_cache
.hashmap
1285 .map(|v
| v
.get(tcx
))
1288 fn insert_candidate_cache(&mut self,
1289 param_env
: ty
::ParamEnv
<'tcx
>,
1290 cache_fresh_trait_pred
: ty
::PolyTraitPredicate
<'tcx
>,
1291 dep_node
: DepNodeIndex
,
1292 candidate
: SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>)
1294 let tcx
= self.tcx();
1295 let trait_ref
= cache_fresh_trait_pred
.0.trait_ref
;
1296 if self.can_use_global_caches(param_env
) {
1297 let mut cache
= tcx
.selection_cache
.hashmap
.borrow_mut();
1298 if let Some(trait_ref
) = tcx
.lift_to_global(&trait_ref
) {
1299 if let Some(candidate
) = tcx
.lift_to_global(&candidate
) {
1301 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) global",
1305 cache
.insert(trait_ref
, WithDepNode
::new(dep_node
, candidate
));
1312 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) local",
1316 self.infcx
.selection_cache
.hashmap
1318 .insert(trait_ref
, WithDepNode
::new(dep_node
, candidate
));
1321 fn assemble_candidates
<'o
>(&mut self,
1322 stack
: &TraitObligationStack
<'o
, 'tcx
>)
1323 -> Result
<SelectionCandidateSet
<'tcx
>, SelectionError
<'tcx
>>
1325 let TraitObligationStack { obligation, .. }
= *stack
;
1326 let ref obligation
= Obligation
{
1327 param_env
: obligation
.param_env
,
1328 cause
: obligation
.cause
.clone(),
1329 recursion_depth
: obligation
.recursion_depth
,
1330 predicate
: self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
)
1333 if obligation
.predicate
.skip_binder().self_ty().is_ty_var() {
1334 // Self is a type variable (e.g. `_: AsRef<str>`).
1336 // This is somewhat problematic, as the current scheme can't really
1337 // handle it turning to be a projection. This does end up as truly
1338 // ambiguous in most cases anyway.
1340 // Take the fast path out - this also improves
1341 // performance by preventing assemble_candidates_from_impls from
1342 // matching every impl for this trait.
1343 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true }
);
1346 let mut candidates
= SelectionCandidateSet
{
1351 // Other bounds. Consider both in-scope bounds from fn decl
1352 // and applicable impls. There is a certain set of precedence rules here.
1354 let def_id
= obligation
.predicate
.def_id();
1355 let lang_items
= self.tcx().lang_items();
1356 if lang_items
.copy_trait() == Some(def_id
) {
1357 debug
!("obligation self ty is {:?}",
1358 obligation
.predicate
.0.self_ty());
1360 // User-defined copy impls are permitted, but only for
1361 // structs and enums.
1362 self.assemble_candidates_from_impls(obligation
, &mut candidates
)?
;
1364 // For other types, we'll use the builtin rules.
1365 let copy_conditions
= self.copy_clone_conditions(obligation
);
1366 self.assemble_builtin_bound_candidates(copy_conditions
, &mut candidates
)?
;
1367 } else if lang_items
.sized_trait() == Some(def_id
) {
1368 // Sized is never implementable by end-users, it is
1369 // always automatically computed.
1370 let sized_conditions
= self.sized_conditions(obligation
);
1371 self.assemble_builtin_bound_candidates(sized_conditions
,
1373 } else if lang_items
.unsize_trait() == Some(def_id
) {
1374 self.assemble_candidates_for_unsizing(obligation
, &mut candidates
);
1376 if lang_items
.clone_trait() == Some(def_id
) {
1377 // Same builtin conditions as `Copy`, i.e. every type which has builtin support
1378 // for `Copy` also has builtin support for `Clone`, + tuples and arrays of `Clone`
1379 // types have builtin support for `Clone`.
1380 let clone_conditions
= self.copy_clone_conditions(obligation
);
1381 self.assemble_builtin_bound_candidates(clone_conditions
, &mut candidates
)?
;
1384 self.assemble_generator_candidates(obligation
, &mut candidates
)?
;
1385 self.assemble_closure_candidates(obligation
, &mut candidates
)?
;
1386 self.assemble_fn_pointer_candidates(obligation
, &mut candidates
)?
;
1387 self.assemble_candidates_from_impls(obligation
, &mut candidates
)?
;
1388 self.assemble_candidates_from_object_ty(obligation
, &mut candidates
);
1391 self.assemble_candidates_from_projected_tys(obligation
, &mut candidates
);
1392 self.assemble_candidates_from_caller_bounds(stack
, &mut candidates
)?
;
1393 // Auto implementations have lower priority, so we only
1394 // consider triggering a default if there is no other impl that can apply.
1395 if candidates
.vec
.is_empty() {
1396 self.assemble_candidates_from_auto_impls(obligation
, &mut candidates
)?
;
1398 debug
!("candidate list size: {}", candidates
.vec
.len());
1402 fn assemble_candidates_from_projected_tys(&mut self,
1403 obligation
: &TraitObligation
<'tcx
>,
1404 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1406 debug
!("assemble_candidates_for_projected_tys({:?})", obligation
);
1408 // before we go into the whole skolemization thing, just
1409 // quickly check if the self-type is a projection at all.
1410 match obligation
.predicate
.0.trait_ref
.self_ty().sty
{
1411 ty
::TyProjection(_
) | ty
::TyAnon(..) => {}
1412 ty
::TyInfer(ty
::TyVar(_
)) => {
1413 span_bug
!(obligation
.cause
.span
,
1414 "Self=_ should have been handled by assemble_candidates");
1419 let result
= self.probe(|this
, snapshot
| {
1420 this
.match_projection_obligation_against_definition_bounds(obligation
,
1425 candidates
.vec
.push(ProjectionCandidate
);
1429 fn match_projection_obligation_against_definition_bounds(
1431 obligation
: &TraitObligation
<'tcx
>,
1432 snapshot
: &infer
::CombinedSnapshot
<'cx
, 'tcx
>)
1435 let poly_trait_predicate
=
1436 self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
);
1437 let (skol_trait_predicate
, skol_map
) =
1438 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate
, snapshot
);
1439 debug
!("match_projection_obligation_against_definition_bounds: \
1440 skol_trait_predicate={:?} skol_map={:?}",
1441 skol_trait_predicate
,
1444 let (def_id
, substs
) = match skol_trait_predicate
.trait_ref
.self_ty().sty
{
1445 ty
::TyProjection(ref data
) =>
1446 (data
.trait_ref(self.tcx()).def_id
, data
.substs
),
1447 ty
::TyAnon(def_id
, substs
) => (def_id
, substs
),
1450 obligation
.cause
.span
,
1451 "match_projection_obligation_against_definition_bounds() called \
1452 but self-ty not a projection: {:?}",
1453 skol_trait_predicate
.trait_ref
.self_ty());
1456 debug
!("match_projection_obligation_against_definition_bounds: \
1457 def_id={:?}, substs={:?}",
1460 let predicates_of
= self.tcx().predicates_of(def_id
);
1461 let bounds
= predicates_of
.instantiate(self.tcx(), substs
);
1462 debug
!("match_projection_obligation_against_definition_bounds: \
1466 let matching_bound
=
1467 util
::elaborate_predicates(self.tcx(), bounds
.predicates
)
1471 |this
, _
| this
.match_projection(obligation
,
1473 skol_trait_predicate
.trait_ref
.clone(),
1477 debug
!("match_projection_obligation_against_definition_bounds: \
1478 matching_bound={:?}",
1480 match matching_bound
{
1483 // Repeat the successful match, if any, this time outside of a probe.
1484 let result
= self.match_projection(obligation
,
1486 skol_trait_predicate
.trait_ref
.clone(),
1490 self.infcx
.pop_skolemized(skol_map
, snapshot
);
1498 fn match_projection(&mut self,
1499 obligation
: &TraitObligation
<'tcx
>,
1500 trait_bound
: ty
::PolyTraitRef
<'tcx
>,
1501 skol_trait_ref
: ty
::TraitRef
<'tcx
>,
1502 skol_map
: &infer
::SkolemizationMap
<'tcx
>,
1503 snapshot
: &infer
::CombinedSnapshot
<'cx
, 'tcx
>)
1506 assert
!(!skol_trait_ref
.has_escaping_regions());
1507 if let Err(_
) = self.infcx
.at(&obligation
.cause
, obligation
.param_env
)
1508 .sup(ty
::Binder(skol_trait_ref
), trait_bound
) {
1512 self.infcx
.leak_check(false, obligation
.cause
.span
, skol_map
, snapshot
).is_ok()
1515 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1516 /// supplied to find out whether it is listed among them.
1518 /// Never affects inference environment.
1519 fn assemble_candidates_from_caller_bounds
<'o
>(&mut self,
1520 stack
: &TraitObligationStack
<'o
, 'tcx
>,
1521 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1522 -> Result
<(),SelectionError
<'tcx
>>
1524 debug
!("assemble_candidates_from_caller_bounds({:?})",
1528 stack
.obligation
.param_env
.caller_bounds
1530 .filter_map(|o
| o
.to_opt_poly_trait_ref());
1532 // micro-optimization: filter out predicates relating to different
1534 let matching_bounds
=
1535 all_bounds
.filter(|p
| p
.def_id() == stack
.obligation
.predicate
.def_id());
1537 let matching_bounds
=
1538 matching_bounds
.filter(
1539 |bound
| self.evaluate_where_clause(stack
, bound
.clone()).may_apply());
1541 let param_candidates
=
1542 matching_bounds
.map(|bound
| ParamCandidate(bound
));
1544 candidates
.vec
.extend(param_candidates
);
1549 fn evaluate_where_clause
<'o
>(&mut self,
1550 stack
: &TraitObligationStack
<'o
, 'tcx
>,
1551 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
1554 self.probe(move |this
, _
| {
1555 match this
.match_where_clause_trait_ref(stack
.obligation
, where_clause_trait_ref
) {
1556 Ok(obligations
) => {
1557 this
.evaluate_predicates_recursively(stack
.list(), obligations
.iter())
1559 Err(()) => EvaluatedToErr
1564 fn assemble_generator_candidates(&mut self,
1565 obligation
: &TraitObligation
<'tcx
>,
1566 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1567 -> Result
<(),SelectionError
<'tcx
>>
1569 if self.tcx().lang_items().gen_trait() != Some(obligation
.predicate
.def_id()) {
1573 // ok to skip binder because the substs on generator types never
1574 // touch bound regions, they just capture the in-scope
1575 // type/region parameters
1576 let self_ty
= *obligation
.self_ty().skip_binder();
1578 ty
::TyGenerator(..) => {
1579 debug
!("assemble_generator_candidates: self_ty={:?} obligation={:?}",
1583 candidates
.vec
.push(GeneratorCandidate
);
1586 ty
::TyInfer(ty
::TyVar(_
)) => {
1587 debug
!("assemble_generator_candidates: ambiguous self-type");
1588 candidates
.ambiguous
= true;
1591 _
=> { return Ok(()); }
1595 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1596 /// FnMut<..>` where `X` is a closure type.
1598 /// Note: the type parameters on a closure candidate are modeled as *output* type
1599 /// parameters and hence do not affect whether this trait is a match or not. They will be
1600 /// unified during the confirmation step.
1601 fn assemble_closure_candidates(&mut self,
1602 obligation
: &TraitObligation
<'tcx
>,
1603 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1604 -> Result
<(),SelectionError
<'tcx
>>
1606 let kind
= match self.tcx().lang_items().fn_trait_kind(obligation
.predicate
.0.def_id()) {
1608 None
=> { return Ok(()); }
1611 // ok to skip binder because the substs on closure types never
1612 // touch bound regions, they just capture the in-scope
1613 // type/region parameters
1614 match obligation
.self_ty().skip_binder().sty
{
1615 ty
::TyClosure(closure_def_id
, closure_substs
) => {
1616 debug
!("assemble_unboxed_candidates: kind={:?} obligation={:?}",
1618 match self.infcx
.closure_kind(closure_def_id
, closure_substs
) {
1619 Some(closure_kind
) => {
1620 debug
!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind
);
1621 if closure_kind
.extends(kind
) {
1622 candidates
.vec
.push(ClosureCandidate
);
1626 debug
!("assemble_unboxed_candidates: closure_kind not yet known");
1627 candidates
.vec
.push(ClosureCandidate
);
1632 ty
::TyInfer(ty
::TyVar(_
)) => {
1633 debug
!("assemble_unboxed_closure_candidates: ambiguous self-type");
1634 candidates
.ambiguous
= true;
1637 _
=> { return Ok(()); }
1641 /// Implement one of the `Fn()` family for a fn pointer.
1642 fn assemble_fn_pointer_candidates(&mut self,
1643 obligation
: &TraitObligation
<'tcx
>,
1644 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1645 -> Result
<(),SelectionError
<'tcx
>>
1647 // We provide impl of all fn traits for fn pointers.
1648 if self.tcx().lang_items().fn_trait_kind(obligation
.predicate
.def_id()).is_none() {
1652 // ok to skip binder because what we are inspecting doesn't involve bound regions
1653 let self_ty
= *obligation
.self_ty().skip_binder();
1655 ty
::TyInfer(ty
::TyVar(_
)) => {
1656 debug
!("assemble_fn_pointer_candidates: ambiguous self-type");
1657 candidates
.ambiguous
= true; // could wind up being a fn() type
1660 // provide an impl, but only for suitable `fn` pointers
1661 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) => {
1662 if let ty
::Binder(ty
::FnSig
{
1663 unsafety
: hir
::Unsafety
::Normal
,
1667 }) = self_ty
.fn_sig(self.tcx()) {
1668 candidates
.vec
.push(FnPointerCandidate
);
1678 /// Search for impls that might apply to `obligation`.
1679 fn assemble_candidates_from_impls(&mut self,
1680 obligation
: &TraitObligation
<'tcx
>,
1681 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1682 -> Result
<(), SelectionError
<'tcx
>>
1684 debug
!("assemble_candidates_from_impls(obligation={:?})", obligation
);
1686 self.tcx().for_each_relevant_impl(
1687 obligation
.predicate
.def_id(),
1688 obligation
.predicate
.0.trait_ref
.self_ty(),
1690 self.probe(|this
, snapshot
| { /* [1] */
1691 match this
.match_impl(impl_def_id
, obligation
, snapshot
) {
1693 candidates
.vec
.push(ImplCandidate(impl_def_id
));
1695 // NB: we can safely drop the skol map
1696 // since we are in a probe [1]
1697 mem
::drop(skol_map
);
1708 fn assemble_candidates_from_auto_impls(&mut self,
1709 obligation
: &TraitObligation
<'tcx
>,
1710 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1711 -> Result
<(), SelectionError
<'tcx
>>
1713 // OK to skip binder here because the tests we do below do not involve bound regions
1714 let self_ty
= *obligation
.self_ty().skip_binder();
1715 debug
!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty
);
1717 let def_id
= obligation
.predicate
.def_id();
1719 if self.tcx().trait_is_auto(def_id
) {
1721 ty
::TyDynamic(..) => {
1722 // For object types, we don't know what the closed
1723 // over types are. This means we conservatively
1724 // say nothing; a candidate may be added by
1725 // `assemble_candidates_from_object_ty`.
1727 ty
::TyForeign(..) => {
1728 // Since the contents of foreign types is unknown,
1729 // we don't add any `..` impl. Default traits could
1730 // still be provided by a manual implementation for
1731 // this trait and type.
1734 ty
::TyProjection(..) => {
1735 // In these cases, we don't know what the actual
1736 // type is. Therefore, we cannot break it down
1737 // into its constituent types. So we don't
1738 // consider the `..` impl but instead just add no
1739 // candidates: this means that typeck will only
1740 // succeed if there is another reason to believe
1741 // that this obligation holds. That could be a
1742 // where-clause or, in the case of an object type,
1743 // it could be that the object type lists the
1744 // trait (e.g. `Foo+Send : Send`). See
1745 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1746 // for an example of a test case that exercises
1749 ty
::TyInfer(ty
::TyVar(_
)) => {
1750 // the auto impl might apply, we don't know
1751 candidates
.ambiguous
= true;
1754 candidates
.vec
.push(AutoImplCandidate(def_id
.clone()))
1762 /// Search for impls that might apply to `obligation`.
1763 fn assemble_candidates_from_object_ty(&mut self,
1764 obligation
: &TraitObligation
<'tcx
>,
1765 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1767 debug
!("assemble_candidates_from_object_ty(self_ty={:?})",
1768 obligation
.self_ty().skip_binder());
1770 // Object-safety candidates are only applicable to object-safe
1771 // traits. Including this check is useful because it helps
1772 // inference in cases of traits like `BorrowFrom`, which are
1773 // not object-safe, and which rely on being able to infer the
1774 // self-type from one of the other inputs. Without this check,
1775 // these cases wind up being considered ambiguous due to a
1776 // (spurious) ambiguity introduced here.
1777 let predicate_trait_ref
= obligation
.predicate
.to_poly_trait_ref();
1778 if !self.tcx().is_object_safe(predicate_trait_ref
.def_id()) {
1782 self.probe(|this
, _snapshot
| {
1783 // the code below doesn't care about regions, and the
1784 // self-ty here doesn't escape this probe, so just erase
1786 let self_ty
= this
.tcx().erase_late_bound_regions(&obligation
.self_ty());
1787 let poly_trait_ref
= match self_ty
.sty
{
1788 ty
::TyDynamic(ref data
, ..) => {
1789 if data
.auto_traits().any(|did
| did
== obligation
.predicate
.def_id()) {
1790 debug
!("assemble_candidates_from_object_ty: matched builtin bound, \
1791 pushing candidate");
1792 candidates
.vec
.push(BuiltinObjectCandidate
);
1796 match data
.principal() {
1797 Some(p
) => p
.with_self_ty(this
.tcx(), self_ty
),
1801 ty
::TyInfer(ty
::TyVar(_
)) => {
1802 debug
!("assemble_candidates_from_object_ty: ambiguous");
1803 candidates
.ambiguous
= true; // could wind up being an object type
1811 debug
!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1814 // Count only those upcast versions that match the trait-ref
1815 // we are looking for. Specifically, do not only check for the
1816 // correct trait, but also the correct type parameters.
1817 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1818 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1819 let upcast_trait_refs
=
1820 util
::supertraits(this
.tcx(), poly_trait_ref
)
1821 .filter(|upcast_trait_ref
| {
1822 this
.probe(|this
, _
| {
1823 let upcast_trait_ref
= upcast_trait_ref
.clone();
1824 this
.match_poly_trait_ref(obligation
, upcast_trait_ref
).is_ok()
1829 if upcast_trait_refs
> 1 {
1830 // can be upcast in many ways; need more type information
1831 candidates
.ambiguous
= true;
1832 } else if upcast_trait_refs
== 1 {
1833 candidates
.vec
.push(ObjectCandidate
);
1838 /// Search for unsizing that might apply to `obligation`.
1839 fn assemble_candidates_for_unsizing(&mut self,
1840 obligation
: &TraitObligation
<'tcx
>,
1841 candidates
: &mut SelectionCandidateSet
<'tcx
>) {
1842 // We currently never consider higher-ranked obligations e.g.
1843 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1844 // because they are a priori invalid, and we could potentially add support
1845 // for them later, it's just that there isn't really a strong need for it.
1846 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1847 // impl, and those are generally applied to concrete types.
1849 // That said, one might try to write a fn with a where clause like
1850 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1851 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1852 // Still, you'd be more likely to write that where clause as
1854 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1855 // obligation above. Should be possible to extend this in the future.
1856 let source
= match obligation
.self_ty().no_late_bound_regions() {
1859 // Don't add any candidates if there are bound regions.
1863 let target
= obligation
.predicate
.skip_binder().trait_ref
.substs
.type_at(1);
1865 debug
!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1868 let may_apply
= match (&source
.sty
, &target
.sty
) {
1869 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1870 (&ty
::TyDynamic(ref data_a
, ..), &ty
::TyDynamic(ref data_b
, ..)) => {
1871 // Upcasts permit two things:
1873 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1874 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1876 // Note that neither of these changes requires any
1877 // change at runtime. Eventually this will be
1880 // We always upcast when we can because of reason
1881 // #2 (region bounds).
1882 match (data_a
.principal(), data_b
.principal()) {
1883 (Some(a
), Some(b
)) => a
.def_id() == b
.def_id() &&
1884 data_b
.auto_traits()
1885 // All of a's auto traits need to be in b's auto traits.
1886 .all(|b
| data_a
.auto_traits().any(|a
| a
== b
)),
1892 (_
, &ty
::TyDynamic(..)) => true,
1894 // Ambiguous handling is below T -> Trait, because inference
1895 // variables can still implement Unsize<Trait> and nested
1896 // obligations will have the final say (likely deferred).
1897 (&ty
::TyInfer(ty
::TyVar(_
)), _
) |
1898 (_
, &ty
::TyInfer(ty
::TyVar(_
))) => {
1899 debug
!("assemble_candidates_for_unsizing: ambiguous");
1900 candidates
.ambiguous
= true;
1905 (&ty
::TyArray(..), &ty
::TySlice(_
)) => true,
1907 // Struct<T> -> Struct<U>.
1908 (&ty
::TyAdt(def_id_a
, _
), &ty
::TyAdt(def_id_b
, _
)) if def_id_a
.is_struct() => {
1909 def_id_a
== def_id_b
1912 // (.., T) -> (.., U).
1913 (&ty
::TyTuple(tys_a
), &ty
::TyTuple(tys_b
)) => {
1914 tys_a
.len() == tys_b
.len()
1921 candidates
.vec
.push(BuiltinUnsizeCandidate
);
1925 ///////////////////////////////////////////////////////////////////////////
1928 // Winnowing is the process of attempting to resolve ambiguity by
1929 // probing further. During the winnowing process, we unify all
1930 // type variables (ignoring skolemization) and then we also
1931 // attempt to evaluate recursive bounds to see if they are
1934 /// Returns true if `candidate_i` should be dropped in favor of
1935 /// `candidate_j`. Generally speaking we will drop duplicate
1936 /// candidates and prefer where-clause candidates.
1937 /// Returns true if `victim` should be dropped in favor of
1938 /// `other`. Generally speaking we will drop duplicate
1939 /// candidates and prefer where-clause candidates.
1941 /// See the comment for "SelectionCandidate" for more details.
1942 fn candidate_should_be_dropped_in_favor_of
<'o
>(
1944 victim
: &EvaluatedCandidate
<'tcx
>,
1945 other
: &EvaluatedCandidate
<'tcx
>)
1948 if victim
.candidate
== other
.candidate
{
1952 match other
.candidate
{
1954 ParamCandidate(_
) | ProjectionCandidate
=> match victim
.candidate
{
1955 AutoImplCandidate(..) => {
1957 "default implementations shouldn't be recorded \
1958 when there are other valid candidates");
1962 GeneratorCandidate
|
1963 FnPointerCandidate
|
1964 BuiltinObjectCandidate
|
1965 BuiltinUnsizeCandidate
|
1966 BuiltinCandidate { .. }
=> {
1967 // We have a where-clause so don't go around looking
1972 ProjectionCandidate
=> {
1973 // Arbitrarily give param candidates priority
1974 // over projection and object candidates.
1977 ParamCandidate(..) => false,
1979 ImplCandidate(other_def
) => {
1980 // See if we can toss out `victim` based on specialization.
1981 // This requires us to know *for sure* that the `other` impl applies
1982 // i.e. EvaluatedToOk:
1983 if other
.evaluation
== EvaluatedToOk
{
1984 if let ImplCandidate(victim_def
) = victim
.candidate
{
1985 let tcx
= self.tcx().global_tcx();
1986 return tcx
.specializes((other_def
, victim_def
)) ||
1987 tcx
.impls_are_allowed_to_overlap(other_def
, victim_def
);
1997 ///////////////////////////////////////////////////////////////////////////
2000 // These cover the traits that are built-in to the language
2001 // itself. This includes `Copy` and `Sized` for sure. For the
2002 // moment, it also includes `Send` / `Sync` and a few others, but
2003 // those will hopefully change to library-defined traits in the
2006 // HACK: if this returns an error, selection exits without considering
2008 fn assemble_builtin_bound_candidates
<'o
>(&mut self,
2009 conditions
: BuiltinImplConditions
<'tcx
>,
2010 candidates
: &mut SelectionCandidateSet
<'tcx
>)
2011 -> Result
<(),SelectionError
<'tcx
>>
2014 BuiltinImplConditions
::Where(nested
) => {
2015 debug
!("builtin_bound: nested={:?}", nested
);
2016 candidates
.vec
.push(BuiltinCandidate
{
2017 has_nested
: nested
.skip_binder().len() > 0
2021 BuiltinImplConditions
::None
=> { Ok(()) }
2022 BuiltinImplConditions
::Ambiguous
=> {
2023 debug
!("assemble_builtin_bound_candidates: ambiguous builtin");
2024 Ok(candidates
.ambiguous
= true)
2026 BuiltinImplConditions
::Never
=> { Err(Unimplemented) }
2030 fn sized_conditions(&mut self, obligation
: &TraitObligation
<'tcx
>)
2031 -> BuiltinImplConditions
<'tcx
>
2033 use self::BuiltinImplConditions
::{Ambiguous, None, Never, Where}
;
2035 // NOTE: binder moved to (*)
2036 let self_ty
= self.infcx
.shallow_resolve(
2037 obligation
.predicate
.skip_binder().self_ty());
2040 ty
::TyInfer(ty
::IntVar(_
)) | ty
::TyInfer(ty
::FloatVar(_
)) |
2041 ty
::TyUint(_
) | ty
::TyInt(_
) | ty
::TyBool
| ty
::TyFloat(_
) |
2042 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) | ty
::TyRawPtr(..) |
2043 ty
::TyChar
| ty
::TyRef(..) | ty
::TyGenerator(..) |
2044 ty
::TyGeneratorWitness(..) | ty
::TyArray(..) | ty
::TyClosure(..) |
2045 ty
::TyNever
| ty
::TyError
=> {
2046 // safe for everything
2047 Where(ty
::Binder(Vec
::new()))
2050 ty
::TyStr
| ty
::TySlice(_
) | ty
::TyDynamic(..) | ty
::TyForeign(..) => Never
,
2052 ty
::TyTuple(tys
) => {
2053 Where(ty
::Binder(tys
.last().into_iter().cloned().collect()))
2056 ty
::TyAdt(def
, substs
) => {
2057 let sized_crit
= def
.sized_constraint(self.tcx());
2058 // (*) binder moved here
2060 sized_crit
.iter().map(|ty
| ty
.subst(self.tcx(), substs
)).collect()
2064 ty
::TyProjection(_
) | ty
::TyParam(_
) | ty
::TyAnon(..) => None
,
2065 ty
::TyInfer(ty
::TyVar(_
)) => Ambiguous
,
2067 ty
::TyInfer(ty
::CanonicalTy(_
)) |
2068 ty
::TyInfer(ty
::FreshTy(_
)) |
2069 ty
::TyInfer(ty
::FreshIntTy(_
)) |
2070 ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
2071 bug
!("asked to assemble builtin bounds of unexpected type: {:?}",
2077 fn copy_clone_conditions(&mut self, obligation
: &TraitObligation
<'tcx
>)
2078 -> BuiltinImplConditions
<'tcx
>
2080 // NOTE: binder moved to (*)
2081 let self_ty
= self.infcx
.shallow_resolve(
2082 obligation
.predicate
.skip_binder().self_ty());
2084 use self::BuiltinImplConditions
::{Ambiguous, None, Never, Where}
;
2087 ty
::TyInfer(ty
::IntVar(_
)) | ty
::TyInfer(ty
::FloatVar(_
)) |
2088 ty
::TyUint(_
) | ty
::TyInt(_
) | ty
::TyBool
| ty
::TyFloat(_
) |
2089 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) | ty
::TyChar
|
2090 ty
::TyRawPtr(..) | ty
::TyError
| ty
::TyNever
|
2091 ty
::TyRef(_
, ty
::TypeAndMut { ty: _, mutbl: hir::MutImmutable }
) => {
2092 Where(ty
::Binder(Vec
::new()))
2095 ty
::TyDynamic(..) | ty
::TyStr
| ty
::TySlice(..) |
2096 ty
::TyGenerator(..) | ty
::TyGeneratorWitness(..) | ty
::TyForeign(..) |
2097 ty
::TyRef(_
, ty
::TypeAndMut { ty: _, mutbl: hir::MutMutable }
) => {
2101 ty
::TyArray(element_ty
, _
) => {
2102 // (*) binder moved here
2103 Where(ty
::Binder(vec
![element_ty
]))
2106 ty
::TyTuple(tys
) => {
2107 // (*) binder moved here
2108 Where(ty
::Binder(tys
.to_vec()))
2111 ty
::TyClosure(def_id
, substs
) => {
2112 let trait_id
= obligation
.predicate
.def_id();
2113 let is_copy_trait
= Some(trait_id
) == self.tcx().lang_items().copy_trait();
2114 let is_clone_trait
= Some(trait_id
) == self.tcx().lang_items().clone_trait();
2115 if is_copy_trait
|| is_clone_trait
{
2116 Where(ty
::Binder(substs
.upvar_tys(def_id
, self.tcx()).collect()))
2122 ty
::TyAdt(..) | ty
::TyProjection(..) | ty
::TyParam(..) | ty
::TyAnon(..) => {
2123 // Fallback to whatever user-defined impls exist in this case.
2127 ty
::TyInfer(ty
::TyVar(_
)) => {
2128 // Unbound type variable. Might or might not have
2129 // applicable impls and so forth, depending on what
2130 // those type variables wind up being bound to.
2134 ty
::TyInfer(ty
::CanonicalTy(_
)) |
2135 ty
::TyInfer(ty
::FreshTy(_
)) |
2136 ty
::TyInfer(ty
::FreshIntTy(_
)) |
2137 ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
2138 bug
!("asked to assemble builtin bounds of unexpected type: {:?}",
2144 /// For default impls, we need to break apart a type into its
2145 /// "constituent types" -- meaning, the types that it contains.
2147 /// Here are some (simple) examples:
2150 /// (i32, u32) -> [i32, u32]
2151 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2152 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2153 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2155 fn constituent_types_for_ty(&self, t
: Ty
<'tcx
>) -> Vec
<Ty
<'tcx
>> {
2165 ty
::TyInfer(ty
::IntVar(_
)) |
2166 ty
::TyInfer(ty
::FloatVar(_
)) |
2175 ty
::TyProjection(..) |
2176 ty
::TyInfer(ty
::CanonicalTy(_
)) |
2177 ty
::TyInfer(ty
::TyVar(_
)) |
2178 ty
::TyInfer(ty
::FreshTy(_
)) |
2179 ty
::TyInfer(ty
::FreshIntTy(_
)) |
2180 ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
2181 bug
!("asked to assemble constituent types of unexpected type: {:?}",
2185 ty
::TyRawPtr(ty
::TypeAndMut { ty: element_ty, ..}
) |
2186 ty
::TyRef(_
, ty
::TypeAndMut { ty: element_ty, ..}
) => {
2190 ty
::TyArray(element_ty
, _
) | ty
::TySlice(element_ty
) => {
2194 ty
::TyTuple(ref tys
) => {
2195 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2199 ty
::TyClosure(def_id
, ref substs
) => {
2200 substs
.upvar_tys(def_id
, self.tcx()).collect()
2203 ty
::TyGenerator(def_id
, ref substs
, interior
) => {
2204 substs
.upvar_tys(def_id
, self.tcx()).chain(iter
::once(interior
.witness
)).collect()
2207 ty
::TyGeneratorWitness(types
) => {
2208 // This is sound because no regions in the witness can refer to
2209 // the binder outside the witness. So we'll effectivly reuse
2210 // the implicit binder around the witness.
2211 types
.skip_binder().to_vec()
2214 // for `PhantomData<T>`, we pass `T`
2215 ty
::TyAdt(def
, substs
) if def
.is_phantom_data() => {
2216 substs
.types().collect()
2219 ty
::TyAdt(def
, substs
) => {
2221 .map(|f
| f
.ty(self.tcx(), substs
))
2225 ty
::TyAnon(def_id
, substs
) => {
2226 // We can resolve the `impl Trait` to its concrete type,
2227 // which enforces a DAG between the functions requiring
2228 // the auto trait bounds in question.
2229 vec
![self.tcx().type_of(def_id
).subst(self.tcx(), substs
)]
2234 fn collect_predicates_for_types(&mut self,
2235 param_env
: ty
::ParamEnv
<'tcx
>,
2236 cause
: ObligationCause
<'tcx
>,
2237 recursion_depth
: usize,
2238 trait_def_id
: DefId
,
2239 types
: ty
::Binder
<Vec
<Ty
<'tcx
>>>)
2240 -> Vec
<PredicateObligation
<'tcx
>>
2242 // Because the types were potentially derived from
2243 // higher-ranked obligations they may reference late-bound
2244 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
2245 // yield a type like `for<'a> &'a int`. In general, we
2246 // maintain the invariant that we never manipulate bound
2247 // regions, so we have to process these bound regions somehow.
2249 // The strategy is to:
2251 // 1. Instantiate those regions to skolemized regions (e.g.,
2252 // `for<'a> &'a int` becomes `&0 int`.
2253 // 2. Produce something like `&'0 int : Copy`
2254 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
2256 types
.skip_binder().into_iter().flat_map(|ty
| { // binder moved -\
2257 let ty
: ty
::Binder
<Ty
<'tcx
>> = ty
::Binder(ty
); // <----------/
2259 self.in_snapshot(|this
, snapshot
| {
2260 let (skol_ty
, skol_map
) =
2261 this
.infcx().skolemize_late_bound_regions(&ty
, snapshot
);
2262 let Normalized { value: normalized_ty, mut obligations }
=
2263 project
::normalize_with_depth(this
,
2268 let skol_obligation
=
2269 this
.tcx().predicate_for_trait_def(param_env
,
2275 obligations
.push(skol_obligation
);
2276 this
.infcx().plug_leaks(skol_map
, snapshot
, obligations
)
2281 ///////////////////////////////////////////////////////////////////////////
2284 // Confirmation unifies the output type parameters of the trait
2285 // with the values found in the obligation, possibly yielding a
2286 // type error. See [rustc guide] for more details.
2289 // https://rust-lang-nursery.github.io/rustc-guide/trait-resolution.html#confirmation
2291 fn confirm_candidate(&mut self,
2292 obligation
: &TraitObligation
<'tcx
>,
2293 candidate
: SelectionCandidate
<'tcx
>)
2294 -> Result
<Selection
<'tcx
>,SelectionError
<'tcx
>>
2296 debug
!("confirm_candidate({:?}, {:?})",
2301 BuiltinCandidate { has_nested }
=> {
2302 let data
= self.confirm_builtin_candidate(obligation
, has_nested
);
2303 Ok(VtableBuiltin(data
))
2306 ParamCandidate(param
) => {
2307 let obligations
= self.confirm_param_candidate(obligation
, param
);
2308 Ok(VtableParam(obligations
))
2311 AutoImplCandidate(trait_def_id
) => {
2312 let data
= self.confirm_auto_impl_candidate(obligation
, trait_def_id
);
2313 Ok(VtableAutoImpl(data
))
2316 ImplCandidate(impl_def_id
) => {
2317 Ok(VtableImpl(self.confirm_impl_candidate(obligation
, impl_def_id
)))
2320 ClosureCandidate
=> {
2321 let vtable_closure
= self.confirm_closure_candidate(obligation
)?
;
2322 Ok(VtableClosure(vtable_closure
))
2325 GeneratorCandidate
=> {
2326 let vtable_generator
= self.confirm_generator_candidate(obligation
)?
;
2327 Ok(VtableGenerator(vtable_generator
))
2330 BuiltinObjectCandidate
=> {
2331 // This indicates something like `(Trait+Send) :
2332 // Send`. In this case, we know that this holds
2333 // because that's what the object type is telling us,
2334 // and there's really no additional obligations to
2335 // prove and no types in particular to unify etc.
2336 Ok(VtableParam(Vec
::new()))
2339 ObjectCandidate
=> {
2340 let data
= self.confirm_object_candidate(obligation
);
2341 Ok(VtableObject(data
))
2344 FnPointerCandidate
=> {
2346 self.confirm_fn_pointer_candidate(obligation
)?
;
2347 Ok(VtableFnPointer(data
))
2350 ProjectionCandidate
=> {
2351 self.confirm_projection_candidate(obligation
);
2352 Ok(VtableParam(Vec
::new()))
2355 BuiltinUnsizeCandidate
=> {
2356 let data
= self.confirm_builtin_unsize_candidate(obligation
)?
;
2357 Ok(VtableBuiltin(data
))
2362 fn confirm_projection_candidate(&mut self,
2363 obligation
: &TraitObligation
<'tcx
>)
2365 self.in_snapshot(|this
, snapshot
| {
2367 this
.match_projection_obligation_against_definition_bounds(obligation
,
2373 fn confirm_param_candidate(&mut self,
2374 obligation
: &TraitObligation
<'tcx
>,
2375 param
: ty
::PolyTraitRef
<'tcx
>)
2376 -> Vec
<PredicateObligation
<'tcx
>>
2378 debug
!("confirm_param_candidate({:?},{:?})",
2382 // During evaluation, we already checked that this
2383 // where-clause trait-ref could be unified with the obligation
2384 // trait-ref. Repeat that unification now without any
2385 // transactional boundary; it should not fail.
2386 match self.match_where_clause_trait_ref(obligation
, param
.clone()) {
2387 Ok(obligations
) => obligations
,
2389 bug
!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2396 fn confirm_builtin_candidate(&mut self,
2397 obligation
: &TraitObligation
<'tcx
>,
2399 -> VtableBuiltinData
<PredicateObligation
<'tcx
>>
2401 debug
!("confirm_builtin_candidate({:?}, {:?})",
2402 obligation
, has_nested
);
2404 let lang_items
= self.tcx().lang_items();
2405 let obligations
= if has_nested
{
2406 let trait_def
= obligation
.predicate
.def_id();
2407 let conditions
= match trait_def
{
2408 _
if Some(trait_def
) == lang_items
.sized_trait() => {
2409 self.sized_conditions(obligation
)
2411 _
if Some(trait_def
) == lang_items
.copy_trait() => {
2412 self.copy_clone_conditions(obligation
)
2414 _
if Some(trait_def
) == lang_items
.clone_trait() => {
2415 self.copy_clone_conditions(obligation
)
2417 _
=> bug
!("unexpected builtin trait {:?}", trait_def
)
2419 let nested
= match conditions
{
2420 BuiltinImplConditions
::Where(nested
) => nested
,
2421 _
=> bug
!("obligation {:?} had matched a builtin impl but now doesn't",
2425 let cause
= obligation
.derived_cause(BuiltinDerivedObligation
);
2426 self.collect_predicates_for_types(obligation
.param_env
,
2428 obligation
.recursion_depth
+1,
2435 debug
!("confirm_builtin_candidate: obligations={:?}",
2438 VtableBuiltinData { nested: obligations }
2441 /// This handles the case where a `auto trait Foo` impl is being used.
2442 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2444 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2445 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2446 fn confirm_auto_impl_candidate(&mut self,
2447 obligation
: &TraitObligation
<'tcx
>,
2448 trait_def_id
: DefId
)
2449 -> VtableAutoImplData
<PredicateObligation
<'tcx
>>
2451 debug
!("confirm_auto_impl_candidate({:?}, {:?})",
2455 // binder is moved below
2456 let self_ty
= self.infcx
.shallow_resolve(obligation
.predicate
.skip_binder().self_ty());
2457 let types
= self.constituent_types_for_ty(self_ty
);
2458 self.vtable_auto_impl(obligation
, trait_def_id
, ty
::Binder(types
))
2461 /// See `confirm_auto_impl_candidate`
2462 fn vtable_auto_impl(&mut self,
2463 obligation
: &TraitObligation
<'tcx
>,
2464 trait_def_id
: DefId
,
2465 nested
: ty
::Binder
<Vec
<Ty
<'tcx
>>>)
2466 -> VtableAutoImplData
<PredicateObligation
<'tcx
>>
2468 debug
!("vtable_auto_impl: nested={:?}", nested
);
2470 let cause
= obligation
.derived_cause(BuiltinDerivedObligation
);
2471 let mut obligations
= self.collect_predicates_for_types(
2472 obligation
.param_env
,
2474 obligation
.recursion_depth
+1,
2478 let trait_obligations
= self.in_snapshot(|this
, snapshot
| {
2479 let poly_trait_ref
= obligation
.predicate
.to_poly_trait_ref();
2480 let (trait_ref
, skol_map
) =
2481 this
.infcx().skolemize_late_bound_regions(&poly_trait_ref
, snapshot
);
2482 let cause
= obligation
.derived_cause(ImplDerivedObligation
);
2483 this
.impl_or_trait_obligations(cause
,
2484 obligation
.recursion_depth
+ 1,
2485 obligation
.param_env
,
2492 obligations
.extend(trait_obligations
);
2494 debug
!("vtable_auto_impl: obligations={:?}", obligations
);
2496 VtableAutoImplData
{
2502 fn confirm_impl_candidate(&mut self,
2503 obligation
: &TraitObligation
<'tcx
>,
2505 -> VtableImplData
<'tcx
, PredicateObligation
<'tcx
>>
2507 debug
!("confirm_impl_candidate({:?},{:?})",
2511 // First, create the substitutions by matching the impl again,
2512 // this time not in a probe.
2513 self.in_snapshot(|this
, snapshot
| {
2514 let (substs
, skol_map
) =
2515 this
.rematch_impl(impl_def_id
, obligation
,
2517 debug
!("confirm_impl_candidate substs={:?}", substs
);
2518 let cause
= obligation
.derived_cause(ImplDerivedObligation
);
2519 this
.vtable_impl(impl_def_id
,
2522 obligation
.recursion_depth
+ 1,
2523 obligation
.param_env
,
2529 fn vtable_impl(&mut self,
2531 mut substs
: Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
2532 cause
: ObligationCause
<'tcx
>,
2533 recursion_depth
: usize,
2534 param_env
: ty
::ParamEnv
<'tcx
>,
2535 skol_map
: infer
::SkolemizationMap
<'tcx
>,
2536 snapshot
: &infer
::CombinedSnapshot
<'cx
, 'tcx
>)
2537 -> VtableImplData
<'tcx
, PredicateObligation
<'tcx
>>
2539 debug
!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2545 let mut impl_obligations
=
2546 self.impl_or_trait_obligations(cause
,
2554 debug
!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2558 // Because of RFC447, the impl-trait-ref and obligations
2559 // are sufficient to determine the impl substs, without
2560 // relying on projections in the impl-trait-ref.
2562 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2563 impl_obligations
.append(&mut substs
.obligations
);
2565 VtableImplData
{ impl_def_id
,
2566 substs
: substs
.value
,
2567 nested
: impl_obligations
}
2570 fn confirm_object_candidate(&mut self,
2571 obligation
: &TraitObligation
<'tcx
>)
2572 -> VtableObjectData
<'tcx
, PredicateObligation
<'tcx
>>
2574 debug
!("confirm_object_candidate({:?})",
2577 // FIXME skipping binder here seems wrong -- we should
2578 // probably flatten the binder from the obligation and the
2579 // binder from the object. Have to try to make a broken test
2580 // case that results. -nmatsakis
2581 let self_ty
= self.infcx
.shallow_resolve(*obligation
.self_ty().skip_binder());
2582 let poly_trait_ref
= match self_ty
.sty
{
2583 ty
::TyDynamic(ref data
, ..) => {
2584 data
.principal().unwrap().with_self_ty(self.tcx(), self_ty
)
2587 span_bug
!(obligation
.cause
.span
,
2588 "object candidate with non-object");
2592 let mut upcast_trait_ref
= None
;
2593 let mut nested
= vec
![];
2597 let tcx
= self.tcx();
2599 // We want to find the first supertrait in the list of
2600 // supertraits that we can unify with, and do that
2601 // unification. We know that there is exactly one in the list
2602 // where we can unify because otherwise select would have
2603 // reported an ambiguity. (When we do find a match, also
2604 // record it for later.)
2606 util
::supertraits(tcx
, poly_trait_ref
)
2610 |this
, _
| this
.match_poly_trait_ref(obligation
, t
))
2612 Ok(obligations
) => {
2613 upcast_trait_ref
= Some(t
);
2614 nested
.extend(obligations
);
2621 // Additionally, for each of the nonmatching predicates that
2622 // we pass over, we sum up the set of number of vtable
2623 // entries, so that we can compute the offset for the selected
2626 nonmatching
.map(|t
| tcx
.count_own_vtable_entries(t
))
2632 upcast_trait_ref
: upcast_trait_ref
.unwrap(),
2638 fn confirm_fn_pointer_candidate(&mut self, obligation
: &TraitObligation
<'tcx
>)
2639 -> Result
<VtableFnPointerData
<'tcx
, PredicateObligation
<'tcx
>>, SelectionError
<'tcx
>>
2641 debug
!("confirm_fn_pointer_candidate({:?})",
2644 // ok to skip binder; it is reintroduced below
2645 let self_ty
= self.infcx
.shallow_resolve(*obligation
.self_ty().skip_binder());
2646 let sig
= self_ty
.fn_sig(self.tcx());
2648 self.tcx().closure_trait_ref_and_return_type(obligation
.predicate
.def_id(),
2651 util
::TupleArgumentsFlag
::Yes
)
2652 .map_bound(|(trait_ref
, _
)| trait_ref
);
2654 let Normalized { value: trait_ref, obligations }
=
2655 project
::normalize_with_depth(self,
2656 obligation
.param_env
,
2657 obligation
.cause
.clone(),
2658 obligation
.recursion_depth
+ 1,
2661 self.confirm_poly_trait_refs(obligation
.cause
.clone(),
2662 obligation
.param_env
,
2663 obligation
.predicate
.to_poly_trait_ref(),
2665 Ok(VtableFnPointerData { fn_ty: self_ty, nested: obligations }
)
2668 fn confirm_generator_candidate(&mut self,
2669 obligation
: &TraitObligation
<'tcx
>)
2670 -> Result
<VtableGeneratorData
<'tcx
, PredicateObligation
<'tcx
>>,
2671 SelectionError
<'tcx
>>
2673 // ok to skip binder because the substs on generator types never
2674 // touch bound regions, they just capture the in-scope
2675 // type/region parameters
2676 let self_ty
= self.infcx
.shallow_resolve(obligation
.self_ty().skip_binder());
2677 let (closure_def_id
, substs
) = match self_ty
.sty
{
2678 ty
::TyGenerator(id
, substs
, _
) => (id
, substs
),
2679 _
=> bug
!("closure candidate for non-closure {:?}", obligation
)
2682 debug
!("confirm_generator_candidate({:?},{:?},{:?})",
2688 self.generator_trait_ref_unnormalized(obligation
, closure_def_id
, substs
);
2692 } = normalize_with_depth(self,
2693 obligation
.param_env
,
2694 obligation
.cause
.clone(),
2695 obligation
.recursion_depth
+1,
2698 debug
!("confirm_generator_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2704 self.confirm_poly_trait_refs(obligation
.cause
.clone(),
2705 obligation
.param_env
,
2706 obligation
.predicate
.to_poly_trait_ref(),
2709 Ok(VtableGeneratorData
{
2710 closure_def_id
: closure_def_id
,
2711 substs
: substs
.clone(),
2716 fn confirm_closure_candidate(&mut self,
2717 obligation
: &TraitObligation
<'tcx
>)
2718 -> Result
<VtableClosureData
<'tcx
, PredicateObligation
<'tcx
>>,
2719 SelectionError
<'tcx
>>
2721 debug
!("confirm_closure_candidate({:?})", obligation
);
2723 let kind
= match self.tcx().lang_items().fn_trait_kind(obligation
.predicate
.0.def_id()) {
2725 None
=> bug
!("closure candidate for non-fn trait {:?}", obligation
)
2728 // ok to skip binder because the substs on closure types never
2729 // touch bound regions, they just capture the in-scope
2730 // type/region parameters
2731 let self_ty
= self.infcx
.shallow_resolve(obligation
.self_ty().skip_binder());
2732 let (closure_def_id
, substs
) = match self_ty
.sty
{
2733 ty
::TyClosure(id
, substs
) => (id
, substs
),
2734 _
=> bug
!("closure candidate for non-closure {:?}", obligation
)
2738 self.closure_trait_ref_unnormalized(obligation
, closure_def_id
, substs
);
2742 } = normalize_with_depth(self,
2743 obligation
.param_env
,
2744 obligation
.cause
.clone(),
2745 obligation
.recursion_depth
+1,
2748 debug
!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2754 self.confirm_poly_trait_refs(obligation
.cause
.clone(),
2755 obligation
.param_env
,
2756 obligation
.predicate
.to_poly_trait_ref(),
2759 obligations
.push(Obligation
::new(
2760 obligation
.cause
.clone(),
2761 obligation
.param_env
,
2762 ty
::Predicate
::ClosureKind(closure_def_id
, substs
, kind
)));
2764 Ok(VtableClosureData
{
2766 substs
: substs
.clone(),
2771 /// In the case of closure types and fn pointers,
2772 /// we currently treat the input type parameters on the trait as
2773 /// outputs. This means that when we have a match we have only
2774 /// considered the self type, so we have to go back and make sure
2775 /// to relate the argument types too. This is kind of wrong, but
2776 /// since we control the full set of impls, also not that wrong,
2777 /// and it DOES yield better error messages (since we don't report
2778 /// errors as if there is no applicable impl, but rather report
2779 /// errors are about mismatched argument types.
2781 /// Here is an example. Imagine we have a closure expression
2782 /// and we desugared it so that the type of the expression is
2783 /// `Closure`, and `Closure` expects an int as argument. Then it
2784 /// is "as if" the compiler generated this impl:
2786 /// impl Fn(int) for Closure { ... }
2788 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2789 /// we have matched the self-type `Closure`. At this point we'll
2790 /// compare the `int` to `usize` and generate an error.
2792 /// Note that this checking occurs *after* the impl has selected,
2793 /// because these output type parameters should not affect the
2794 /// selection of the impl. Therefore, if there is a mismatch, we
2795 /// report an error to the user.
2796 fn confirm_poly_trait_refs(&mut self,
2797 obligation_cause
: ObligationCause
<'tcx
>,
2798 obligation_param_env
: ty
::ParamEnv
<'tcx
>,
2799 obligation_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
2800 expected_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
2801 -> Result
<Vec
<PredicateObligation
<'tcx
>>, SelectionError
<'tcx
>>
2803 let obligation_trait_ref
= obligation_trait_ref
.clone();
2805 .at(&obligation_cause
, obligation_param_env
)
2806 .sup(obligation_trait_ref
, expected_trait_ref
)
2807 .map(|InferOk { obligations, .. }
| obligations
)
2808 .map_err(|e
| OutputTypeParameterMismatch(expected_trait_ref
, obligation_trait_ref
, e
))
2811 fn confirm_builtin_unsize_candidate(&mut self,
2812 obligation
: &TraitObligation
<'tcx
>,)
2813 -> Result
<VtableBuiltinData
<PredicateObligation
<'tcx
>>, SelectionError
<'tcx
>>
2815 let tcx
= self.tcx();
2817 // assemble_candidates_for_unsizing should ensure there are no late bound
2818 // regions here. See the comment there for more details.
2819 let source
= self.infcx
.shallow_resolve(
2820 obligation
.self_ty().no_late_bound_regions().unwrap());
2821 let target
= obligation
.predicate
.skip_binder().trait_ref
.substs
.type_at(1);
2822 let target
= self.infcx
.shallow_resolve(target
);
2824 debug
!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2827 let mut nested
= vec
![];
2828 match (&source
.sty
, &target
.sty
) {
2829 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2830 (&ty
::TyDynamic(ref data_a
, r_a
), &ty
::TyDynamic(ref data_b
, r_b
)) => {
2831 // See assemble_candidates_for_unsizing for more info.
2832 // Binders reintroduced below in call to mk_existential_predicates.
2833 let principal
= data_a
.skip_binder().principal();
2834 let iter
= principal
.into_iter().map(ty
::ExistentialPredicate
::Trait
)
2835 .chain(data_a
.skip_binder().projection_bounds()
2836 .map(|x
| ty
::ExistentialPredicate
::Projection(x
)))
2837 .chain(data_b
.auto_traits().map(ty
::ExistentialPredicate
::AutoTrait
));
2838 let new_trait
= tcx
.mk_dynamic(
2839 ty
::Binder(tcx
.mk_existential_predicates(iter
)), r_b
);
2840 let InferOk { obligations, .. }
=
2841 self.infcx
.at(&obligation
.cause
, obligation
.param_env
)
2842 .eq(target
, new_trait
)
2843 .map_err(|_
| Unimplemented
)?
;
2844 nested
.extend(obligations
);
2846 // Register one obligation for 'a: 'b.
2847 let cause
= ObligationCause
::new(obligation
.cause
.span
,
2848 obligation
.cause
.body_id
,
2849 ObjectCastObligation(target
));
2850 let outlives
= ty
::OutlivesPredicate(r_a
, r_b
);
2851 nested
.push(Obligation
::with_depth(cause
,
2852 obligation
.recursion_depth
+ 1,
2853 obligation
.param_env
,
2854 ty
::Binder(outlives
).to_predicate()));
2858 (_
, &ty
::TyDynamic(ref data
, r
)) => {
2859 let mut object_dids
=
2860 data
.auto_traits().chain(data
.principal().map(|p
| p
.def_id()));
2861 if let Some(did
) = object_dids
.find(|did
| {
2862 !tcx
.is_object_safe(*did
)
2864 return Err(TraitNotObjectSafe(did
))
2867 let cause
= ObligationCause
::new(obligation
.cause
.span
,
2868 obligation
.cause
.body_id
,
2869 ObjectCastObligation(target
));
2870 let mut push
= |predicate
| {
2871 nested
.push(Obligation
::with_depth(cause
.clone(),
2872 obligation
.recursion_depth
+ 1,
2873 obligation
.param_env
,
2877 // Create obligations:
2878 // - Casting T to Trait
2879 // - For all the various builtin bounds attached to the object cast. (In other
2880 // words, if the object type is Foo+Send, this would create an obligation for the
2882 // - Projection predicates
2883 for predicate
in data
.iter() {
2884 push(predicate
.with_self_ty(tcx
, source
));
2887 // We can only make objects from sized types.
2888 let tr
= ty
::TraitRef
{
2889 def_id
: tcx
.require_lang_item(lang_items
::SizedTraitLangItem
),
2890 substs
: tcx
.mk_substs_trait(source
, &[]),
2892 push(tr
.to_predicate());
2894 // If the type is `Foo+'a`, ensures that the type
2895 // being cast to `Foo+'a` outlives `'a`:
2896 let outlives
= ty
::OutlivesPredicate(source
, r
);
2897 push(ty
::Binder(outlives
).to_predicate());
2901 (&ty
::TyArray(a
, _
), &ty
::TySlice(b
)) => {
2902 let InferOk { obligations, .. }
=
2903 self.infcx
.at(&obligation
.cause
, obligation
.param_env
)
2905 .map_err(|_
| Unimplemented
)?
;
2906 nested
.extend(obligations
);
2909 // Struct<T> -> Struct<U>.
2910 (&ty
::TyAdt(def
, substs_a
), &ty
::TyAdt(_
, substs_b
)) => {
2913 .map(|f
| tcx
.type_of(f
.did
))
2914 .collect
::<Vec
<_
>>();
2916 // The last field of the structure has to exist and contain type parameters.
2917 let field
= if let Some(&field
) = fields
.last() {
2920 return Err(Unimplemented
);
2922 let mut ty_params
= BitVector
::new(substs_a
.types().count());
2923 let mut found
= false;
2924 for ty
in field
.walk() {
2925 if let ty
::TyParam(p
) = ty
.sty
{
2926 ty_params
.insert(p
.idx
as usize);
2931 return Err(Unimplemented
);
2934 // Replace type parameters used in unsizing with
2935 // TyError and ensure they do not affect any other fields.
2936 // This could be checked after type collection for any struct
2937 // with a potentially unsized trailing field.
2938 let params
= substs_a
.iter().enumerate().map(|(i
, &k
)| {
2939 if ty_params
.contains(i
) {
2940 Kind
::from(tcx
.types
.err
)
2945 let substs
= tcx
.mk_substs(params
);
2946 for &ty
in fields
.split_last().unwrap().1 {
2947 if ty
.subst(tcx
, substs
).references_error() {
2948 return Err(Unimplemented
);
2952 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2953 let inner_source
= field
.subst(tcx
, substs_a
);
2954 let inner_target
= field
.subst(tcx
, substs_b
);
2956 // Check that the source struct with the target's
2957 // unsized parameters is equal to the target.
2958 let params
= substs_a
.iter().enumerate().map(|(i
, &k
)| {
2959 if ty_params
.contains(i
) {
2960 substs_b
.type_at(i
).into()
2965 let new_struct
= tcx
.mk_adt(def
, tcx
.mk_substs(params
));
2966 let InferOk { obligations, .. }
=
2967 self.infcx
.at(&obligation
.cause
, obligation
.param_env
)
2968 .eq(target
, new_struct
)
2969 .map_err(|_
| Unimplemented
)?
;
2970 nested
.extend(obligations
);
2972 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2973 nested
.push(tcx
.predicate_for_trait_def(
2974 obligation
.param_env
,
2975 obligation
.cause
.clone(),
2976 obligation
.predicate
.def_id(),
2977 obligation
.recursion_depth
+ 1,
2982 // (.., T) -> (.., U).
2983 (&ty
::TyTuple(tys_a
), &ty
::TyTuple(tys_b
)) => {
2984 assert_eq
!(tys_a
.len(), tys_b
.len());
2986 // The last field of the tuple has to exist.
2987 let (a_last
, a_mid
) = if let Some(x
) = tys_a
.split_last() {
2990 return Err(Unimplemented
);
2992 let b_last
= tys_b
.last().unwrap();
2994 // Check that the source tuple with the target's
2995 // last element is equal to the target.
2996 let new_tuple
= tcx
.mk_tup(a_mid
.iter().chain(Some(b_last
)));
2997 let InferOk { obligations, .. }
=
2998 self.infcx
.at(&obligation
.cause
, obligation
.param_env
)
2999 .eq(target
, new_tuple
)
3000 .map_err(|_
| Unimplemented
)?
;
3001 nested
.extend(obligations
);
3003 // Construct the nested T: Unsize<U> predicate.
3004 nested
.push(tcx
.predicate_for_trait_def(
3005 obligation
.param_env
,
3006 obligation
.cause
.clone(),
3007 obligation
.predicate
.def_id(),
3008 obligation
.recursion_depth
+ 1,
3016 Ok(VtableBuiltinData { nested: nested }
)
3019 ///////////////////////////////////////////////////////////////////////////
3022 // Matching is a common path used for both evaluation and
3023 // confirmation. It basically unifies types that appear in impls
3024 // and traits. This does affect the surrounding environment;
3025 // therefore, when used during evaluation, match routines must be
3026 // run inside of a `probe()` so that their side-effects are
3029 fn rematch_impl(&mut self,
3031 obligation
: &TraitObligation
<'tcx
>,
3032 snapshot
: &infer
::CombinedSnapshot
<'cx
, 'tcx
>)
3033 -> (Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
3034 infer
::SkolemizationMap
<'tcx
>)
3036 match self.match_impl(impl_def_id
, obligation
, snapshot
) {
3037 Ok((substs
, skol_map
)) => (substs
, skol_map
),
3039 bug
!("Impl {:?} was matchable against {:?} but now is not",
3046 fn match_impl(&mut self,
3048 obligation
: &TraitObligation
<'tcx
>,
3049 snapshot
: &infer
::CombinedSnapshot
<'cx
, 'tcx
>)
3050 -> Result
<(Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
3051 infer
::SkolemizationMap
<'tcx
>), ()>
3053 let impl_trait_ref
= self.tcx().impl_trait_ref(impl_def_id
).unwrap();
3055 // Before we create the substitutions and everything, first
3056 // consider a "quick reject". This avoids creating more types
3057 // and so forth that we need to.
3058 if self.fast_reject_trait_refs(obligation
, &impl_trait_ref
) {
3062 let (skol_obligation
, skol_map
) = self.infcx().skolemize_late_bound_regions(
3063 &obligation
.predicate
,
3065 let skol_obligation_trait_ref
= skol_obligation
.trait_ref
;
3067 let impl_substs
= self.infcx
.fresh_substs_for_item(obligation
.cause
.span
,
3070 let impl_trait_ref
= impl_trait_ref
.subst(self.tcx(),
3073 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations }
=
3074 project
::normalize_with_depth(self,
3075 obligation
.param_env
,
3076 obligation
.cause
.clone(),
3077 obligation
.recursion_depth
+ 1,
3080 debug
!("match_impl(impl_def_id={:?}, obligation={:?}, \
3081 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
3085 skol_obligation_trait_ref
);
3087 let InferOk { obligations, .. }
=
3088 self.infcx
.at(&obligation
.cause
, obligation
.param_env
)
3089 .eq(skol_obligation_trait_ref
, impl_trait_ref
)
3091 debug
!("match_impl: failed eq_trait_refs due to `{}`", e
);
3094 nested_obligations
.extend(obligations
);
3096 if let Err(e
) = self.infcx
.leak_check(false,
3097 obligation
.cause
.span
,
3100 debug
!("match_impl: failed leak check due to `{}`", e
);
3104 debug
!("match_impl: success impl_substs={:?}", impl_substs
);
3107 obligations
: nested_obligations
3111 fn fast_reject_trait_refs(&mut self,
3112 obligation
: &TraitObligation
,
3113 impl_trait_ref
: &ty
::TraitRef
)
3116 // We can avoid creating type variables and doing the full
3117 // substitution if we find that any of the input types, when
3118 // simplified, do not match.
3120 obligation
.predicate
.skip_binder().input_types()
3121 .zip(impl_trait_ref
.input_types())
3122 .any(|(obligation_ty
, impl_ty
)| {
3123 let simplified_obligation_ty
=
3124 fast_reject
::simplify_type(self.tcx(), obligation_ty
, true);
3125 let simplified_impl_ty
=
3126 fast_reject
::simplify_type(self.tcx(), impl_ty
, false);
3128 simplified_obligation_ty
.is_some() &&
3129 simplified_impl_ty
.is_some() &&
3130 simplified_obligation_ty
!= simplified_impl_ty
3134 /// Normalize `where_clause_trait_ref` and try to match it against
3135 /// `obligation`. If successful, return any predicates that
3136 /// result from the normalization. Normalization is necessary
3137 /// because where-clauses are stored in the parameter environment
3139 fn match_where_clause_trait_ref(&mut self,
3140 obligation
: &TraitObligation
<'tcx
>,
3141 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
3142 -> Result
<Vec
<PredicateObligation
<'tcx
>>,()>
3144 self.match_poly_trait_ref(obligation
, where_clause_trait_ref
)
3147 /// Returns `Ok` if `poly_trait_ref` being true implies that the
3148 /// obligation is satisfied.
3149 fn match_poly_trait_ref(&mut self,
3150 obligation
: &TraitObligation
<'tcx
>,
3151 poly_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
3152 -> Result
<Vec
<PredicateObligation
<'tcx
>>,()>
3154 debug
!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
3158 self.infcx
.at(&obligation
.cause
, obligation
.param_env
)
3159 .sup(obligation
.predicate
.to_poly_trait_ref(), poly_trait_ref
)
3160 .map(|InferOk { obligations, .. }
| obligations
)
3164 ///////////////////////////////////////////////////////////////////////////
3167 fn match_fresh_trait_refs(&self,
3168 previous
: &ty
::PolyTraitRef
<'tcx
>,
3169 current
: &ty
::PolyTraitRef
<'tcx
>)
3172 let mut matcher
= ty
::_match
::Match
::new(self.tcx());
3173 matcher
.relate(previous
, current
).is_ok()
3176 fn push_stack
<'o
,'s
:'o
>(&mut self,
3177 previous_stack
: TraitObligationStackList
<'s
, 'tcx
>,
3178 obligation
: &'o TraitObligation
<'tcx
>)
3179 -> TraitObligationStack
<'o
, 'tcx
>
3181 let fresh_trait_ref
=
3182 obligation
.predicate
.to_poly_trait_ref().fold_with(&mut self.freshener
);
3184 TraitObligationStack
{
3187 previous
: previous_stack
,
3191 fn closure_trait_ref_unnormalized(&mut self,
3192 obligation
: &TraitObligation
<'tcx
>,
3193 closure_def_id
: DefId
,
3194 substs
: ty
::ClosureSubsts
<'tcx
>)
3195 -> ty
::PolyTraitRef
<'tcx
>
3197 let closure_type
= self.infcx
.closure_sig(closure_def_id
, substs
);
3198 let ty
::Binder((trait_ref
, _
)) =
3199 self.tcx().closure_trait_ref_and_return_type(obligation
.predicate
.def_id(),
3200 obligation
.predicate
.0.self_ty(), // (1)
3202 util
::TupleArgumentsFlag
::No
);
3203 // (1) Feels icky to skip the binder here, but OTOH we know
3204 // that the self-type is an unboxed closure type and hence is
3205 // in fact unparameterized (or at least does not reference any
3206 // regions bound in the obligation). Still probably some
3207 // refactoring could make this nicer.
3209 ty
::Binder(trait_ref
)
3212 fn generator_trait_ref_unnormalized(&mut self,
3213 obligation
: &TraitObligation
<'tcx
>,
3214 closure_def_id
: DefId
,
3215 substs
: ty
::ClosureSubsts
<'tcx
>)
3216 -> ty
::PolyTraitRef
<'tcx
>
3218 let gen_sig
= substs
.generator_poly_sig(closure_def_id
, self.tcx());
3219 let ty
::Binder((trait_ref
, ..)) =
3220 self.tcx().generator_trait_ref_and_outputs(obligation
.predicate
.def_id(),
3221 obligation
.predicate
.0.self_ty(), // (1)
3223 // (1) Feels icky to skip the binder here, but OTOH we know
3224 // that the self-type is an generator type and hence is
3225 // in fact unparameterized (or at least does not reference any
3226 // regions bound in the obligation). Still probably some
3227 // refactoring could make this nicer.
3229 ty
::Binder(trait_ref
)
3232 /// Returns the obligations that are implied by instantiating an
3233 /// impl or trait. The obligations are substituted and fully
3234 /// normalized. This is used when confirming an impl or default
3236 fn impl_or_trait_obligations(&mut self,
3237 cause
: ObligationCause
<'tcx
>,
3238 recursion_depth
: usize,
3239 param_env
: ty
::ParamEnv
<'tcx
>,
3240 def_id
: DefId
, // of impl or trait
3241 substs
: &Substs
<'tcx
>, // for impl or trait
3242 skol_map
: infer
::SkolemizationMap
<'tcx
>,
3243 snapshot
: &infer
::CombinedSnapshot
<'cx
, 'tcx
>)
3244 -> Vec
<PredicateObligation
<'tcx
>>
3246 debug
!("impl_or_trait_obligations(def_id={:?})", def_id
);
3247 let tcx
= self.tcx();
3249 // To allow for one-pass evaluation of the nested obligation,
3250 // each predicate must be preceded by the obligations required
3252 // for example, if we have:
3253 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
3254 // the impl will have the following predicates:
3255 // <V as Iterator>::Item = U,
3256 // U: Iterator, U: Sized,
3257 // V: Iterator, V: Sized,
3258 // <U as Iterator>::Item: Copy
3259 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
3260 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
3261 // `$1: Copy`, so we must ensure the obligations are emitted in
3263 let predicates
= tcx
.predicates_of(def_id
);
3264 assert_eq
!(predicates
.parent
, None
);
3265 let mut predicates
: Vec
<_
> = predicates
.predicates
.iter().flat_map(|predicate
| {
3266 let predicate
= normalize_with_depth(self, param_env
, cause
.clone(), recursion_depth
,
3267 &predicate
.subst(tcx
, substs
));
3268 predicate
.obligations
.into_iter().chain(
3270 cause
: cause
.clone(),
3273 predicate
: predicate
.value
3276 // We are performing deduplication here to avoid exponential blowups
3277 // (#38528) from happening, but the real cause of the duplication is
3278 // unknown. What we know is that the deduplication avoids exponential
3279 // amount of predicates being propogated when processing deeply nested
3281 let mut seen
= FxHashSet();
3282 predicates
.retain(|i
| seen
.insert(i
.clone()));
3283 self.infcx().plug_leaks(skol_map
, snapshot
, predicates
)
3287 impl<'tcx
> TraitObligation
<'tcx
> {
3288 #[allow(unused_comparisons)]
3289 pub fn derived_cause(&self,
3290 variant
: fn(DerivedObligationCause
<'tcx
>) -> ObligationCauseCode
<'tcx
>)
3291 -> ObligationCause
<'tcx
>
3294 * Creates a cause for obligations that are derived from
3295 * `obligation` by a recursive search (e.g., for a builtin
3296 * bound, or eventually a `auto trait Foo`). If `obligation`
3297 * is itself a derived obligation, this is just a clone, but
3298 * otherwise we create a "derived obligation" cause so as to
3299 * keep track of the original root obligation for error
3303 let obligation
= self;
3305 // NOTE(flaper87): As of now, it keeps track of the whole error
3306 // chain. Ideally, we should have a way to configure this either
3307 // by using -Z verbose or just a CLI argument.
3308 if obligation
.recursion_depth
>= 0 {
3309 let derived_cause
= DerivedObligationCause
{
3310 parent_trait_ref
: obligation
.predicate
.to_poly_trait_ref(),
3311 parent_code
: Rc
::new(obligation
.cause
.code
.clone())
3313 let derived_code
= variant(derived_cause
);
3314 ObligationCause
::new(obligation
.cause
.span
, obligation
.cause
.body_id
, derived_code
)
3316 obligation
.cause
.clone()
3321 impl<'tcx
> SelectionCache
<'tcx
> {
3322 pub fn new() -> SelectionCache
<'tcx
> {
3324 hashmap
: RefCell
::new(FxHashMap())
3328 pub fn clear(&self) {
3329 *self.hashmap
.borrow_mut() = FxHashMap()
3333 impl<'tcx
> EvaluationCache
<'tcx
> {
3334 pub fn new() -> EvaluationCache
<'tcx
> {
3336 hashmap
: RefCell
::new(FxHashMap())
3340 pub fn clear(&self) {
3341 *self.hashmap
.borrow_mut() = FxHashMap()
3345 impl<'o
,'tcx
> TraitObligationStack
<'o
,'tcx
> {
3346 fn list(&'o
self) -> TraitObligationStackList
<'o
,'tcx
> {
3347 TraitObligationStackList
::with(self)
3350 fn iter(&'o
self) -> TraitObligationStackList
<'o
,'tcx
> {
3355 #[derive(Copy, Clone)]
3356 struct TraitObligationStackList
<'o
,'tcx
:'o
> {
3357 head
: Option
<&'o TraitObligationStack
<'o
,'tcx
>>
3360 impl<'o
,'tcx
> TraitObligationStackList
<'o
,'tcx
> {
3361 fn empty() -> TraitObligationStackList
<'o
,'tcx
> {
3362 TraitObligationStackList { head: None }
3365 fn with(r
: &'o TraitObligationStack
<'o
,'tcx
>) -> TraitObligationStackList
<'o
,'tcx
> {
3366 TraitObligationStackList { head: Some(r) }
3370 impl<'o
,'tcx
> Iterator
for TraitObligationStackList
<'o
,'tcx
>{
3371 type Item
= &'o TraitObligationStack
<'o
,'tcx
>;
3373 fn next(&mut self) -> Option
<&'o TraitObligationStack
<'o
,'tcx
>> {
3384 impl<'o
,'tcx
> fmt
::Debug
for TraitObligationStack
<'o
,'tcx
> {
3385 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
3386 write
!(f
, "TraitObligationStack({:?})", self.obligation
)
3391 pub struct WithDepNode
<T
> {
3392 dep_node
: DepNodeIndex
,
3396 impl<T
: Clone
> WithDepNode
<T
> {
3397 pub fn new(dep_node
: DepNodeIndex
, cached_value
: T
) -> Self {
3398 WithDepNode { dep_node, cached_value }
3401 pub fn get(&self, tcx
: TyCtxt
) -> T
{
3402 tcx
.dep_graph
.read_index(self.dep_node
);
3403 self.cached_value
.clone()