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 `README.md` for high-level documentation
13 pub use self::MethodMatchResult
::*;
14 pub use self::MethodMatchedData
::*;
15 use self::SelectionCandidate
::*;
16 use self::EvaluationResult
::*;
19 use super::DerivedObligationCause
;
21 use super::project
::{normalize_with_depth, Normalized}
;
22 use super::{PredicateObligation, TraitObligation, ObligationCause}
;
23 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation}
;
24 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch}
;
25 use super::{ObjectCastObligation, Obligation}
;
27 use super::TraitNotObjectSafe
;
29 use super::SelectionResult
;
30 use super::{VtableBuiltin
, VtableImpl
, VtableParam
, VtableClosure
,
31 VtableFnPointer
, VtableObject
, VtableDefaultImpl
};
32 use super::{VtableImplData
, VtableObjectData
, VtableBuiltinData
,
33 VtableClosureData
, VtableDefaultImplData
, VtableFnPointerData
};
36 use hir
::def_id
::DefId
;
38 use infer
::{InferCtxt, InferOk, TypeFreshener, TypeOrigin}
;
39 use ty
::subst
::{Subst, Substs, TypeSpace}
;
40 use ty
::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable}
;
43 use ty
::relate
::TypeRelation
;
45 use rustc_data_structures
::snapshot_vec
::{SnapshotVecDelegate, SnapshotVec}
;
46 use std
::cell
::RefCell
;
48 use std
::marker
::PhantomData
;
53 use util
::nodemap
::FnvHashMap
;
55 struct InferredObligationsSnapshotVecDelegate
<'tcx
> {
56 phantom
: PhantomData
<&'tcx
i32>,
58 impl<'tcx
> SnapshotVecDelegate
for InferredObligationsSnapshotVecDelegate
<'tcx
> {
59 type Value
= PredicateObligation
<'tcx
>;
61 fn reverse(_
: &mut Vec
<Self::Value
>, _
: Self::Undo
) {}
64 pub struct SelectionContext
<'cx
, 'gcx
: 'cx
+'tcx
, 'tcx
: 'cx
> {
65 infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>,
67 /// Freshener used specifically for skolemizing entries on the
68 /// obligation stack. This ensures that all entries on the stack
69 /// at one time will have the same set of skolemized entries,
70 /// which is important for checking for trait bounds that
71 /// recursively require themselves.
72 freshener
: TypeFreshener
<'cx
, 'gcx
, 'tcx
>,
74 /// If true, indicates that the evaluation should be conservative
75 /// and consider the possibility of types outside this crate.
76 /// This comes up primarily when resolving ambiguity. Imagine
77 /// there is some trait reference `$0 : Bar` where `$0` is an
78 /// inference variable. If `intercrate` is true, then we can never
79 /// say for sure that this reference is not implemented, even if
80 /// there are *no impls at all for `Bar`*, because `$0` could be
81 /// bound to some type that in a downstream crate that implements
82 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
83 /// though, we set this to false, because we are only interested
84 /// in types that the user could actually have written --- in
85 /// other words, we consider `$0 : Bar` to be unimplemented if
86 /// there is no type that the user could *actually name* that
87 /// would satisfy it. This avoids crippling inference, basically.
90 inferred_obligations
: SnapshotVec
<InferredObligationsSnapshotVecDelegate
<'tcx
>>,
93 // A stack that walks back up the stack frame.
94 struct TraitObligationStack
<'prev
, 'tcx
: 'prev
> {
95 obligation
: &'prev TraitObligation
<'tcx
>,
97 /// Trait ref from `obligation` but skolemized with the
98 /// selection-context's freshener. Used to check for recursion.
99 fresh_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
101 previous
: TraitObligationStackList
<'prev
, 'tcx
>,
105 pub struct SelectionCache
<'tcx
> {
106 hashmap
: RefCell
<FnvHashMap
<ty
::TraitRef
<'tcx
>,
107 SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>>>,
110 pub enum MethodMatchResult
{
111 MethodMatched(MethodMatchedData
),
112 MethodAmbiguous(/* list of impls that could apply */ Vec
<DefId
>),
116 #[derive(Copy, Clone, Debug)]
117 pub enum MethodMatchedData
{
118 // In the case of a precise match, we don't really need to store
119 // how the match was found. So don't.
122 // In the case of a coercion, we need to know the precise impl so
123 // that we can determine the type to which things were coerced.
124 CoerciveMethodMatch(/* impl we matched */ DefId
)
127 /// The selection process begins by considering all impls, where
128 /// clauses, and so forth that might resolve an obligation. Sometimes
129 /// we'll be able to say definitively that (e.g.) an impl does not
130 /// apply to the obligation: perhaps it is defined for `usize` but the
131 /// obligation is for `int`. In that case, we drop the impl out of the
132 /// list. But the other cases are considered *candidates*.
134 /// For selection to succeed, there must be exactly one matching
135 /// candidate. If the obligation is fully known, this is guaranteed
136 /// by coherence. However, if the obligation contains type parameters
137 /// or variables, there may be multiple such impls.
139 /// It is not a real problem if multiple matching impls exist because
140 /// of type variables - it just means the obligation isn't sufficiently
141 /// elaborated. In that case we report an ambiguity, and the caller can
142 /// try again after more type information has been gathered or report a
143 /// "type annotations required" error.
145 /// However, with type parameters, this can be a real problem - type
146 /// parameters don't unify with regular types, but they *can* unify
147 /// with variables from blanket impls, and (unless we know its bounds
148 /// will always be satisfied) picking the blanket impl will be wrong
149 /// for at least *some* substitutions. To make this concrete, if we have
151 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
152 /// impl<T: fmt::Debug> AsDebug for T {
154 /// fn debug(self) -> fmt::Debug { self }
156 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
158 /// we can't just use the impl to resolve the <T as AsDebug> obligation
159 /// - a type from another crate (that doesn't implement fmt::Debug) could
160 /// implement AsDebug.
162 /// Because where-clauses match the type exactly, multiple clauses can
163 /// only match if there are unresolved variables, and we can mostly just
164 /// report this ambiguity in that case. This is still a problem - we can't
165 /// *do anything* with ambiguities that involve only regions. This is issue
168 /// If a single where-clause matches and there are no inference
169 /// variables left, then it definitely matches and we can just select
172 /// In fact, we even select the where-clause when the obligation contains
173 /// inference variables. The can lead to inference making "leaps of logic",
174 /// for example in this situation:
176 /// pub trait Foo<T> { fn foo(&self) -> T; }
177 /// impl<T> Foo<()> for T { fn foo(&self) { } }
178 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
180 /// pub fn foo<T>(t: T) where T: Foo<bool> {
181 /// println!("{:?}", <T as Foo<_>>::foo(&t));
183 /// fn main() { foo(false); }
185 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
186 /// impl and the where-clause. We select the where-clause and unify $0=bool,
187 /// so the program prints "false". However, if the where-clause is omitted,
188 /// the blanket impl is selected, we unify $0=(), and the program prints
191 /// Exactly the same issues apply to projection and object candidates, except
192 /// that we can have both a projection candidate and a where-clause candidate
193 /// for the same obligation. In that case either would do (except that
194 /// different "leaps of logic" would occur if inference variables are
195 /// present), and we just pick the where-clause. This is, for example,
196 /// required for associated types to work in default impls, as the bounds
197 /// are visible both as projection bounds and as where-clauses from the
198 /// parameter environment.
199 #[derive(PartialEq,Eq,Debug,Clone)]
200 enum SelectionCandidate
<'tcx
> {
201 BuiltinCandidate { has_nested: bool }
,
202 ParamCandidate(ty
::PolyTraitRef
<'tcx
>),
203 ImplCandidate(DefId
),
204 DefaultImplCandidate(DefId
),
205 DefaultImplObjectCandidate(DefId
),
207 /// This is a trait matching with a projected type as `Self`, and
208 /// we found an applicable bound in the trait definition.
211 /// Implementation of a `Fn`-family trait by one of the anonymous types
212 /// generated for a `||` expression. The ty::ClosureKind informs the
213 /// confirmation step what ClosureKind obligation to emit.
214 ClosureCandidate(/* closure */ DefId
, ty
::ClosureSubsts
<'tcx
>, ty
::ClosureKind
),
216 /// Implementation of a `Fn`-family trait by one of the anonymous
217 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
222 BuiltinObjectCandidate
,
224 BuiltinUnsizeCandidate
,
227 impl<'a
, 'tcx
> ty
::Lift
<'tcx
> for SelectionCandidate
<'a
> {
228 type Lifted
= SelectionCandidate
<'tcx
>;
229 fn lift_to_tcx
<'b
, 'gcx
>(&self, tcx
: TyCtxt
<'b
, 'gcx
, 'tcx
>) -> Option
<Self::Lifted
> {
231 BuiltinCandidate { has_nested }
=> {
233 has_nested
: has_nested
236 ImplCandidate(def_id
) => ImplCandidate(def_id
),
237 DefaultImplCandidate(def_id
) => DefaultImplCandidate(def_id
),
238 DefaultImplObjectCandidate(def_id
) => {
239 DefaultImplObjectCandidate(def_id
)
241 ProjectionCandidate
=> ProjectionCandidate
,
242 FnPointerCandidate
=> FnPointerCandidate
,
243 ObjectCandidate
=> ObjectCandidate
,
244 BuiltinObjectCandidate
=> BuiltinObjectCandidate
,
245 BuiltinUnsizeCandidate
=> BuiltinUnsizeCandidate
,
247 ParamCandidate(ref trait_ref
) => {
248 return tcx
.lift(trait_ref
).map(ParamCandidate
);
250 ClosureCandidate(def_id
, ref substs
, kind
) => {
251 return tcx
.lift(substs
).map(|substs
| {
252 ClosureCandidate(def_id
, substs
, kind
)
259 struct SelectionCandidateSet
<'tcx
> {
260 // a list of candidates that definitely apply to the current
261 // obligation (meaning: types unify).
262 vec
: Vec
<SelectionCandidate
<'tcx
>>,
264 // if this is true, then there were candidates that might or might
265 // not have applied, but we couldn't tell. This occurs when some
266 // of the input types are type variables, in which case there are
267 // various "builtin" rules that might or might not trigger.
271 #[derive(PartialEq,Eq,Debug,Clone)]
272 struct EvaluatedCandidate
<'tcx
> {
273 candidate
: SelectionCandidate
<'tcx
>,
274 evaluation
: EvaluationResult
,
277 /// When does the builtin impl for `T: Trait` apply?
278 enum BuiltinImplConditions
<'tcx
> {
279 /// The impl is conditional on T1,T2,.. : Trait
280 Where(ty
::Binder
<Vec
<Ty
<'tcx
>>>),
281 /// There is no built-in impl. There may be some other
282 /// candidate (a where-clause or user-defined impl).
284 /// There is *no* impl for this, builtin or not. Ignore
285 /// all where-clauses.
287 /// It is unknown whether there is an impl.
291 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
292 /// The result of trait evaluation. The order is important
293 /// here as the evaluation of a list is the maximum of the
295 enum EvaluationResult
{
296 /// Evaluation successful
298 /// Evaluation failed because of recursion - treated as ambiguous
300 /// Evaluation is known to be ambiguous
302 /// Evaluation failed
307 pub struct EvaluationCache
<'tcx
> {
308 hashmap
: RefCell
<FnvHashMap
<ty
::PolyTraitRef
<'tcx
>, EvaluationResult
>>
311 impl<'cx
, 'gcx
, 'tcx
> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
312 pub fn new(infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>) -> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
315 freshener
: infcx
.freshener(),
317 inferred_obligations
: SnapshotVec
::new(),
321 pub fn intercrate(infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>) -> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
324 freshener
: infcx
.freshener(),
326 inferred_obligations
: SnapshotVec
::new(),
330 pub fn infcx(&self) -> &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
> {
334 pub fn tcx(&self) -> TyCtxt
<'cx
, 'gcx
, 'tcx
> {
338 pub fn param_env(&self) -> &'cx ty
::ParameterEnvironment
<'tcx
> {
339 self.infcx
.param_env()
342 pub fn closure_typer(&self) -> &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
> {
346 pub fn projection_mode(&self) -> Reveal
{
347 self.infcx
.projection_mode()
350 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
352 fn in_snapshot
<R
, F
>(&mut self, f
: F
) -> R
353 where F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
) -> R
355 // The irrefutable nature of the operation means we don't need to snapshot the
356 // inferred_obligations vector.
357 self.infcx
.in_snapshot(|snapshot
| f(self, snapshot
))
360 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
362 fn probe
<R
, F
>(&mut self, f
: F
) -> R
363 where F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
) -> R
365 let inferred_obligations_snapshot
= self.inferred_obligations
.start_snapshot();
366 let result
= self.infcx
.probe(|snapshot
| f(self, snapshot
));
367 self.inferred_obligations
.rollback_to(inferred_obligations_snapshot
);
371 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
372 /// the transaction fails and s.t. old obligations are retained.
373 fn commit_if_ok
<T
, E
, F
>(&mut self, f
: F
) -> Result
<T
, E
> where
374 F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
) -> Result
<T
, E
>
376 let inferred_obligations_snapshot
= self.inferred_obligations
.start_snapshot();
377 match self.infcx
.commit_if_ok(|snapshot
| f(self, snapshot
)) {
379 self.inferred_obligations
.commit(inferred_obligations_snapshot
);
383 self.inferred_obligations
.rollback_to(inferred_obligations_snapshot
);
390 ///////////////////////////////////////////////////////////////////////////
393 // The selection phase tries to identify *how* an obligation will
394 // be resolved. For example, it will identify which impl or
395 // parameter bound is to be used. The process can be inconclusive
396 // if the self type in the obligation is not fully inferred. Selection
397 // can result in an error in one of two ways:
399 // 1. If no applicable impl or parameter bound can be found.
400 // 2. If the output type parameters in the obligation do not match
401 // those specified by the impl/bound. For example, if the obligation
402 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
403 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
405 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
406 /// type environment by performing unification.
407 pub fn select(&mut self, obligation
: &TraitObligation
<'tcx
>)
408 -> SelectionResult
<'tcx
, Selection
<'tcx
>> {
409 debug
!("select({:?})", obligation
);
410 assert
!(!obligation
.predicate
.has_escaping_regions());
412 let dep_node
= obligation
.predicate
.dep_node();
413 let _task
= self.tcx().dep_graph
.in_task(dep_node
);
415 let stack
= self.push_stack(TraitObligationStackList
::empty(), obligation
);
416 match self.candidate_from_obligation(&stack
)?
{
419 let mut candidate
= self.confirm_candidate(obligation
, candidate
)?
;
420 // FIXME(#32730) remove this assertion once inferred obligations are propagated
422 assert
!(self.inferred_obligations
.len() == 0);
423 let inferred_obligations
= (*self.inferred_obligations
).into_iter().cloned();
424 candidate
.nested_obligations_mut().extend(inferred_obligations
);
430 ///////////////////////////////////////////////////////////////////////////
433 // Tests whether an obligation can be selected or whether an impl
434 // can be applied to particular types. It skips the "confirmation"
435 // step and hence completely ignores output type parameters.
437 // The result is "true" if the obligation *may* hold and "false" if
438 // we can be sure it does not.
440 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
441 pub fn evaluate_obligation(&mut self,
442 obligation
: &PredicateObligation
<'tcx
>)
445 debug
!("evaluate_obligation({:?})",
448 self.probe(|this
, _
| {
449 this
.evaluate_predicate_recursively(TraitObligationStackList
::empty(), obligation
)
454 /// Evaluates whether the obligation `obligation` can be satisfied,
455 /// and returns `false` if not certain. However, this is not entirely
456 /// accurate if inference variables are involved.
457 pub fn evaluate_obligation_conservatively(&mut self,
458 obligation
: &PredicateObligation
<'tcx
>)
461 debug
!("evaluate_obligation_conservatively({:?})",
464 self.probe(|this
, _
| {
465 this
.evaluate_predicate_recursively(TraitObligationStackList
::empty(), obligation
)
470 /// Evaluates the predicates in `predicates` recursively. Note that
471 /// this applies projections in the predicates, and therefore
472 /// is run within an inference probe.
473 fn evaluate_predicates_recursively
<'a
,'o
,I
>(&mut self,
474 stack
: TraitObligationStackList
<'o
, 'tcx
>,
477 where I
: Iterator
<Item
=&'a PredicateObligation
<'tcx
>>, 'tcx
:'a
479 let mut result
= EvaluatedToOk
;
480 for obligation
in predicates
{
481 let eval
= self.evaluate_predicate_recursively(stack
, obligation
);
482 debug
!("evaluate_predicate_recursively({:?}) = {:?}",
485 EvaluatedToErr
=> { return EvaluatedToErr; }
486 EvaluatedToAmbig
=> { result = EvaluatedToAmbig; }
487 EvaluatedToUnknown
=> {
488 if result
< EvaluatedToUnknown
{
489 result
= EvaluatedToUnknown
;
498 fn evaluate_predicate_recursively
<'o
>(&mut self,
499 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
500 obligation
: &PredicateObligation
<'tcx
>)
503 debug
!("evaluate_predicate_recursively({:?})",
506 // Check the cache from the tcx of predicates that we know
507 // have been proven elsewhere. This cache only contains
508 // predicates that are global in scope and hence unaffected by
509 // the current environment.
510 if self.tcx().fulfilled_predicates
.borrow().check_duplicate(&obligation
.predicate
) {
511 return EvaluatedToOk
;
514 match obligation
.predicate
{
515 ty
::Predicate
::Rfc1592(..) => EvaluatedToOk
,
517 ty
::Predicate
::Trait(ref t
) => {
518 assert
!(!t
.has_escaping_regions());
519 let obligation
= obligation
.with(t
.clone());
520 self.evaluate_obligation_recursively(previous_stack
, &obligation
)
523 ty
::Predicate
::Equate(ref p
) => {
524 // does this code ever run?
525 match self.infcx
.equality_predicate(obligation
.cause
.span
, p
) {
526 Ok(InferOk { obligations, .. }
) => {
527 self.inferred_obligations
.extend(obligations
);
530 Err(_
) => EvaluatedToErr
534 ty
::Predicate
::WellFormed(ty
) => {
535 match ty
::wf
::obligations(self.infcx
, obligation
.cause
.body_id
,
536 ty
, obligation
.cause
.span
) {
538 self.evaluate_predicates_recursively(previous_stack
, obligations
.iter()),
544 ty
::Predicate
::TypeOutlives(..) | ty
::Predicate
::RegionOutlives(..) => {
545 // we do not consider region relationships when
546 // evaluating trait matches
550 ty
::Predicate
::ObjectSafe(trait_def_id
) => {
551 if self.tcx().is_object_safe(trait_def_id
) {
558 ty
::Predicate
::Projection(ref data
) => {
559 let project_obligation
= obligation
.with(data
.clone());
560 match project
::poly_project_and_unify_type(self, &project_obligation
) {
561 Ok(Some(subobligations
)) => {
562 self.evaluate_predicates_recursively(previous_stack
,
563 subobligations
.iter())
574 ty
::Predicate
::ClosureKind(closure_def_id
, kind
) => {
575 match self.infcx
.closure_kind(closure_def_id
) {
576 Some(closure_kind
) => {
577 if closure_kind
.extends(kind
) {
591 fn evaluate_obligation_recursively
<'o
>(&mut self,
592 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
593 obligation
: &TraitObligation
<'tcx
>)
596 debug
!("evaluate_obligation_recursively({:?})",
599 let stack
= self.push_stack(previous_stack
, obligation
);
600 let fresh_trait_ref
= stack
.fresh_trait_ref
;
601 if let Some(result
) = self.check_evaluation_cache(fresh_trait_ref
) {
602 debug
!("CACHE HIT: EVAL({:?})={:?}",
608 let result
= self.evaluate_stack(&stack
);
610 debug
!("CACHE MISS: EVAL({:?})={:?}",
613 self.insert_evaluation_cache(fresh_trait_ref
, result
);
618 fn evaluate_stack
<'o
>(&mut self,
619 stack
: &TraitObligationStack
<'o
, 'tcx
>)
622 // In intercrate mode, whenever any of the types are unbound,
623 // there can always be an impl. Even if there are no impls in
624 // this crate, perhaps the type would be unified with
625 // something from another crate that does provide an impl.
627 // In intra mode, we must still be conservative. The reason is
628 // that we want to avoid cycles. Imagine an impl like:
630 // impl<T:Eq> Eq for Vec<T>
632 // and a trait reference like `$0 : Eq` where `$0` is an
633 // unbound variable. When we evaluate this trait-reference, we
634 // will unify `$0` with `Vec<$1>` (for some fresh variable
635 // `$1`), on the condition that `$1 : Eq`. We will then wind
636 // up with many candidates (since that are other `Eq` impls
637 // that apply) and try to winnow things down. This results in
638 // a recursive evaluation that `$1 : Eq` -- as you can
639 // imagine, this is just where we started. To avoid that, we
640 // check for unbound variables and return an ambiguous (hence possible)
641 // match if we've seen this trait before.
643 // This suffices to allow chains like `FnMut` implemented in
644 // terms of `Fn` etc, but we could probably make this more
646 let input_types
= stack
.fresh_trait_ref
.0.input_types
();
647 let unbound_input_types
= input_types
.iter().any(|ty
| ty
.is_fresh());
648 if unbound_input_types
&& self.intercrate
{
649 debug
!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
650 stack
.fresh_trait_ref
);
651 return EvaluatedToAmbig
;
653 if unbound_input_types
&&
654 stack
.iter().skip(1).any(
655 |prev
| self.match_fresh_trait_refs(&stack
.fresh_trait_ref
,
656 &prev
.fresh_trait_ref
))
658 debug
!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
659 stack
.fresh_trait_ref
);
660 return EvaluatedToUnknown
;
663 // If there is any previous entry on the stack that precisely
664 // matches this obligation, then we can assume that the
665 // obligation is satisfied for now (still all other conditions
666 // must be met of course). One obvious case this comes up is
667 // marker traits like `Send`. Think of a linked list:
669 // struct List<T> { data: T, next: Option<Box<List<T>>> {
671 // `Box<List<T>>` will be `Send` if `T` is `Send` and
672 // `Option<Box<List<T>>>` is `Send`, and in turn
673 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
676 // Note that we do this comparison using the `fresh_trait_ref`
677 // fields. Because these have all been skolemized using
678 // `self.freshener`, we can be sure that (a) this will not
679 // affect the inferencer state and (b) that if we see two
680 // skolemized types with the same index, they refer to the
681 // same unbound type variable.
684 .skip(1) // skip top-most frame
685 .any(|prev
| stack
.fresh_trait_ref
== prev
.fresh_trait_ref
)
687 debug
!("evaluate_stack({:?}) --> recursive",
688 stack
.fresh_trait_ref
);
689 return EvaluatedToOk
;
692 match self.candidate_from_obligation(stack
) {
693 Ok(Some(c
)) => self.evaluate_candidate(stack
, &c
),
694 Ok(None
) => EvaluatedToAmbig
,
695 Err(..) => EvaluatedToErr
699 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
700 /// obligations are met. Returns true if `candidate` remains viable after this further
702 fn evaluate_candidate
<'o
>(&mut self,
703 stack
: &TraitObligationStack
<'o
, 'tcx
>,
704 candidate
: &SelectionCandidate
<'tcx
>)
707 debug
!("evaluate_candidate: depth={} candidate={:?}",
708 stack
.obligation
.recursion_depth
, candidate
);
709 let result
= self.probe(|this
, _
| {
710 let candidate
= (*candidate
).clone();
711 match this
.confirm_candidate(stack
.obligation
, candidate
) {
713 this
.evaluate_predicates_recursively(
715 selection
.nested_obligations().iter())
717 Err(..) => EvaluatedToErr
720 debug
!("evaluate_candidate: depth={} result={:?}",
721 stack
.obligation
.recursion_depth
, result
);
725 fn check_evaluation_cache(&self, trait_ref
: ty
::PolyTraitRef
<'tcx
>)
726 -> Option
<EvaluationResult
>
728 if self.can_use_global_caches() {
729 let cache
= self.tcx().evaluation_cache
.hashmap
.borrow();
730 if let Some(cached
) = cache
.get(&trait_ref
) {
731 return Some(cached
.clone());
734 self.infcx
.evaluation_cache
.hashmap
.borrow().get(&trait_ref
).cloned()
737 fn insert_evaluation_cache(&mut self,
738 trait_ref
: ty
::PolyTraitRef
<'tcx
>,
739 result
: EvaluationResult
)
741 // Avoid caching results that depend on more than just the trait-ref:
742 // The stack can create EvaluatedToUnknown, and closure signatures
743 // being yet uninferred can create "spurious" EvaluatedToAmbig
744 // and EvaluatedToOk.
745 if result
== EvaluatedToUnknown
||
746 ((result
== EvaluatedToAmbig
|| result
== EvaluatedToOk
)
747 && trait_ref
.has_closure_types())
752 if self.can_use_global_caches() {
753 let mut cache
= self.tcx().evaluation_cache
.hashmap
.borrow_mut();
754 if let Some(trait_ref
) = self.tcx().lift_to_global(&trait_ref
) {
755 cache
.insert(trait_ref
, result
);
760 self.infcx
.evaluation_cache
.hashmap
.borrow_mut().insert(trait_ref
, result
);
763 ///////////////////////////////////////////////////////////////////////////
764 // CANDIDATE ASSEMBLY
766 // The selection process begins by examining all in-scope impls,
767 // caller obligations, and so forth and assembling a list of
768 // candidates. See `README.md` and the `Candidate` type for more
771 fn candidate_from_obligation
<'o
>(&mut self,
772 stack
: &TraitObligationStack
<'o
, 'tcx
>)
773 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>
775 // Watch out for overflow. This intentionally bypasses (and does
776 // not update) the cache.
777 let recursion_limit
= self.infcx
.tcx
.sess
.recursion_limit
.get();
778 if stack
.obligation
.recursion_depth
>= recursion_limit
{
779 self.infcx().report_overflow_error(&stack
.obligation
, true);
782 // Check the cache. Note that we skolemize the trait-ref
783 // separately rather than using `stack.fresh_trait_ref` -- this
784 // is because we want the unbound variables to be replaced
785 // with fresh skolemized types starting from index 0.
786 let cache_fresh_trait_pred
=
787 self.infcx
.freshen(stack
.obligation
.predicate
.clone());
788 debug
!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
789 cache_fresh_trait_pred
,
791 assert
!(!stack
.obligation
.predicate
.has_escaping_regions());
793 match self.check_candidate_cache(&cache_fresh_trait_pred
) {
795 debug
!("CACHE HIT: SELECT({:?})={:?}",
796 cache_fresh_trait_pred
,
803 // If no match, compute result and insert into cache.
804 let candidate
= self.candidate_from_obligation_no_cache(stack
);
806 if self.should_update_candidate_cache(&cache_fresh_trait_pred
, &candidate
) {
807 debug
!("CACHE MISS: SELECT({:?})={:?}",
808 cache_fresh_trait_pred
, candidate
);
809 self.insert_candidate_cache(cache_fresh_trait_pred
, candidate
.clone());
815 // Treat negative impls as unimplemented
816 fn filter_negative_impls(&self, candidate
: SelectionCandidate
<'tcx
>)
817 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>> {
818 if let ImplCandidate(def_id
) = candidate
{
819 if self.tcx().trait_impl_polarity(def_id
) == Some(hir
::ImplPolarity
::Negative
) {
820 return Err(Unimplemented
)
826 fn candidate_from_obligation_no_cache
<'o
>(&mut self,
827 stack
: &TraitObligationStack
<'o
, 'tcx
>)
828 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>
830 if stack
.obligation
.predicate
.references_error() {
831 // If we encounter a `TyError`, we generally prefer the
832 // most "optimistic" result in response -- that is, the
833 // one least likely to report downstream errors. But
834 // because this routine is shared by coherence and by
835 // trait selection, there isn't an obvious "right" choice
836 // here in that respect, so we opt to just return
837 // ambiguity and let the upstream clients sort it out.
841 if !self.is_knowable(stack
) {
842 debug
!("coherence stage: not knowable");
846 let candidate_set
= self.assemble_candidates(stack
)?
;
848 if candidate_set
.ambiguous
{
849 debug
!("candidate set contains ambig");
853 let mut candidates
= candidate_set
.vec
;
855 debug
!("assembled {} candidates for {:?}: {:?}",
860 // At this point, we know that each of the entries in the
861 // candidate set is *individually* applicable. Now we have to
862 // figure out if they contain mutual incompatibilities. This
863 // frequently arises if we have an unconstrained input type --
864 // for example, we are looking for $0:Eq where $0 is some
865 // unconstrained type variable. In that case, we'll get a
866 // candidate which assumes $0 == int, one that assumes $0 ==
867 // usize, etc. This spells an ambiguity.
869 // If there is more than one candidate, first winnow them down
870 // by considering extra conditions (nested obligations and so
871 // forth). We don't winnow if there is exactly one
872 // candidate. This is a relatively minor distinction but it
873 // can lead to better inference and error-reporting. An
874 // example would be if there was an impl:
876 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
878 // and we were to see some code `foo.push_clone()` where `boo`
879 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
880 // we were to winnow, we'd wind up with zero candidates.
881 // Instead, we select the right impl now but report `Bar does
882 // not implement Clone`.
883 if candidates
.len() == 1 {
884 return self.filter_negative_impls(candidates
.pop().unwrap());
887 // Winnow, but record the exact outcome of evaluation, which
888 // is needed for specialization.
889 let mut candidates
: Vec
<_
> = candidates
.into_iter().filter_map(|c
| {
890 let eval
= self.evaluate_candidate(stack
, &c
);
891 if eval
.may_apply() {
892 Some(EvaluatedCandidate
{
901 // If there are STILL multiple candidate, we can further
902 // reduce the list by dropping duplicates -- including
903 // resolving specializations.
904 if candidates
.len() > 1 {
906 while i
< candidates
.len() {
908 (0..candidates
.len())
910 .any(|j
| self.candidate_should_be_dropped_in_favor_of(&candidates
[i
],
913 debug
!("Dropping candidate #{}/{}: {:?}",
914 i
, candidates
.len(), candidates
[i
]);
915 candidates
.swap_remove(i
);
917 debug
!("Retaining candidate #{}/{}: {:?}",
918 i
, candidates
.len(), candidates
[i
]);
924 // If there are *STILL* multiple candidates, give up and
926 if candidates
.len() > 1 {
927 debug
!("multiple matches, ambig");
931 // If there are *NO* candidates, then there are no impls --
932 // that we know of, anyway. Note that in the case where there
933 // are unbound type variables within the obligation, it might
934 // be the case that you could still satisfy the obligation
935 // from another crate by instantiating the type variables with
936 // a type from another crate that does have an impl. This case
937 // is checked for in `evaluate_stack` (and hence users
938 // who might care about this case, like coherence, should use
940 if candidates
.is_empty() {
941 return Err(Unimplemented
);
944 // Just one candidate left.
945 self.filter_negative_impls(candidates
.pop().unwrap().candidate
)
948 fn is_knowable
<'o
>(&mut self,
949 stack
: &TraitObligationStack
<'o
, 'tcx
>)
952 debug
!("is_knowable(intercrate={})", self.intercrate
);
954 if !self.intercrate
{
958 let obligation
= &stack
.obligation
;
959 let predicate
= self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
);
961 // ok to skip binder because of the nature of the
962 // trait-ref-is-knowable check, which does not care about
964 let trait_ref
= &predicate
.skip_binder().trait_ref
;
966 coherence
::trait_ref_is_knowable(self.tcx(), trait_ref
)
969 /// Returns true if the global caches can be used.
970 /// Do note that if the type itself is not in the
971 /// global tcx, the local caches will be used.
972 fn can_use_global_caches(&self) -> bool
{
973 // If there are any where-clauses in scope, then we always use
974 // a cache local to this particular scope. Otherwise, we
975 // switch to a global cache. We used to try and draw
976 // finer-grained distinctions, but that led to a serious of
977 // annoying and weird bugs like #22019 and #18290. This simple
978 // rule seems to be pretty clearly safe and also still retains
979 // a very high hit rate (~95% when compiling rustc).
980 if !self.param_env().caller_bounds
.is_empty() {
984 // Avoid using the master cache during coherence and just rely
985 // on the local cache. This effectively disables caching
986 // during coherence. It is really just a simplification to
987 // avoid us having to fear that coherence results "pollute"
988 // the master cache. Since coherence executes pretty quickly,
989 // it's not worth going to more trouble to increase the
990 // hit-rate I don't think.
995 // Otherwise, we can use the global cache.
999 fn check_candidate_cache(&mut self,
1000 cache_fresh_trait_pred
: &ty
::PolyTraitPredicate
<'tcx
>)
1001 -> Option
<SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>>
1003 let trait_ref
= &cache_fresh_trait_pred
.0.trait_ref
;
1004 if self.can_use_global_caches() {
1005 let cache
= self.tcx().selection_cache
.hashmap
.borrow();
1006 if let Some(cached
) = cache
.get(&trait_ref
) {
1007 return Some(cached
.clone());
1010 self.infcx
.selection_cache
.hashmap
.borrow().get(trait_ref
).cloned()
1013 fn insert_candidate_cache(&mut self,
1014 cache_fresh_trait_pred
: ty
::PolyTraitPredicate
<'tcx
>,
1015 candidate
: SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>)
1017 let trait_ref
= cache_fresh_trait_pred
.0.trait_ref
;
1018 if self.can_use_global_caches() {
1019 let mut cache
= self.tcx().selection_cache
.hashmap
.borrow_mut();
1020 if let Some(trait_ref
) = self.tcx().lift_to_global(&trait_ref
) {
1021 if let Some(candidate
) = self.tcx().lift_to_global(&candidate
) {
1022 cache
.insert(trait_ref
, candidate
);
1028 self.infcx
.selection_cache
.hashmap
.borrow_mut().insert(trait_ref
, candidate
);
1031 fn should_update_candidate_cache(&mut self,
1032 cache_fresh_trait_pred
: &ty
::PolyTraitPredicate
<'tcx
>,
1033 candidate
: &SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>)
1036 // In general, it's a good idea to cache results, even
1037 // ambiguous ones, to save us some trouble later. But we have
1038 // to be careful not to cache results that could be
1039 // invalidated later by advances in inference. Normally, this
1040 // is not an issue, because any inference variables whose
1041 // types are not yet bound are "freshened" in the cache key,
1042 // which means that if we later get the same request once that
1043 // type variable IS bound, we'll have a different cache key.
1044 // For example, if we have `Vec<_#0t> : Foo`, and `_#0t` is
1045 // not yet known, we may cache the result as `None`. But if
1046 // later `_#0t` is bound to `Bar`, then when we freshen we'll
1047 // have `Vec<Bar> : Foo` as the cache key.
1049 // HOWEVER, it CAN happen that we get an ambiguity result in
1050 // one particular case around closures where the cache key
1051 // would not change. That is when the precise types of the
1052 // upvars that a closure references have not yet been figured
1053 // out (i.e., because it is not yet known if they are captured
1054 // by ref, and if by ref, what kind of ref). In these cases,
1055 // when matching a builtin bound, we will yield back an
1056 // ambiguous result. But the *cache key* is just the closure type,
1057 // it doesn't capture the state of the upvar computation.
1059 // To avoid this trap, just don't cache ambiguous results if
1060 // the self-type contains no inference byproducts (that really
1061 // shouldn't happen in other circumstances anyway, given
1065 Ok(Some(_
)) | Err(_
) => true,
1067 cache_fresh_trait_pred
.0.trait_ref
.substs
.types
.has_infer_types()
1072 fn assemble_candidates
<'o
>(&mut self,
1073 stack
: &TraitObligationStack
<'o
, 'tcx
>)
1074 -> Result
<SelectionCandidateSet
<'tcx
>, SelectionError
<'tcx
>>
1076 let TraitObligationStack { obligation, .. }
= *stack
;
1077 let ref obligation
= Obligation
{
1078 cause
: obligation
.cause
.clone(),
1079 recursion_depth
: obligation
.recursion_depth
,
1080 predicate
: self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
)
1083 if obligation
.predicate
.skip_binder().self_ty().is_ty_var() {
1084 // FIXME(#20297): Self is a type variable (e.g. `_: AsRef<str>`).
1086 // This is somewhat problematic, as the current scheme can't really
1087 // handle it turning to be a projection. This does end up as truly
1088 // ambiguous in most cases anyway.
1090 // Until this is fixed, take the fast path out - this also improves
1091 // performance by preventing assemble_candidates_from_impls from
1092 // matching every impl for this trait.
1093 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true }
);
1096 let mut candidates
= SelectionCandidateSet
{
1101 // Other bounds. Consider both in-scope bounds from fn decl
1102 // and applicable impls. There is a certain set of precedence rules here.
1104 match self.tcx().lang_items
.to_builtin_kind(obligation
.predicate
.def_id()) {
1105 Some(ty
::BoundCopy
) => {
1106 debug
!("obligation self ty is {:?}",
1107 obligation
.predicate
.0.self_ty());
1109 // User-defined copy impls are permitted, but only for
1110 // structs and enums.
1111 self.assemble_candidates_from_impls(obligation
, &mut candidates
)?
;
1113 // For other types, we'll use the builtin rules.
1114 let copy_conditions
= self.copy_conditions(obligation
);
1115 self.assemble_builtin_bound_candidates(copy_conditions
, &mut candidates
)?
;
1117 Some(ty
::BoundSized
) => {
1118 // Sized is never implementable by end-users, it is
1119 // always automatically computed.
1120 let sized_conditions
= self.sized_conditions(obligation
);
1121 self.assemble_builtin_bound_candidates(sized_conditions
,
1125 None
if self.tcx().lang_items
.unsize_trait() ==
1126 Some(obligation
.predicate
.def_id()) => {
1127 self.assemble_candidates_for_unsizing(obligation
, &mut candidates
);
1130 Some(ty
::BoundSend
) |
1131 Some(ty
::BoundSync
) |
1133 self.assemble_closure_candidates(obligation
, &mut candidates
)?
;
1134 self.assemble_fn_pointer_candidates(obligation
, &mut candidates
)?
;
1135 self.assemble_candidates_from_impls(obligation
, &mut candidates
)?
;
1136 self.assemble_candidates_from_object_ty(obligation
, &mut candidates
);
1140 self.assemble_candidates_from_projected_tys(obligation
, &mut candidates
);
1141 self.assemble_candidates_from_caller_bounds(stack
, &mut candidates
)?
;
1142 // Default implementations have lower priority, so we only
1143 // consider triggering a default if there is no other impl that can apply.
1144 if candidates
.vec
.is_empty() {
1145 self.assemble_candidates_from_default_impls(obligation
, &mut candidates
)?
;
1147 debug
!("candidate list size: {}", candidates
.vec
.len());
1151 fn assemble_candidates_from_projected_tys(&mut self,
1152 obligation
: &TraitObligation
<'tcx
>,
1153 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1155 debug
!("assemble_candidates_for_projected_tys({:?})", obligation
);
1157 // FIXME(#20297) -- just examining the self-type is very simplistic
1159 // before we go into the whole skolemization thing, just
1160 // quickly check if the self-type is a projection at all.
1161 match obligation
.predicate
.0.trait_ref
.self_ty().sty
{
1162 ty
::TyProjection(_
) | ty
::TyAnon(..) => {}
1163 ty
::TyInfer(ty
::TyVar(_
)) => {
1164 span_bug
!(obligation
.cause
.span
,
1165 "Self=_ should have been handled by assemble_candidates");
1170 let result
= self.probe(|this
, snapshot
| {
1171 this
.match_projection_obligation_against_definition_bounds(obligation
,
1176 candidates
.vec
.push(ProjectionCandidate
);
1180 fn match_projection_obligation_against_definition_bounds(
1182 obligation
: &TraitObligation
<'tcx
>,
1183 snapshot
: &infer
::CombinedSnapshot
)
1186 let poly_trait_predicate
=
1187 self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
);
1188 let (skol_trait_predicate
, skol_map
) =
1189 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate
, snapshot
);
1190 debug
!("match_projection_obligation_against_definition_bounds: \
1191 skol_trait_predicate={:?} skol_map={:?}",
1192 skol_trait_predicate
,
1195 let (def_id
, substs
) = match skol_trait_predicate
.trait_ref
.self_ty().sty
{
1196 ty
::TyProjection(ref data
) => (data
.trait_ref
.def_id
, data
.trait_ref
.substs
),
1197 ty
::TyAnon(def_id
, substs
) => (def_id
, substs
),
1200 obligation
.cause
.span
,
1201 "match_projection_obligation_against_definition_bounds() called \
1202 but self-ty not a projection: {:?}",
1203 skol_trait_predicate
.trait_ref
.self_ty());
1206 debug
!("match_projection_obligation_against_definition_bounds: \
1207 def_id={:?}, substs={:?}",
1210 let item_predicates
= self.tcx().lookup_predicates(def_id
);
1211 let bounds
= item_predicates
.instantiate(self.tcx(), substs
);
1212 debug
!("match_projection_obligation_against_definition_bounds: \
1216 let matching_bound
=
1217 util
::elaborate_predicates(self.tcx(), bounds
.predicates
.into_vec())
1221 |this
, _
| this
.match_projection(obligation
,
1223 skol_trait_predicate
.trait_ref
.clone(),
1227 debug
!("match_projection_obligation_against_definition_bounds: \
1228 matching_bound={:?}",
1230 match matching_bound
{
1233 // Repeat the successful match, if any, this time outside of a probe.
1234 let result
= self.match_projection(obligation
,
1236 skol_trait_predicate
.trait_ref
.clone(),
1240 self.infcx
.pop_skolemized(skol_map
, snapshot
);
1248 fn match_projection(&mut self,
1249 obligation
: &TraitObligation
<'tcx
>,
1250 trait_bound
: ty
::PolyTraitRef
<'tcx
>,
1251 skol_trait_ref
: ty
::TraitRef
<'tcx
>,
1252 skol_map
: &infer
::SkolemizationMap
,
1253 snapshot
: &infer
::CombinedSnapshot
)
1256 assert
!(!skol_trait_ref
.has_escaping_regions());
1257 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation
.cause
.span
);
1258 match self.infcx
.sub_poly_trait_refs(false,
1260 trait_bound
.clone(),
1261 ty
::Binder(skol_trait_ref
.clone())) {
1262 Ok(InferOk { obligations, .. }
) => {
1263 self.inferred_obligations
.extend(obligations
);
1265 Err(_
) => { return false; }
1268 self.infcx
.leak_check(false, obligation
.cause
.span
, skol_map
, snapshot
).is_ok()
1271 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1272 /// supplied to find out whether it is listed among them.
1274 /// Never affects inference environment.
1275 fn assemble_candidates_from_caller_bounds
<'o
>(&mut self,
1276 stack
: &TraitObligationStack
<'o
, 'tcx
>,
1277 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1278 -> Result
<(),SelectionError
<'tcx
>>
1280 debug
!("assemble_candidates_from_caller_bounds({:?})",
1284 self.param_env().caller_bounds
1286 .filter_map(|o
| o
.to_opt_poly_trait_ref());
1288 let matching_bounds
=
1290 |bound
| self.evaluate_where_clause(stack
, bound
.clone()).may_apply());
1292 let param_candidates
=
1293 matching_bounds
.map(|bound
| ParamCandidate(bound
));
1295 candidates
.vec
.extend(param_candidates
);
1300 fn evaluate_where_clause
<'o
>(&mut self,
1301 stack
: &TraitObligationStack
<'o
, 'tcx
>,
1302 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
1305 self.probe(move |this
, _
| {
1306 match this
.match_where_clause_trait_ref(stack
.obligation
, where_clause_trait_ref
) {
1307 Ok(obligations
) => {
1308 this
.evaluate_predicates_recursively(stack
.list(), obligations
.iter())
1310 Err(()) => EvaluatedToErr
1315 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1316 /// FnMut<..>` where `X` is a closure type.
1318 /// Note: the type parameters on a closure candidate are modeled as *output* type
1319 /// parameters and hence do not affect whether this trait is a match or not. They will be
1320 /// unified during the confirmation step.
1321 fn assemble_closure_candidates(&mut self,
1322 obligation
: &TraitObligation
<'tcx
>,
1323 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1324 -> Result
<(),SelectionError
<'tcx
>>
1326 let kind
= match self.tcx().lang_items
.fn_trait_kind(obligation
.predicate
.0.def_id()) {
1328 None
=> { return Ok(()); }
1331 // ok to skip binder because the substs on closure types never
1332 // touch bound regions, they just capture the in-scope
1333 // type/region parameters
1334 let self_ty
= *obligation
.self_ty().skip_binder();
1335 let (closure_def_id
, substs
) = match self_ty
.sty
{
1336 ty
::TyClosure(id
, substs
) => (id
, substs
),
1337 ty
::TyInfer(ty
::TyVar(_
)) => {
1338 debug
!("assemble_unboxed_closure_candidates: ambiguous self-type");
1339 candidates
.ambiguous
= true;
1342 _
=> { return Ok(()); }
1345 debug
!("assemble_unboxed_candidates: self_ty={:?} kind={:?} obligation={:?}",
1350 match self.infcx
.closure_kind(closure_def_id
) {
1351 Some(closure_kind
) => {
1352 debug
!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind
);
1353 if closure_kind
.extends(kind
) {
1354 candidates
.vec
.push(ClosureCandidate(closure_def_id
, substs
, kind
));
1358 debug
!("assemble_unboxed_candidates: closure_kind not yet known");
1359 candidates
.vec
.push(ClosureCandidate(closure_def_id
, substs
, kind
));
1366 /// Implement one of the `Fn()` family for a fn pointer.
1367 fn assemble_fn_pointer_candidates(&mut self,
1368 obligation
: &TraitObligation
<'tcx
>,
1369 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1370 -> Result
<(),SelectionError
<'tcx
>>
1372 // We provide impl of all fn traits for fn pointers.
1373 if self.tcx().lang_items
.fn_trait_kind(obligation
.predicate
.def_id()).is_none() {
1377 // ok to skip binder because what we are inspecting doesn't involve bound regions
1378 let self_ty
= *obligation
.self_ty().skip_binder();
1380 ty
::TyInfer(ty
::TyVar(_
)) => {
1381 debug
!("assemble_fn_pointer_candidates: ambiguous self-type");
1382 candidates
.ambiguous
= true; // could wind up being a fn() type
1385 // provide an impl, but only for suitable `fn` pointers
1386 ty
::TyFnDef(_
, _
, &ty
::BareFnTy
{
1387 unsafety
: hir
::Unsafety
::Normal
,
1389 sig
: ty
::Binder(ty
::FnSig
{
1395 ty
::TyFnPtr(&ty
::BareFnTy
{
1396 unsafety
: hir
::Unsafety
::Normal
,
1398 sig
: ty
::Binder(ty
::FnSig
{
1404 candidates
.vec
.push(FnPointerCandidate
);
1413 /// Search for impls that might apply to `obligation`.
1414 fn assemble_candidates_from_impls(&mut self,
1415 obligation
: &TraitObligation
<'tcx
>,
1416 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1417 -> Result
<(), SelectionError
<'tcx
>>
1419 debug
!("assemble_candidates_from_impls(obligation={:?})", obligation
);
1421 let def
= self.tcx().lookup_trait_def(obligation
.predicate
.def_id());
1423 def
.for_each_relevant_impl(
1425 obligation
.predicate
.0.trait_ref
.self_ty(),
1427 self.probe(|this
, snapshot
| { /* [1] */
1428 match this
.match_impl(impl_def_id
, obligation
, snapshot
) {
1430 candidates
.vec
.push(ImplCandidate(impl_def_id
));
1432 // NB: we can safely drop the skol map
1433 // since we are in a probe [1]
1434 mem
::drop(skol_map
);
1445 fn assemble_candidates_from_default_impls(&mut self,
1446 obligation
: &TraitObligation
<'tcx
>,
1447 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1448 -> Result
<(), SelectionError
<'tcx
>>
1450 // OK to skip binder here because the tests we do below do not involve bound regions
1451 let self_ty
= *obligation
.self_ty().skip_binder();
1452 debug
!("assemble_candidates_from_default_impls(self_ty={:?})", self_ty
);
1454 let def_id
= obligation
.predicate
.def_id();
1456 if self.tcx().trait_has_default_impl(def_id
) {
1458 ty
::TyTrait(..) => {
1459 // For object types, we don't know what the closed
1460 // over types are. For most traits, this means we
1461 // conservatively say nothing; a candidate may be
1462 // added by `assemble_candidates_from_object_ty`.
1463 // However, for the kind of magic reflect trait,
1464 // we consider it to be implemented even for
1465 // object types, because it just lets you reflect
1466 // onto the object type, not into the object's
1468 if self.tcx().has_attr(def_id
, "rustc_reflect_like") {
1469 candidates
.vec
.push(DefaultImplObjectCandidate(def_id
));
1473 ty
::TyProjection(..) |
1475 // In these cases, we don't know what the actual
1476 // type is. Therefore, we cannot break it down
1477 // into its constituent types. So we don't
1478 // consider the `..` impl but instead just add no
1479 // candidates: this means that typeck will only
1480 // succeed if there is another reason to believe
1481 // that this obligation holds. That could be a
1482 // where-clause or, in the case of an object type,
1483 // it could be that the object type lists the
1484 // trait (e.g. `Foo+Send : Send`). See
1485 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1486 // for an example of a test case that exercises
1489 ty
::TyInfer(ty
::TyVar(_
)) => {
1490 // the defaulted impl might apply, we don't know
1491 candidates
.ambiguous
= true;
1494 candidates
.vec
.push(DefaultImplCandidate(def_id
.clone()))
1502 /// Search for impls that might apply to `obligation`.
1503 fn assemble_candidates_from_object_ty(&mut self,
1504 obligation
: &TraitObligation
<'tcx
>,
1505 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1507 debug
!("assemble_candidates_from_object_ty(self_ty={:?})",
1508 obligation
.self_ty().skip_binder());
1510 // Object-safety candidates are only applicable to object-safe
1511 // traits. Including this check is useful because it helps
1512 // inference in cases of traits like `BorrowFrom`, which are
1513 // not object-safe, and which rely on being able to infer the
1514 // self-type from one of the other inputs. Without this check,
1515 // these cases wind up being considered ambiguous due to a
1516 // (spurious) ambiguity introduced here.
1517 let predicate_trait_ref
= obligation
.predicate
.to_poly_trait_ref();
1518 if !self.tcx().is_object_safe(predicate_trait_ref
.def_id()) {
1522 self.probe(|this
, _snapshot
| {
1523 // the code below doesn't care about regions, and the
1524 // self-ty here doesn't escape this probe, so just erase
1526 let self_ty
= this
.tcx().erase_late_bound_regions(&obligation
.self_ty());
1527 let poly_trait_ref
= match self_ty
.sty
{
1528 ty
::TyTrait(ref data
) => {
1529 match this
.tcx().lang_items
.to_builtin_kind(obligation
.predicate
.def_id()) {
1530 Some(bound @ ty
::BoundSend
) | Some(bound @ ty
::BoundSync
) => {
1531 if data
.bounds
.builtin_bounds
.contains(&bound
) {
1532 debug
!("assemble_candidates_from_object_ty: matched builtin bound, \
1533 pushing candidate");
1534 candidates
.vec
.push(BuiltinObjectCandidate
);
1541 data
.principal_trait_ref_with_self_ty(this
.tcx(), self_ty
)
1543 ty
::TyInfer(ty
::TyVar(_
)) => {
1544 debug
!("assemble_candidates_from_object_ty: ambiguous");
1545 candidates
.ambiguous
= true; // could wind up being an object type
1553 debug
!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1556 // Count only those upcast versions that match the trait-ref
1557 // we are looking for. Specifically, do not only check for the
1558 // correct trait, but also the correct type parameters.
1559 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1560 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1561 let upcast_trait_refs
=
1562 util
::supertraits(this
.tcx(), poly_trait_ref
)
1563 .filter(|upcast_trait_ref
| {
1564 this
.probe(|this
, _
| {
1565 let upcast_trait_ref
= upcast_trait_ref
.clone();
1566 this
.match_poly_trait_ref(obligation
, upcast_trait_ref
).is_ok()
1571 if upcast_trait_refs
> 1 {
1572 // can be upcast in many ways; need more type information
1573 candidates
.ambiguous
= true;
1574 } else if upcast_trait_refs
== 1 {
1575 candidates
.vec
.push(ObjectCandidate
);
1580 /// Search for unsizing that might apply to `obligation`.
1581 fn assemble_candidates_for_unsizing(&mut self,
1582 obligation
: &TraitObligation
<'tcx
>,
1583 candidates
: &mut SelectionCandidateSet
<'tcx
>) {
1584 // We currently never consider higher-ranked obligations e.g.
1585 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1586 // because they are a priori invalid, and we could potentially add support
1587 // for them later, it's just that there isn't really a strong need for it.
1588 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1589 // impl, and those are generally applied to concrete types.
1591 // That said, one might try to write a fn with a where clause like
1592 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1593 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1594 // Still, you'd be more likely to write that where clause as
1596 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1597 // obligation above. Should be possible to extend this in the future.
1598 let source
= match self.tcx().no_late_bound_regions(&obligation
.self_ty()) {
1601 // Don't add any candidates if there are bound regions.
1605 let target
= obligation
.predicate
.0.input_types
()[0];
1607 debug
!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1610 let may_apply
= match (&source
.sty
, &target
.sty
) {
1611 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1612 (&ty
::TyTrait(ref data_a
), &ty
::TyTrait(ref data_b
)) => {
1613 // Upcasts permit two things:
1615 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1616 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1618 // Note that neither of these changes requires any
1619 // change at runtime. Eventually this will be
1622 // We always upcast when we can because of reason
1623 // #2 (region bounds).
1624 data_a
.principal
.def_id() == data_a
.principal
.def_id() &&
1625 data_a
.bounds
.builtin_bounds
.is_superset(&data_b
.bounds
.builtin_bounds
)
1629 (_
, &ty
::TyTrait(_
)) => true,
1631 // Ambiguous handling is below T -> Trait, because inference
1632 // variables can still implement Unsize<Trait> and nested
1633 // obligations will have the final say (likely deferred).
1634 (&ty
::TyInfer(ty
::TyVar(_
)), _
) |
1635 (_
, &ty
::TyInfer(ty
::TyVar(_
))) => {
1636 debug
!("assemble_candidates_for_unsizing: ambiguous");
1637 candidates
.ambiguous
= true;
1642 (&ty
::TyArray(_
, _
), &ty
::TySlice(_
)) => true,
1644 // Struct<T> -> Struct<U>.
1645 (&ty
::TyStruct(def_id_a
, _
), &ty
::TyStruct(def_id_b
, _
)) => {
1646 def_id_a
== def_id_b
1653 candidates
.vec
.push(BuiltinUnsizeCandidate
);
1657 ///////////////////////////////////////////////////////////////////////////
1660 // Winnowing is the process of attempting to resolve ambiguity by
1661 // probing further. During the winnowing process, we unify all
1662 // type variables (ignoring skolemization) and then we also
1663 // attempt to evaluate recursive bounds to see if they are
1666 /// Returns true if `candidate_i` should be dropped in favor of
1667 /// `candidate_j`. Generally speaking we will drop duplicate
1668 /// candidates and prefer where-clause candidates.
1669 /// Returns true if `victim` should be dropped in favor of
1670 /// `other`. Generally speaking we will drop duplicate
1671 /// candidates and prefer where-clause candidates.
1673 /// See the comment for "SelectionCandidate" for more details.
1674 fn candidate_should_be_dropped_in_favor_of
<'o
>(
1676 victim
: &EvaluatedCandidate
<'tcx
>,
1677 other
: &EvaluatedCandidate
<'tcx
>)
1680 if victim
.candidate
== other
.candidate
{
1684 match other
.candidate
{
1686 ParamCandidate(_
) | ProjectionCandidate
=> match victim
.candidate
{
1687 DefaultImplCandidate(..) => {
1689 "default implementations shouldn't be recorded \
1690 when there are other valid candidates");
1693 ClosureCandidate(..) |
1694 FnPointerCandidate
|
1695 BuiltinObjectCandidate
|
1696 BuiltinUnsizeCandidate
|
1697 DefaultImplObjectCandidate(..) |
1698 BuiltinCandidate { .. }
=> {
1699 // We have a where-clause so don't go around looking
1704 ProjectionCandidate
=> {
1705 // Arbitrarily give param candidates priority
1706 // over projection and object candidates.
1709 ParamCandidate(..) => false,
1711 ImplCandidate(other_def
) => {
1712 // See if we can toss out `victim` based on specialization.
1713 // This requires us to know *for sure* that the `other` impl applies
1714 // i.e. EvaluatedToOk:
1715 if other
.evaluation
== EvaluatedToOk
{
1716 if let ImplCandidate(victim_def
) = victim
.candidate
{
1717 let tcx
= self.tcx().global_tcx();
1718 return traits
::specializes(tcx
, other_def
, victim_def
);
1728 ///////////////////////////////////////////////////////////////////////////
1731 // These cover the traits that are built-in to the language
1732 // itself. This includes `Copy` and `Sized` for sure. For the
1733 // moment, it also includes `Send` / `Sync` and a few others, but
1734 // those will hopefully change to library-defined traits in the
1737 // HACK: if this returns an error, selection exits without considering
1739 fn assemble_builtin_bound_candidates
<'o
>(&mut self,
1740 conditions
: BuiltinImplConditions
<'tcx
>,
1741 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1742 -> Result
<(),SelectionError
<'tcx
>>
1745 BuiltinImplConditions
::Where(nested
) => {
1746 debug
!("builtin_bound: nested={:?}", nested
);
1747 candidates
.vec
.push(BuiltinCandidate
{
1748 has_nested
: nested
.skip_binder().len() > 0
1752 BuiltinImplConditions
::None
=> { Ok(()) }
1753 BuiltinImplConditions
::Ambiguous
=> {
1754 debug
!("assemble_builtin_bound_candidates: ambiguous builtin");
1755 Ok(candidates
.ambiguous
= true)
1757 BuiltinImplConditions
::Never
=> { Err(Unimplemented) }
1761 fn sized_conditions(&mut self, obligation
: &TraitObligation
<'tcx
>)
1762 -> BuiltinImplConditions
<'tcx
>
1764 use self::BuiltinImplConditions
::{Ambiguous, None, Never, Where}
;
1766 // NOTE: binder moved to (*)
1767 let self_ty
= self.infcx
.shallow_resolve(
1768 obligation
.predicate
.skip_binder().self_ty());
1771 ty
::TyInfer(ty
::IntVar(_
)) | ty
::TyInfer(ty
::FloatVar(_
)) |
1772 ty
::TyUint(_
) | ty
::TyInt(_
) | ty
::TyBool
| ty
::TyFloat(_
) |
1773 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) | ty
::TyRawPtr(..) |
1774 ty
::TyChar
| ty
::TyBox(_
) | ty
::TyRef(..) |
1775 ty
::TyArray(..) | ty
::TyClosure(..) | ty
::TyNever
|
1777 // safe for everything
1778 Where(ty
::Binder(Vec
::new()))
1781 ty
::TyStr
| ty
::TySlice(_
) | ty
::TyTrait(..) => Never
,
1783 ty
::TyTuple(tys
) => {
1784 // FIXME(#33242) we only need to constrain the last field
1785 Where(ty
::Binder(tys
.to_vec()))
1788 ty
::TyStruct(def
, substs
) | ty
::TyEnum(def
, substs
) => {
1789 let sized_crit
= def
.sized_constraint(self.tcx());
1790 // (*) binder moved here
1791 Where(ty
::Binder(match sized_crit
.sty
{
1792 ty
::TyTuple(tys
) => tys
.to_vec().subst(self.tcx(), substs
),
1793 ty
::TyBool
=> vec
![],
1794 _
=> vec
![sized_crit
.subst(self.tcx(), substs
)]
1798 ty
::TyProjection(_
) | ty
::TyParam(_
) | ty
::TyAnon(..) => None
,
1799 ty
::TyInfer(ty
::TyVar(_
)) => Ambiguous
,
1801 ty
::TyInfer(ty
::FreshTy(_
))
1802 | ty
::TyInfer(ty
::FreshIntTy(_
))
1803 | ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
1804 bug
!("asked to assemble builtin bounds of unexpected type: {:?}",
1810 fn copy_conditions(&mut self, obligation
: &TraitObligation
<'tcx
>)
1811 -> BuiltinImplConditions
<'tcx
>
1813 // NOTE: binder moved to (*)
1814 let self_ty
= self.infcx
.shallow_resolve(
1815 obligation
.predicate
.skip_binder().self_ty());
1817 use self::BuiltinImplConditions
::{Ambiguous, None, Never, Where}
;
1820 ty
::TyInfer(ty
::IntVar(_
)) | ty
::TyInfer(ty
::FloatVar(_
)) |
1821 ty
::TyUint(_
) | ty
::TyInt(_
) | ty
::TyBool
| ty
::TyFloat(_
) |
1822 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) | ty
::TyChar
|
1823 ty
::TyRawPtr(..) | ty
::TyError
| ty
::TyNever
|
1824 ty
::TyRef(_
, ty
::TypeAndMut { ty: _, mutbl: hir::MutImmutable }
) => {
1825 Where(ty
::Binder(Vec
::new()))
1828 ty
::TyBox(_
) | ty
::TyTrait(..) | ty
::TyStr
| ty
::TySlice(..) |
1830 ty
::TyRef(_
, ty
::TypeAndMut { ty: _, mutbl: hir::MutMutable }
) => {
1834 ty
::TyArray(element_ty
, _
) => {
1835 // (*) binder moved here
1836 Where(ty
::Binder(vec
![element_ty
]))
1839 ty
::TyTuple(tys
) => {
1840 // (*) binder moved here
1841 Where(ty
::Binder(tys
.to_vec()))
1844 ty
::TyStruct(..) | ty
::TyEnum(..) |
1845 ty
::TyProjection(..) | ty
::TyParam(..) | ty
::TyAnon(..) => {
1846 // Fallback to whatever user-defined impls exist in this case.
1850 ty
::TyInfer(ty
::TyVar(_
)) => {
1851 // Unbound type variable. Might or might not have
1852 // applicable impls and so forth, depending on what
1853 // those type variables wind up being bound to.
1857 ty
::TyInfer(ty
::FreshTy(_
))
1858 | ty
::TyInfer(ty
::FreshIntTy(_
))
1859 | ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
1860 bug
!("asked to assemble builtin bounds of unexpected type: {:?}",
1866 /// For default impls, we need to break apart a type into its
1867 /// "constituent types" -- meaning, the types that it contains.
1869 /// Here are some (simple) examples:
1872 /// (i32, u32) -> [i32, u32]
1873 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1874 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1875 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1877 fn constituent_types_for_ty(&self, t
: Ty
<'tcx
>) -> Vec
<Ty
<'tcx
>> {
1887 ty
::TyInfer(ty
::IntVar(_
)) |
1888 ty
::TyInfer(ty
::FloatVar(_
)) |
1896 ty
::TyProjection(..) |
1898 ty
::TyInfer(ty
::TyVar(_
)) |
1899 ty
::TyInfer(ty
::FreshTy(_
)) |
1900 ty
::TyInfer(ty
::FreshIntTy(_
)) |
1901 ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
1902 bug
!("asked to assemble constituent types of unexpected type: {:?}",
1906 ty
::TyBox(referent_ty
) => { // Box<T>
1910 ty
::TyRawPtr(ty
::TypeAndMut { ty: element_ty, ..}
) |
1911 ty
::TyRef(_
, ty
::TypeAndMut { ty: element_ty, ..}
) => {
1915 ty
::TyArray(element_ty
, _
) | ty
::TySlice(element_ty
) => {
1919 ty
::TyTuple(ref tys
) => {
1920 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1924 ty
::TyClosure(_
, ref substs
) => {
1925 // FIXME(#27086). We are invariant w/r/t our
1926 // substs.func_substs, but we don't see them as
1927 // constituent types; this seems RIGHT but also like
1928 // something that a normal type couldn't simulate. Is
1929 // this just a gap with the way that PhantomData and
1930 // OIBIT interact? That is, there is no way to say
1931 // "make me invariant with respect to this TYPE, but
1932 // do not act as though I can reach it"
1933 substs
.upvar_tys
.to_vec()
1936 // for `PhantomData<T>`, we pass `T`
1937 ty
::TyStruct(def
, substs
) if def
.is_phantom_data() => {
1938 substs
.types
.get_slice(TypeSpace
).to_vec()
1941 ty
::TyStruct(def
, substs
) | ty
::TyEnum(def
, substs
) => {
1943 .map(|f
| f
.ty(self.tcx(), substs
))
1949 fn collect_predicates_for_types(&mut self,
1950 cause
: ObligationCause
<'tcx
>,
1951 recursion_depth
: usize,
1952 trait_def_id
: DefId
,
1953 types
: ty
::Binder
<Vec
<Ty
<'tcx
>>>)
1954 -> Vec
<PredicateObligation
<'tcx
>>
1956 // Because the types were potentially derived from
1957 // higher-ranked obligations they may reference late-bound
1958 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
1959 // yield a type like `for<'a> &'a int`. In general, we
1960 // maintain the invariant that we never manipulate bound
1961 // regions, so we have to process these bound regions somehow.
1963 // The strategy is to:
1965 // 1. Instantiate those regions to skolemized regions (e.g.,
1966 // `for<'a> &'a int` becomes `&0 int`.
1967 // 2. Produce something like `&'0 int : Copy`
1968 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
1970 types
.skip_binder().into_iter().flat_map(|ty
| { // binder moved -\
1971 let ty
: ty
::Binder
<Ty
<'tcx
>> = ty
::Binder(ty
); // <----------/
1973 self.in_snapshot(|this
, snapshot
| {
1974 let (skol_ty
, skol_map
) =
1975 this
.infcx().skolemize_late_bound_regions(&ty
, snapshot
);
1976 let Normalized { value: normalized_ty, mut obligations }
=
1977 project
::normalize_with_depth(this
,
1981 let skol_obligation
=
1982 this
.tcx().predicate_for_trait_def(
1988 obligations
.push(skol_obligation
);
1989 this
.infcx().plug_leaks(skol_map
, snapshot
, &obligations
)
1994 ///////////////////////////////////////////////////////////////////////////
1997 // Confirmation unifies the output type parameters of the trait
1998 // with the values found in the obligation, possibly yielding a
1999 // type error. See `README.md` for more details.
2001 fn confirm_candidate(&mut self,
2002 obligation
: &TraitObligation
<'tcx
>,
2003 candidate
: SelectionCandidate
<'tcx
>)
2004 -> Result
<Selection
<'tcx
>,SelectionError
<'tcx
>>
2006 debug
!("confirm_candidate({:?}, {:?})",
2011 BuiltinCandidate { has_nested }
=> {
2013 self.confirm_builtin_candidate(obligation
, has_nested
)))
2016 ParamCandidate(param
) => {
2017 let obligations
= self.confirm_param_candidate(obligation
, param
);
2018 Ok(VtableParam(obligations
))
2021 DefaultImplCandidate(trait_def_id
) => {
2022 let data
= self.confirm_default_impl_candidate(obligation
, trait_def_id
);
2023 Ok(VtableDefaultImpl(data
))
2026 DefaultImplObjectCandidate(trait_def_id
) => {
2027 let data
= self.confirm_default_impl_object_candidate(obligation
, trait_def_id
);
2028 Ok(VtableDefaultImpl(data
))
2031 ImplCandidate(impl_def_id
) => {
2032 Ok(VtableImpl(self.confirm_impl_candidate(obligation
, impl_def_id
)))
2035 ClosureCandidate(closure_def_id
, substs
, kind
) => {
2036 let vtable_closure
=
2037 self.confirm_closure_candidate(obligation
, closure_def_id
, substs
, kind
)?
;
2038 Ok(VtableClosure(vtable_closure
))
2041 BuiltinObjectCandidate
=> {
2042 // This indicates something like `(Trait+Send) :
2043 // Send`. In this case, we know that this holds
2044 // because that's what the object type is telling us,
2045 // and there's really no additional obligations to
2046 // prove and no types in particular to unify etc.
2047 Ok(VtableParam(Vec
::new()))
2050 ObjectCandidate
=> {
2051 let data
= self.confirm_object_candidate(obligation
);
2052 Ok(VtableObject(data
))
2055 FnPointerCandidate
=> {
2057 self.confirm_fn_pointer_candidate(obligation
)?
;
2058 Ok(VtableFnPointer(data
))
2061 ProjectionCandidate
=> {
2062 self.confirm_projection_candidate(obligation
);
2063 Ok(VtableParam(Vec
::new()))
2066 BuiltinUnsizeCandidate
=> {
2067 let data
= self.confirm_builtin_unsize_candidate(obligation
)?
;
2068 Ok(VtableBuiltin(data
))
2073 fn confirm_projection_candidate(&mut self,
2074 obligation
: &TraitObligation
<'tcx
>)
2076 self.in_snapshot(|this
, snapshot
| {
2078 this
.match_projection_obligation_against_definition_bounds(obligation
,
2084 fn confirm_param_candidate(&mut self,
2085 obligation
: &TraitObligation
<'tcx
>,
2086 param
: ty
::PolyTraitRef
<'tcx
>)
2087 -> Vec
<PredicateObligation
<'tcx
>>
2089 debug
!("confirm_param_candidate({:?},{:?})",
2093 // During evaluation, we already checked that this
2094 // where-clause trait-ref could be unified with the obligation
2095 // trait-ref. Repeat that unification now without any
2096 // transactional boundary; it should not fail.
2097 match self.match_where_clause_trait_ref(obligation
, param
.clone()) {
2098 Ok(obligations
) => obligations
,
2100 bug
!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2107 fn confirm_builtin_candidate(&mut self,
2108 obligation
: &TraitObligation
<'tcx
>,
2110 -> VtableBuiltinData
<PredicateObligation
<'tcx
>>
2112 debug
!("confirm_builtin_candidate({:?}, {:?})",
2113 obligation
, has_nested
);
2115 let obligations
= if has_nested
{
2116 let trait_def
= obligation
.predicate
.def_id();
2117 let conditions
= match trait_def
{
2118 _
if Some(trait_def
) == self.tcx().lang_items
.sized_trait() => {
2119 self.sized_conditions(obligation
)
2121 _
if Some(trait_def
) == self.tcx().lang_items
.copy_trait() => {
2122 self.copy_conditions(obligation
)
2124 _
=> bug
!("unexpected builtin trait {:?}", trait_def
)
2126 let nested
= match conditions
{
2127 BuiltinImplConditions
::Where(nested
) => nested
,
2128 _
=> bug
!("obligation {:?} had matched a builtin impl but now doesn't",
2132 let cause
= obligation
.derived_cause(BuiltinDerivedObligation
);
2133 self.collect_predicates_for_types(cause
,
2134 obligation
.recursion_depth
+1,
2141 debug
!("confirm_builtin_candidate: obligations={:?}",
2143 VtableBuiltinData { nested: obligations }
2146 /// This handles the case where a `impl Foo for ..` impl is being used.
2147 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2149 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2150 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2151 fn confirm_default_impl_candidate(&mut self,
2152 obligation
: &TraitObligation
<'tcx
>,
2153 trait_def_id
: DefId
)
2154 -> VtableDefaultImplData
<PredicateObligation
<'tcx
>>
2156 debug
!("confirm_default_impl_candidate({:?}, {:?})",
2160 // binder is moved below
2161 let self_ty
= self.infcx
.shallow_resolve(obligation
.predicate
.skip_binder().self_ty());
2162 let types
= self.constituent_types_for_ty(self_ty
);
2163 self.vtable_default_impl(obligation
, trait_def_id
, ty
::Binder(types
))
2166 fn confirm_default_impl_object_candidate(&mut self,
2167 obligation
: &TraitObligation
<'tcx
>,
2168 trait_def_id
: DefId
)
2169 -> VtableDefaultImplData
<PredicateObligation
<'tcx
>>
2171 debug
!("confirm_default_impl_object_candidate({:?}, {:?})",
2175 assert
!(self.tcx().has_attr(trait_def_id
, "rustc_reflect_like"));
2177 // OK to skip binder, it is reintroduced below
2178 let self_ty
= self.infcx
.shallow_resolve(obligation
.predicate
.skip_binder().self_ty());
2180 ty
::TyTrait(ref data
) => {
2181 // OK to skip the binder, it is reintroduced below
2182 let input_types
= data
.principal
.skip_binder().substs
.types
.get_slice(TypeSpace
);
2183 let assoc_types
= data
.bounds
.projection_bounds
2185 .map(|pb
| pb
.skip_binder().ty
);
2186 let all_types
: Vec
<_
> = input_types
.iter().cloned()
2190 // reintroduce the two binding levels we skipped, then flatten into one
2191 let all_types
= ty
::Binder(ty
::Binder(all_types
));
2192 let all_types
= self.tcx().flatten_late_bound_regions(&all_types
);
2194 self.vtable_default_impl(obligation
, trait_def_id
, all_types
)
2197 bug
!("asked to confirm default object implementation for non-object type: {:?}",
2203 /// See `confirm_default_impl_candidate`
2204 fn vtable_default_impl(&mut self,
2205 obligation
: &TraitObligation
<'tcx
>,
2206 trait_def_id
: DefId
,
2207 nested
: ty
::Binder
<Vec
<Ty
<'tcx
>>>)
2208 -> VtableDefaultImplData
<PredicateObligation
<'tcx
>>
2210 debug
!("vtable_default_impl: nested={:?}", nested
);
2212 let cause
= obligation
.derived_cause(BuiltinDerivedObligation
);
2213 let mut obligations
= self.collect_predicates_for_types(
2215 obligation
.recursion_depth
+1,
2219 let trait_obligations
= self.in_snapshot(|this
, snapshot
| {
2220 let poly_trait_ref
= obligation
.predicate
.to_poly_trait_ref();
2221 let (trait_ref
, skol_map
) =
2222 this
.infcx().skolemize_late_bound_regions(&poly_trait_ref
, snapshot
);
2223 let cause
= obligation
.derived_cause(ImplDerivedObligation
);
2224 this
.impl_or_trait_obligations(cause
,
2225 obligation
.recursion_depth
+ 1,
2232 obligations
.extend(trait_obligations
);
2234 debug
!("vtable_default_impl: obligations={:?}", obligations
);
2236 VtableDefaultImplData
{
2237 trait_def_id
: trait_def_id
,
2242 fn confirm_impl_candidate(&mut self,
2243 obligation
: &TraitObligation
<'tcx
>,
2245 -> VtableImplData
<'tcx
, PredicateObligation
<'tcx
>>
2247 debug
!("confirm_impl_candidate({:?},{:?})",
2251 // First, create the substitutions by matching the impl again,
2252 // this time not in a probe.
2253 self.in_snapshot(|this
, snapshot
| {
2254 let (substs
, skol_map
) =
2255 this
.rematch_impl(impl_def_id
, obligation
,
2257 debug
!("confirm_impl_candidate substs={:?}", substs
);
2258 let cause
= obligation
.derived_cause(ImplDerivedObligation
);
2259 this
.vtable_impl(impl_def_id
, substs
, cause
,
2260 obligation
.recursion_depth
+ 1,
2265 fn vtable_impl(&mut self,
2267 mut substs
: Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
2268 cause
: ObligationCause
<'tcx
>,
2269 recursion_depth
: usize,
2270 skol_map
: infer
::SkolemizationMap
,
2271 snapshot
: &infer
::CombinedSnapshot
)
2272 -> VtableImplData
<'tcx
, PredicateObligation
<'tcx
>>
2274 debug
!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2280 let mut impl_obligations
=
2281 self.impl_or_trait_obligations(cause
,
2288 debug
!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2292 // Because of RFC447, the impl-trait-ref and obligations
2293 // are sufficient to determine the impl substs, without
2294 // relying on projections in the impl-trait-ref.
2296 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2297 impl_obligations
.append(&mut substs
.obligations
);
2299 VtableImplData
{ impl_def_id
: impl_def_id
,
2300 substs
: substs
.value
,
2301 nested
: impl_obligations
}
2304 fn confirm_object_candidate(&mut self,
2305 obligation
: &TraitObligation
<'tcx
>)
2306 -> VtableObjectData
<'tcx
, PredicateObligation
<'tcx
>>
2308 debug
!("confirm_object_candidate({:?})",
2311 // FIXME skipping binder here seems wrong -- we should
2312 // probably flatten the binder from the obligation and the
2313 // binder from the object. Have to try to make a broken test
2314 // case that results. -nmatsakis
2315 let self_ty
= self.infcx
.shallow_resolve(*obligation
.self_ty().skip_binder());
2316 let poly_trait_ref
= match self_ty
.sty
{
2317 ty
::TyTrait(ref data
) => {
2318 data
.principal_trait_ref_with_self_ty(self.tcx(), self_ty
)
2321 span_bug
!(obligation
.cause
.span
,
2322 "object candidate with non-object");
2326 let mut upcast_trait_ref
= None
;
2330 let tcx
= self.tcx();
2332 // We want to find the first supertrait in the list of
2333 // supertraits that we can unify with, and do that
2334 // unification. We know that there is exactly one in the list
2335 // where we can unify because otherwise select would have
2336 // reported an ambiguity. (When we do find a match, also
2337 // record it for later.)
2339 util
::supertraits(tcx
, poly_trait_ref
)
2343 |this
, _
| this
.match_poly_trait_ref(obligation
, t
))
2345 Ok(_
) => { upcast_trait_ref = Some(t); false }
2350 // Additionally, for each of the nonmatching predicates that
2351 // we pass over, we sum up the set of number of vtable
2352 // entries, so that we can compute the offset for the selected
2355 nonmatching
.map(|t
| tcx
.count_own_vtable_entries(t
))
2361 upcast_trait_ref
: upcast_trait_ref
.unwrap(),
2362 vtable_base
: vtable_base
,
2367 fn confirm_fn_pointer_candidate(&mut self, obligation
: &TraitObligation
<'tcx
>)
2368 -> Result
<VtableFnPointerData
<'tcx
, PredicateObligation
<'tcx
>>, SelectionError
<'tcx
>>
2370 debug
!("confirm_fn_pointer_candidate({:?})",
2373 // ok to skip binder; it is reintroduced below
2374 let self_ty
= self.infcx
.shallow_resolve(*obligation
.self_ty().skip_binder());
2375 let sig
= self_ty
.fn_sig();
2377 self.tcx().closure_trait_ref_and_return_type(obligation
.predicate
.def_id(),
2380 util
::TupleArgumentsFlag
::Yes
)
2381 .map_bound(|(trait_ref
, _
)| trait_ref
);
2383 self.confirm_poly_trait_refs(obligation
.cause
.clone(),
2384 obligation
.predicate
.to_poly_trait_ref(),
2386 Ok(VtableFnPointerData { fn_ty: self_ty, nested: vec![] }
)
2389 fn confirm_closure_candidate(&mut self,
2390 obligation
: &TraitObligation
<'tcx
>,
2391 closure_def_id
: DefId
,
2392 substs
: ty
::ClosureSubsts
<'tcx
>,
2393 kind
: ty
::ClosureKind
)
2394 -> Result
<VtableClosureData
<'tcx
, PredicateObligation
<'tcx
>>,
2395 SelectionError
<'tcx
>>
2397 debug
!("confirm_closure_candidate({:?},{:?},{:?})",
2405 } = self.closure_trait_ref(obligation
, closure_def_id
, substs
);
2407 debug
!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2412 self.confirm_poly_trait_refs(obligation
.cause
.clone(),
2413 obligation
.predicate
.to_poly_trait_ref(),
2416 obligations
.push(Obligation
::new(
2417 obligation
.cause
.clone(),
2418 ty
::Predicate
::ClosureKind(closure_def_id
, kind
)));
2420 Ok(VtableClosureData
{
2421 closure_def_id
: closure_def_id
,
2422 substs
: substs
.clone(),
2427 /// In the case of closure types and fn pointers,
2428 /// we currently treat the input type parameters on the trait as
2429 /// outputs. This means that when we have a match we have only
2430 /// considered the self type, so we have to go back and make sure
2431 /// to relate the argument types too. This is kind of wrong, but
2432 /// since we control the full set of impls, also not that wrong,
2433 /// and it DOES yield better error messages (since we don't report
2434 /// errors as if there is no applicable impl, but rather report
2435 /// errors are about mismatched argument types.
2437 /// Here is an example. Imagine we have a closure expression
2438 /// and we desugared it so that the type of the expression is
2439 /// `Closure`, and `Closure` expects an int as argument. Then it
2440 /// is "as if" the compiler generated this impl:
2442 /// impl Fn(int) for Closure { ... }
2444 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2445 /// we have matched the self-type `Closure`. At this point we'll
2446 /// compare the `int` to `usize` and generate an error.
2448 /// Note that this checking occurs *after* the impl has selected,
2449 /// because these output type parameters should not affect the
2450 /// selection of the impl. Therefore, if there is a mismatch, we
2451 /// report an error to the user.
2452 fn confirm_poly_trait_refs(&mut self,
2453 obligation_cause
: ObligationCause
,
2454 obligation_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
2455 expected_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
2456 -> Result
<(), SelectionError
<'tcx
>>
2458 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation_cause
.span
);
2460 let obligation_trait_ref
= obligation_trait_ref
.clone();
2461 self.infcx
.sub_poly_trait_refs(false,
2463 expected_trait_ref
.clone(),
2464 obligation_trait_ref
.clone())
2465 .map(|InferOk { obligations, .. }
| self.inferred_obligations
.extend(obligations
))
2466 .map_err(|e
| OutputTypeParameterMismatch(expected_trait_ref
, obligation_trait_ref
, e
))
2469 fn confirm_builtin_unsize_candidate(&mut self,
2470 obligation
: &TraitObligation
<'tcx
>,)
2471 -> Result
<VtableBuiltinData
<PredicateObligation
<'tcx
>>,
2472 SelectionError
<'tcx
>> {
2473 let tcx
= self.tcx();
2475 // assemble_candidates_for_unsizing should ensure there are no late bound
2476 // regions here. See the comment there for more details.
2477 let source
= self.infcx
.shallow_resolve(
2478 tcx
.no_late_bound_regions(&obligation
.self_ty()).unwrap());
2479 let target
= self.infcx
.shallow_resolve(obligation
.predicate
.0.input_types
()[0]);
2481 debug
!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2484 let mut nested
= vec
![];
2485 match (&source
.sty
, &target
.sty
) {
2486 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2487 (&ty
::TyTrait(ref data_a
), &ty
::TyTrait(ref data_b
)) => {
2488 // See assemble_candidates_for_unsizing for more info.
2489 let bounds
= ty
::ExistentialBounds
{
2490 region_bound
: data_b
.bounds
.region_bound
,
2491 builtin_bounds
: data_b
.bounds
.builtin_bounds
,
2492 projection_bounds
: data_a
.bounds
.projection_bounds
.clone(),
2495 let new_trait
= tcx
.mk_trait(data_a
.principal
.clone(), bounds
);
2496 let origin
= TypeOrigin
::Misc(obligation
.cause
.span
);
2497 let InferOk { obligations, .. }
=
2498 self.infcx
.sub_types(false, origin
, new_trait
, target
)
2499 .map_err(|_
| Unimplemented
)?
;
2500 self.inferred_obligations
.extend(obligations
);
2502 // Register one obligation for 'a: 'b.
2503 let cause
= ObligationCause
::new(obligation
.cause
.span
,
2504 obligation
.cause
.body_id
,
2505 ObjectCastObligation(target
));
2506 let outlives
= ty
::OutlivesPredicate(data_a
.bounds
.region_bound
,
2507 data_b
.bounds
.region_bound
);
2508 nested
.push(Obligation
::with_depth(cause
,
2509 obligation
.recursion_depth
+ 1,
2510 ty
::Binder(outlives
).to_predicate()));
2514 (_
, &ty
::TyTrait(ref data
)) => {
2515 let mut object_dids
= Some(data
.principal_def_id()).into_iter();
2517 // data.bounds.builtin_bounds.iter().flat_map(|bound| {
2518 // tcx.lang_items.from_builtin_kind(bound).ok()
2520 // .chain(Some(data.principal_def_id()));
2521 if let Some(did
) = object_dids
.find(|did
| {
2522 !tcx
.is_object_safe(*did
)
2524 return Err(TraitNotObjectSafe(did
))
2527 let cause
= ObligationCause
::new(obligation
.cause
.span
,
2528 obligation
.cause
.body_id
,
2529 ObjectCastObligation(target
));
2530 let mut push
= |predicate
| {
2531 nested
.push(Obligation
::with_depth(cause
.clone(),
2532 obligation
.recursion_depth
+ 1,
2536 // Create the obligation for casting from T to Trait.
2537 push(data
.principal_trait_ref_with_self_ty(tcx
, source
).to_predicate());
2539 // We can only make objects from sized types.
2540 let mut builtin_bounds
= data
.bounds
.builtin_bounds
;
2541 builtin_bounds
.insert(ty
::BoundSized
);
2543 // Create additional obligations for all the various builtin
2544 // bounds attached to the object cast. (In other words, if the
2545 // object type is Foo+Send, this would create an obligation
2546 // for the Send check.)
2547 for bound
in &builtin_bounds
{
2548 if let Ok(tr
) = tcx
.trait_ref_for_builtin_bound(bound
, source
) {
2549 push(tr
.to_predicate());
2551 return Err(Unimplemented
);
2555 // Create obligations for the projection predicates.
2556 for bound
in data
.projection_bounds_with_self_ty(tcx
, source
) {
2557 push(bound
.to_predicate());
2560 // If the type is `Foo+'a`, ensures that the type
2561 // being cast to `Foo+'a` outlives `'a`:
2562 let outlives
= ty
::OutlivesPredicate(source
,
2563 data
.bounds
.region_bound
);
2564 push(ty
::Binder(outlives
).to_predicate());
2568 (&ty
::TyArray(a
, _
), &ty
::TySlice(b
)) => {
2569 let origin
= TypeOrigin
::Misc(obligation
.cause
.span
);
2570 let InferOk { obligations, .. }
=
2571 self.infcx
.sub_types(false, origin
, a
, b
)
2572 .map_err(|_
| Unimplemented
)?
;
2573 self.inferred_obligations
.extend(obligations
);
2576 // Struct<T> -> Struct<U>.
2577 (&ty
::TyStruct(def
, substs_a
), &ty
::TyStruct(_
, substs_b
)) => {
2580 .map(|f
| f
.unsubst_ty())
2581 .collect
::<Vec
<_
>>();
2583 // The last field of the structure has to exist and contain type parameters.
2584 let field
= if let Some(&field
) = fields
.last() {
2587 return Err(Unimplemented
);
2589 let mut ty_params
= vec
![];
2590 for ty
in field
.walk() {
2591 if let ty
::TyParam(p
) = ty
.sty
{
2592 assert
!(p
.space
== TypeSpace
);
2593 let idx
= p
.idx
as usize;
2594 if !ty_params
.contains(&idx
) {
2595 ty_params
.push(idx
);
2599 if ty_params
.is_empty() {
2600 return Err(Unimplemented
);
2603 // Replace type parameters used in unsizing with
2604 // TyError and ensure they do not affect any other fields.
2605 // This could be checked after type collection for any struct
2606 // with a potentially unsized trailing field.
2607 let mut new_substs
= substs_a
.clone();
2608 for &i
in &ty_params
{
2609 new_substs
.types
.get_mut_slice(TypeSpace
)[i
] = tcx
.types
.err
;
2611 for &ty
in fields
.split_last().unwrap().1 {
2612 if ty
.subst(tcx
, &new_substs
).references_error() {
2613 return Err(Unimplemented
);
2617 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2618 let inner_source
= field
.subst(tcx
, substs_a
);
2619 let inner_target
= field
.subst(tcx
, substs_b
);
2621 // Check that the source structure with the target's
2622 // type parameters is a subtype of the target.
2623 for &i
in &ty_params
{
2624 let param_b
= *substs_b
.types
.get(TypeSpace
, i
);
2625 new_substs
.types
.get_mut_slice(TypeSpace
)[i
] = param_b
;
2627 let new_struct
= tcx
.mk_struct(def
, tcx
.mk_substs(new_substs
));
2628 let origin
= TypeOrigin
::Misc(obligation
.cause
.span
);
2629 let InferOk { obligations, .. }
=
2630 self.infcx
.sub_types(false, origin
, new_struct
, target
)
2631 .map_err(|_
| Unimplemented
)?
;
2632 self.inferred_obligations
.extend(obligations
);
2634 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2635 nested
.push(tcx
.predicate_for_trait_def(
2636 obligation
.cause
.clone(),
2637 obligation
.predicate
.def_id(),
2638 obligation
.recursion_depth
+ 1,
2640 vec
![inner_target
]));
2646 Ok(VtableBuiltinData { nested: nested }
)
2649 ///////////////////////////////////////////////////////////////////////////
2652 // Matching is a common path used for both evaluation and
2653 // confirmation. It basically unifies types that appear in impls
2654 // and traits. This does affect the surrounding environment;
2655 // therefore, when used during evaluation, match routines must be
2656 // run inside of a `probe()` so that their side-effects are
2659 fn rematch_impl(&mut self,
2661 obligation
: &TraitObligation
<'tcx
>,
2662 snapshot
: &infer
::CombinedSnapshot
)
2663 -> (Normalized
<'tcx
, &'tcx Substs
<'tcx
>>, infer
::SkolemizationMap
)
2665 match self.match_impl(impl_def_id
, obligation
, snapshot
) {
2666 Ok((substs
, skol_map
)) => (substs
, skol_map
),
2668 bug
!("Impl {:?} was matchable against {:?} but now is not",
2675 fn match_impl(&mut self,
2677 obligation
: &TraitObligation
<'tcx
>,
2678 snapshot
: &infer
::CombinedSnapshot
)
2679 -> Result
<(Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
2680 infer
::SkolemizationMap
), ()>
2682 let impl_trait_ref
= self.tcx().impl_trait_ref(impl_def_id
).unwrap();
2684 // Before we create the substitutions and everything, first
2685 // consider a "quick reject". This avoids creating more types
2686 // and so forth that we need to.
2687 if self.fast_reject_trait_refs(obligation
, &impl_trait_ref
) {
2691 let (skol_obligation
, skol_map
) = self.infcx().skolemize_late_bound_regions(
2692 &obligation
.predicate
,
2694 let skol_obligation_trait_ref
= skol_obligation
.trait_ref
;
2696 let impl_substs
= util
::fresh_type_vars_for_impl(self.infcx
,
2697 obligation
.cause
.span
,
2700 let impl_trait_ref
= impl_trait_ref
.subst(self.tcx(),
2703 let impl_trait_ref
=
2704 project
::normalize_with_depth(self,
2705 obligation
.cause
.clone(),
2706 obligation
.recursion_depth
+ 1,
2709 debug
!("match_impl(impl_def_id={:?}, obligation={:?}, \
2710 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
2714 skol_obligation_trait_ref
);
2716 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation
.cause
.span
);
2717 let InferOk { obligations, .. }
=
2718 self.infcx
.eq_trait_refs(false,
2720 impl_trait_ref
.value
.clone(),
2721 skol_obligation_trait_ref
)
2723 debug
!("match_impl: failed eq_trait_refs due to `{}`", e
);
2726 self.inferred_obligations
.extend(obligations
);
2728 if let Err(e
) = self.infcx
.leak_check(false,
2729 obligation
.cause
.span
,
2732 debug
!("match_impl: failed leak check due to `{}`", e
);
2736 debug
!("match_impl: success impl_substs={:?}", impl_substs
);
2739 obligations
: impl_trait_ref
.obligations
2743 fn fast_reject_trait_refs(&mut self,
2744 obligation
: &TraitObligation
,
2745 impl_trait_ref
: &ty
::TraitRef
)
2748 // We can avoid creating type variables and doing the full
2749 // substitution if we find that any of the input types, when
2750 // simplified, do not match.
2752 obligation
.predicate
.0.input_types
().iter()
2753 .zip(impl_trait_ref
.input_types())
2754 .any(|(&obligation_ty
, &impl_ty
)| {
2755 let simplified_obligation_ty
=
2756 fast_reject
::simplify_type(self.tcx(), obligation_ty
, true);
2757 let simplified_impl_ty
=
2758 fast_reject
::simplify_type(self.tcx(), impl_ty
, false);
2760 simplified_obligation_ty
.is_some() &&
2761 simplified_impl_ty
.is_some() &&
2762 simplified_obligation_ty
!= simplified_impl_ty
2766 /// Normalize `where_clause_trait_ref` and try to match it against
2767 /// `obligation`. If successful, return any predicates that
2768 /// result from the normalization. Normalization is necessary
2769 /// because where-clauses are stored in the parameter environment
2771 fn match_where_clause_trait_ref(&mut self,
2772 obligation
: &TraitObligation
<'tcx
>,
2773 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
2774 -> Result
<Vec
<PredicateObligation
<'tcx
>>,()>
2776 self.match_poly_trait_ref(obligation
, where_clause_trait_ref
)?
;
2780 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2781 /// obligation is satisfied.
2782 fn match_poly_trait_ref(&mut self,
2783 obligation
: &TraitObligation
<'tcx
>,
2784 poly_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
2787 debug
!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
2791 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation
.cause
.span
);
2792 self.infcx
.sub_poly_trait_refs(false,
2795 obligation
.predicate
.to_poly_trait_ref())
2796 .map(|InferOk { obligations, .. }
| self.inferred_obligations
.extend(obligations
))
2800 ///////////////////////////////////////////////////////////////////////////
2803 fn match_fresh_trait_refs(&self,
2804 previous
: &ty
::PolyTraitRef
<'tcx
>,
2805 current
: &ty
::PolyTraitRef
<'tcx
>)
2808 let mut matcher
= ty
::_match
::Match
::new(self.tcx());
2809 matcher
.relate(previous
, current
).is_ok()
2812 fn push_stack
<'o
,'s
:'o
>(&mut self,
2813 previous_stack
: TraitObligationStackList
<'s
, 'tcx
>,
2814 obligation
: &'o TraitObligation
<'tcx
>)
2815 -> TraitObligationStack
<'o
, 'tcx
>
2817 let fresh_trait_ref
=
2818 obligation
.predicate
.to_poly_trait_ref().fold_with(&mut self.freshener
);
2820 TraitObligationStack
{
2821 obligation
: obligation
,
2822 fresh_trait_ref
: fresh_trait_ref
,
2823 previous
: previous_stack
,
2827 fn closure_trait_ref_unnormalized(&mut self,
2828 obligation
: &TraitObligation
<'tcx
>,
2829 closure_def_id
: DefId
,
2830 substs
: ty
::ClosureSubsts
<'tcx
>)
2831 -> ty
::PolyTraitRef
<'tcx
>
2833 let closure_type
= self.infcx
.closure_type(closure_def_id
, substs
);
2834 let ty
::Binder((trait_ref
, _
)) =
2835 self.tcx().closure_trait_ref_and_return_type(obligation
.predicate
.def_id(),
2836 obligation
.predicate
.0.self_ty(), // (1)
2838 util
::TupleArgumentsFlag
::No
);
2839 // (1) Feels icky to skip the binder here, but OTOH we know
2840 // that the self-type is an unboxed closure type and hence is
2841 // in fact unparameterized (or at least does not reference any
2842 // regions bound in the obligation). Still probably some
2843 // refactoring could make this nicer.
2845 ty
::Binder(trait_ref
)
2848 fn closure_trait_ref(&mut self,
2849 obligation
: &TraitObligation
<'tcx
>,
2850 closure_def_id
: DefId
,
2851 substs
: ty
::ClosureSubsts
<'tcx
>)
2852 -> Normalized
<'tcx
, ty
::PolyTraitRef
<'tcx
>>
2854 let trait_ref
= self.closure_trait_ref_unnormalized(
2855 obligation
, closure_def_id
, substs
);
2857 // A closure signature can contain associated types which
2858 // must be normalized.
2859 normalize_with_depth(self,
2860 obligation
.cause
.clone(),
2861 obligation
.recursion_depth
+1,
2865 /// Returns the obligations that are implied by instantiating an
2866 /// impl or trait. The obligations are substituted and fully
2867 /// normalized. This is used when confirming an impl or default
2869 fn impl_or_trait_obligations(&mut self,
2870 cause
: ObligationCause
<'tcx
>,
2871 recursion_depth
: usize,
2872 def_id
: DefId
, // of impl or trait
2873 substs
: &Substs
<'tcx
>, // for impl or trait
2874 skol_map
: infer
::SkolemizationMap
,
2875 snapshot
: &infer
::CombinedSnapshot
)
2876 -> Vec
<PredicateObligation
<'tcx
>>
2878 debug
!("impl_or_trait_obligations(def_id={:?})", def_id
);
2879 let tcx
= self.tcx();
2881 // To allow for one-pass evaluation of the nested obligation,
2882 // each predicate must be preceded by the obligations required
2884 // for example, if we have:
2885 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
2886 // the impl will have the following predicates:
2887 // <V as Iterator>::Item = U,
2888 // U: Iterator, U: Sized,
2889 // V: Iterator, V: Sized,
2890 // <U as Iterator>::Item: Copy
2891 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2892 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2893 // `$1: Copy`, so we must ensure the obligations are emitted in
2895 let predicates
= tcx
2896 .lookup_predicates(def_id
)
2898 .flat_map(|predicate
| {
2900 normalize_with_depth(self, cause
.clone(), recursion_depth
,
2901 &predicate
.subst(tcx
, substs
));
2902 predicate
.obligations
.into_iter().chain(
2904 cause
: cause
.clone(),
2905 recursion_depth
: recursion_depth
,
2906 predicate
: predicate
.value
2909 self.infcx().plug_leaks(skol_map
, snapshot
, &predicates
)
2913 impl<'tcx
> TraitObligation
<'tcx
> {
2914 #[allow(unused_comparisons)]
2915 pub fn derived_cause(&self,
2916 variant
: fn(DerivedObligationCause
<'tcx
>) -> ObligationCauseCode
<'tcx
>)
2917 -> ObligationCause
<'tcx
>
2920 * Creates a cause for obligations that are derived from
2921 * `obligation` by a recursive search (e.g., for a builtin
2922 * bound, or eventually a `impl Foo for ..`). If `obligation`
2923 * is itself a derived obligation, this is just a clone, but
2924 * otherwise we create a "derived obligation" cause so as to
2925 * keep track of the original root obligation for error
2929 let obligation
= self;
2931 // NOTE(flaper87): As of now, it keeps track of the whole error
2932 // chain. Ideally, we should have a way to configure this either
2933 // by using -Z verbose or just a CLI argument.
2934 if obligation
.recursion_depth
>= 0 {
2935 let derived_cause
= DerivedObligationCause
{
2936 parent_trait_ref
: obligation
.predicate
.to_poly_trait_ref(),
2937 parent_code
: Rc
::new(obligation
.cause
.code
.clone())
2939 let derived_code
= variant(derived_cause
);
2940 ObligationCause
::new(obligation
.cause
.span
, obligation
.cause
.body_id
, derived_code
)
2942 obligation
.cause
.clone()
2947 impl<'tcx
> SelectionCache
<'tcx
> {
2948 pub fn new() -> SelectionCache
<'tcx
> {
2950 hashmap
: RefCell
::new(FnvHashMap())
2955 impl<'tcx
> EvaluationCache
<'tcx
> {
2956 pub fn new() -> EvaluationCache
<'tcx
> {
2958 hashmap
: RefCell
::new(FnvHashMap())
2963 impl<'o
,'tcx
> TraitObligationStack
<'o
,'tcx
> {
2964 fn list(&'o
self) -> TraitObligationStackList
<'o
,'tcx
> {
2965 TraitObligationStackList
::with(self)
2968 fn iter(&'o
self) -> TraitObligationStackList
<'o
,'tcx
> {
2973 #[derive(Copy, Clone)]
2974 struct TraitObligationStackList
<'o
,'tcx
:'o
> {
2975 head
: Option
<&'o TraitObligationStack
<'o
,'tcx
>>
2978 impl<'o
,'tcx
> TraitObligationStackList
<'o
,'tcx
> {
2979 fn empty() -> TraitObligationStackList
<'o
,'tcx
> {
2980 TraitObligationStackList { head: None }
2983 fn with(r
: &'o TraitObligationStack
<'o
,'tcx
>) -> TraitObligationStackList
<'o
,'tcx
> {
2984 TraitObligationStackList { head: Some(r) }
2988 impl<'o
,'tcx
> Iterator
for TraitObligationStackList
<'o
,'tcx
>{
2989 type Item
= &'o TraitObligationStack
<'o
,'tcx
>;
2991 fn next(&mut self) -> Option
<&'o TraitObligationStack
<'o
,'tcx
>> {
3002 impl<'o
,'tcx
> fmt
::Debug
for TraitObligationStack
<'o
,'tcx
> {
3003 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
3004 write
!(f
, "TraitObligationStack({:?})", self.obligation
)
3008 impl EvaluationResult
{
3009 fn may_apply(&self) -> bool
{
3013 EvaluatedToUnknown
=> true,
3015 EvaluatedToErr
=> false
3020 impl MethodMatchResult
{
3021 pub fn may_apply(&self) -> bool
{
3023 MethodMatched(_
) => true,
3024 MethodAmbiguous(_
) => true,
3025 MethodDidNotMatch
=> false,