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
::{Kind, Subst, Substs}
;
40 use ty
::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable}
;
43 use ty
::relate
::TypeRelation
;
45 use rustc_data_structures
::bitvec
::BitVector
;
46 use rustc_data_structures
::snapshot_vec
::{SnapshotVecDelegate, SnapshotVec}
;
47 use std
::cell
::RefCell
;
49 use std
::marker
::PhantomData
;
54 use util
::nodemap
::FnvHashMap
;
56 struct InferredObligationsSnapshotVecDelegate
<'tcx
> {
57 phantom
: PhantomData
<&'tcx
i32>,
59 impl<'tcx
> SnapshotVecDelegate
for InferredObligationsSnapshotVecDelegate
<'tcx
> {
60 type Value
= PredicateObligation
<'tcx
>;
62 fn reverse(_
: &mut Vec
<Self::Value
>, _
: Self::Undo
) {}
65 pub struct SelectionContext
<'cx
, 'gcx
: 'cx
+'tcx
, 'tcx
: 'cx
> {
66 infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>,
68 /// Freshener used specifically for skolemizing entries on the
69 /// obligation stack. This ensures that all entries on the stack
70 /// at one time will have the same set of skolemized entries,
71 /// which is important for checking for trait bounds that
72 /// recursively require themselves.
73 freshener
: TypeFreshener
<'cx
, 'gcx
, 'tcx
>,
75 /// If true, indicates that the evaluation should be conservative
76 /// and consider the possibility of types outside this crate.
77 /// This comes up primarily when resolving ambiguity. Imagine
78 /// there is some trait reference `$0 : Bar` where `$0` is an
79 /// inference variable. If `intercrate` is true, then we can never
80 /// say for sure that this reference is not implemented, even if
81 /// there are *no impls at all for `Bar`*, because `$0` could be
82 /// bound to some type that in a downstream crate that implements
83 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
84 /// though, we set this to false, because we are only interested
85 /// in types that the user could actually have written --- in
86 /// other words, we consider `$0 : Bar` to be unimplemented if
87 /// there is no type that the user could *actually name* that
88 /// would satisfy it. This avoids crippling inference, basically.
91 inferred_obligations
: SnapshotVec
<InferredObligationsSnapshotVecDelegate
<'tcx
>>,
94 // A stack that walks back up the stack frame.
95 struct TraitObligationStack
<'prev
, 'tcx
: 'prev
> {
96 obligation
: &'prev TraitObligation
<'tcx
>,
98 /// Trait ref from `obligation` but skolemized with the
99 /// selection-context's freshener. Used to check for recursion.
100 fresh_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
102 previous
: TraitObligationStackList
<'prev
, 'tcx
>,
106 pub struct SelectionCache
<'tcx
> {
107 hashmap
: RefCell
<FnvHashMap
<ty
::TraitRef
<'tcx
>,
108 SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>>>,
111 pub enum MethodMatchResult
{
112 MethodMatched(MethodMatchedData
),
113 MethodAmbiguous(/* list of impls that could apply */ Vec
<DefId
>),
117 #[derive(Copy, Clone, Debug)]
118 pub enum MethodMatchedData
{
119 // In the case of a precise match, we don't really need to store
120 // how the match was found. So don't.
123 // In the case of a coercion, we need to know the precise impl so
124 // that we can determine the type to which things were coerced.
125 CoerciveMethodMatch(/* impl we matched */ DefId
)
128 /// The selection process begins by considering all impls, where
129 /// clauses, and so forth that might resolve an obligation. Sometimes
130 /// we'll be able to say definitively that (e.g.) an impl does not
131 /// apply to the obligation: perhaps it is defined for `usize` but the
132 /// obligation is for `int`. In that case, we drop the impl out of the
133 /// list. But the other cases are considered *candidates*.
135 /// For selection to succeed, there must be exactly one matching
136 /// candidate. If the obligation is fully known, this is guaranteed
137 /// by coherence. However, if the obligation contains type parameters
138 /// or variables, there may be multiple such impls.
140 /// It is not a real problem if multiple matching impls exist because
141 /// of type variables - it just means the obligation isn't sufficiently
142 /// elaborated. In that case we report an ambiguity, and the caller can
143 /// try again after more type information has been gathered or report a
144 /// "type annotations required" error.
146 /// However, with type parameters, this can be a real problem - type
147 /// parameters don't unify with regular types, but they *can* unify
148 /// with variables from blanket impls, and (unless we know its bounds
149 /// will always be satisfied) picking the blanket impl will be wrong
150 /// for at least *some* substitutions. To make this concrete, if we have
152 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
153 /// impl<T: fmt::Debug> AsDebug for T {
155 /// fn debug(self) -> fmt::Debug { self }
157 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
159 /// we can't just use the impl to resolve the <T as AsDebug> obligation
160 /// - a type from another crate (that doesn't implement fmt::Debug) could
161 /// implement AsDebug.
163 /// Because where-clauses match the type exactly, multiple clauses can
164 /// only match if there are unresolved variables, and we can mostly just
165 /// report this ambiguity in that case. This is still a problem - we can't
166 /// *do anything* with ambiguities that involve only regions. This is issue
169 /// If a single where-clause matches and there are no inference
170 /// variables left, then it definitely matches and we can just select
173 /// In fact, we even select the where-clause when the obligation contains
174 /// inference variables. The can lead to inference making "leaps of logic",
175 /// for example in this situation:
177 /// pub trait Foo<T> { fn foo(&self) -> T; }
178 /// impl<T> Foo<()> for T { fn foo(&self) { } }
179 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
181 /// pub fn foo<T>(t: T) where T: Foo<bool> {
182 /// println!("{:?}", <T as Foo<_>>::foo(&t));
184 /// fn main() { foo(false); }
186 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
187 /// impl and the where-clause. We select the where-clause and unify $0=bool,
188 /// so the program prints "false". However, if the where-clause is omitted,
189 /// the blanket impl is selected, we unify $0=(), and the program prints
192 /// Exactly the same issues apply to projection and object candidates, except
193 /// that we can have both a projection candidate and a where-clause candidate
194 /// for the same obligation. In that case either would do (except that
195 /// different "leaps of logic" would occur if inference variables are
196 /// present), and we just pick the where-clause. This is, for example,
197 /// required for associated types to work in default impls, as the bounds
198 /// are visible both as projection bounds and as where-clauses from the
199 /// parameter environment.
200 #[derive(PartialEq,Eq,Debug,Clone)]
201 enum SelectionCandidate
<'tcx
> {
202 BuiltinCandidate { has_nested: bool }
,
203 ParamCandidate(ty
::PolyTraitRef
<'tcx
>),
204 ImplCandidate(DefId
),
205 DefaultImplCandidate(DefId
),
206 DefaultImplObjectCandidate(DefId
),
208 /// This is a trait matching with a projected type as `Self`, and
209 /// we found an applicable bound in the trait definition.
212 /// Implementation of a `Fn`-family trait by one of the anonymous types
213 /// generated for a `||` expression. The ty::ClosureKind informs the
214 /// confirmation step what ClosureKind obligation to emit.
215 ClosureCandidate(/* closure */ DefId
, ty
::ClosureSubsts
<'tcx
>, ty
::ClosureKind
),
217 /// Implementation of a `Fn`-family trait by one of the anonymous
218 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
223 BuiltinObjectCandidate
,
225 BuiltinUnsizeCandidate
,
228 impl<'a
, 'tcx
> ty
::Lift
<'tcx
> for SelectionCandidate
<'a
> {
229 type Lifted
= SelectionCandidate
<'tcx
>;
230 fn lift_to_tcx
<'b
, 'gcx
>(&self, tcx
: TyCtxt
<'b
, 'gcx
, 'tcx
>) -> Option
<Self::Lifted
> {
232 BuiltinCandidate { has_nested }
=> {
234 has_nested
: has_nested
237 ImplCandidate(def_id
) => ImplCandidate(def_id
),
238 DefaultImplCandidate(def_id
) => DefaultImplCandidate(def_id
),
239 DefaultImplObjectCandidate(def_id
) => {
240 DefaultImplObjectCandidate(def_id
)
242 ProjectionCandidate
=> ProjectionCandidate
,
243 FnPointerCandidate
=> FnPointerCandidate
,
244 ObjectCandidate
=> ObjectCandidate
,
245 BuiltinObjectCandidate
=> BuiltinObjectCandidate
,
246 BuiltinUnsizeCandidate
=> BuiltinUnsizeCandidate
,
248 ParamCandidate(ref trait_ref
) => {
249 return tcx
.lift(trait_ref
).map(ParamCandidate
);
251 ClosureCandidate(def_id
, ref substs
, kind
) => {
252 return tcx
.lift(substs
).map(|substs
| {
253 ClosureCandidate(def_id
, substs
, kind
)
260 struct SelectionCandidateSet
<'tcx
> {
261 // a list of candidates that definitely apply to the current
262 // obligation (meaning: types unify).
263 vec
: Vec
<SelectionCandidate
<'tcx
>>,
265 // if this is true, then there were candidates that might or might
266 // not have applied, but we couldn't tell. This occurs when some
267 // of the input types are type variables, in which case there are
268 // various "builtin" rules that might or might not trigger.
272 #[derive(PartialEq,Eq,Debug,Clone)]
273 struct EvaluatedCandidate
<'tcx
> {
274 candidate
: SelectionCandidate
<'tcx
>,
275 evaluation
: EvaluationResult
,
278 /// When does the builtin impl for `T: Trait` apply?
279 enum BuiltinImplConditions
<'tcx
> {
280 /// The impl is conditional on T1,T2,.. : Trait
281 Where(ty
::Binder
<Vec
<Ty
<'tcx
>>>),
282 /// There is no built-in impl. There may be some other
283 /// candidate (a where-clause or user-defined impl).
285 /// There is *no* impl for this, builtin or not. Ignore
286 /// all where-clauses.
288 /// It is unknown whether there is an impl.
292 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
293 /// The result of trait evaluation. The order is important
294 /// here as the evaluation of a list is the maximum of the
296 enum EvaluationResult
{
297 /// Evaluation successful
299 /// Evaluation failed because of recursion - treated as ambiguous
301 /// Evaluation is known to be ambiguous
303 /// Evaluation failed
308 pub struct EvaluationCache
<'tcx
> {
309 hashmap
: RefCell
<FnvHashMap
<ty
::PolyTraitRef
<'tcx
>, EvaluationResult
>>
312 impl<'cx
, 'gcx
, 'tcx
> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
313 pub fn new(infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>) -> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
316 freshener
: infcx
.freshener(),
318 inferred_obligations
: SnapshotVec
::new(),
322 pub fn intercrate(infcx
: &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
>) -> SelectionContext
<'cx
, 'gcx
, 'tcx
> {
325 freshener
: infcx
.freshener(),
327 inferred_obligations
: SnapshotVec
::new(),
331 pub fn infcx(&self) -> &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
> {
335 pub fn tcx(&self) -> TyCtxt
<'cx
, 'gcx
, 'tcx
> {
339 pub fn param_env(&self) -> &'cx ty
::ParameterEnvironment
<'gcx
> {
340 self.infcx
.param_env()
343 pub fn closure_typer(&self) -> &'cx InferCtxt
<'cx
, 'gcx
, 'tcx
> {
347 pub fn projection_mode(&self) -> Reveal
{
348 self.infcx
.projection_mode()
351 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
353 fn in_snapshot
<R
, F
>(&mut self, f
: F
) -> R
354 where F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
) -> R
356 // The irrefutable nature of the operation means we don't need to snapshot the
357 // inferred_obligations vector.
358 self.infcx
.in_snapshot(|snapshot
| f(self, snapshot
))
361 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
363 fn probe
<R
, F
>(&mut self, f
: F
) -> R
364 where F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
) -> R
366 let inferred_obligations_snapshot
= self.inferred_obligations
.start_snapshot();
367 let result
= self.infcx
.probe(|snapshot
| f(self, snapshot
));
368 self.inferred_obligations
.rollback_to(inferred_obligations_snapshot
);
372 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
373 /// the transaction fails and s.t. old obligations are retained.
374 fn commit_if_ok
<T
, E
, F
>(&mut self, f
: F
) -> Result
<T
, E
> where
375 F
: FnOnce(&mut Self, &infer
::CombinedSnapshot
) -> Result
<T
, E
>
377 let inferred_obligations_snapshot
= self.inferred_obligations
.start_snapshot();
378 match self.infcx
.commit_if_ok(|snapshot
| f(self, snapshot
)) {
380 self.inferred_obligations
.commit(inferred_obligations_snapshot
);
384 self.inferred_obligations
.rollback_to(inferred_obligations_snapshot
);
391 ///////////////////////////////////////////////////////////////////////////
394 // The selection phase tries to identify *how* an obligation will
395 // be resolved. For example, it will identify which impl or
396 // parameter bound is to be used. The process can be inconclusive
397 // if the self type in the obligation is not fully inferred. Selection
398 // can result in an error in one of two ways:
400 // 1. If no applicable impl or parameter bound can be found.
401 // 2. If the output type parameters in the obligation do not match
402 // those specified by the impl/bound. For example, if the obligation
403 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
404 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
406 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
407 /// type environment by performing unification.
408 pub fn select(&mut self, obligation
: &TraitObligation
<'tcx
>)
409 -> SelectionResult
<'tcx
, Selection
<'tcx
>> {
410 debug
!("select({:?})", obligation
);
411 assert
!(!obligation
.predicate
.has_escaping_regions());
413 let dep_node
= obligation
.predicate
.dep_node();
414 let _task
= self.tcx().dep_graph
.in_task(dep_node
);
416 let stack
= self.push_stack(TraitObligationStackList
::empty(), obligation
);
417 match self.candidate_from_obligation(&stack
)?
{
420 let mut candidate
= self.confirm_candidate(obligation
, candidate
)?
;
421 // FIXME(#32730) remove this assertion once inferred obligations are propagated
423 assert
!(self.inferred_obligations
.len() == 0);
424 let inferred_obligations
= (*self.inferred_obligations
).into_iter().cloned();
425 candidate
.nested_obligations_mut().extend(inferred_obligations
);
431 ///////////////////////////////////////////////////////////////////////////
434 // Tests whether an obligation can be selected or whether an impl
435 // can be applied to particular types. It skips the "confirmation"
436 // step and hence completely ignores output type parameters.
438 // The result is "true" if the obligation *may* hold and "false" if
439 // we can be sure it does not.
441 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
442 pub fn evaluate_obligation(&mut self,
443 obligation
: &PredicateObligation
<'tcx
>)
446 debug
!("evaluate_obligation({:?})",
449 self.probe(|this
, _
| {
450 this
.evaluate_predicate_recursively(TraitObligationStackList
::empty(), obligation
)
455 /// Evaluates whether the obligation `obligation` can be satisfied,
456 /// and returns `false` if not certain. However, this is not entirely
457 /// accurate if inference variables are involved.
458 pub fn evaluate_obligation_conservatively(&mut self,
459 obligation
: &PredicateObligation
<'tcx
>)
462 debug
!("evaluate_obligation_conservatively({:?})",
465 self.probe(|this
, _
| {
466 this
.evaluate_predicate_recursively(TraitObligationStackList
::empty(), obligation
)
471 /// Evaluates the predicates in `predicates` recursively. Note that
472 /// this applies projections in the predicates, and therefore
473 /// is run within an inference probe.
474 fn evaluate_predicates_recursively
<'a
,'o
,I
>(&mut self,
475 stack
: TraitObligationStackList
<'o
, 'tcx
>,
478 where I
: Iterator
<Item
=&'a PredicateObligation
<'tcx
>>, 'tcx
:'a
480 let mut result
= EvaluatedToOk
;
481 for obligation
in predicates
{
482 let eval
= self.evaluate_predicate_recursively(stack
, obligation
);
483 debug
!("evaluate_predicate_recursively({:?}) = {:?}",
486 EvaluatedToErr
=> { return EvaluatedToErr; }
487 EvaluatedToAmbig
=> { result = EvaluatedToAmbig; }
488 EvaluatedToUnknown
=> {
489 if result
< EvaluatedToUnknown
{
490 result
= EvaluatedToUnknown
;
499 fn evaluate_predicate_recursively
<'o
>(&mut self,
500 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
501 obligation
: &PredicateObligation
<'tcx
>)
504 debug
!("evaluate_predicate_recursively({:?})",
507 // Check the cache from the tcx of predicates that we know
508 // have been proven elsewhere. This cache only contains
509 // predicates that are global in scope and hence unaffected by
510 // the current environment.
511 if self.tcx().fulfilled_predicates
.borrow().check_duplicate(&obligation
.predicate
) {
512 return EvaluatedToOk
;
515 match obligation
.predicate
{
516 ty
::Predicate
::Trait(ref t
) => {
517 assert
!(!t
.has_escaping_regions());
518 let obligation
= obligation
.with(t
.clone());
519 self.evaluate_obligation_recursively(previous_stack
, &obligation
)
522 ty
::Predicate
::Equate(ref p
) => {
523 // does this code ever run?
524 match self.infcx
.equality_predicate(obligation
.cause
.span
, p
) {
525 Ok(InferOk { obligations, .. }
) => {
526 self.inferred_obligations
.extend(obligations
);
529 Err(_
) => EvaluatedToErr
533 ty
::Predicate
::WellFormed(ty
) => {
534 match ty
::wf
::obligations(self.infcx
, obligation
.cause
.body_id
,
535 ty
, obligation
.cause
.span
) {
537 self.evaluate_predicates_recursively(previous_stack
, obligations
.iter()),
543 ty
::Predicate
::TypeOutlives(..) | ty
::Predicate
::RegionOutlives(..) => {
544 // we do not consider region relationships when
545 // evaluating trait matches
549 ty
::Predicate
::ObjectSafe(trait_def_id
) => {
550 if self.tcx().is_object_safe(trait_def_id
) {
557 ty
::Predicate
::Projection(ref data
) => {
558 let project_obligation
= obligation
.with(data
.clone());
559 match project
::poly_project_and_unify_type(self, &project_obligation
) {
560 Ok(Some(subobligations
)) => {
561 self.evaluate_predicates_recursively(previous_stack
,
562 subobligations
.iter())
573 ty
::Predicate
::ClosureKind(closure_def_id
, kind
) => {
574 match self.infcx
.closure_kind(closure_def_id
) {
575 Some(closure_kind
) => {
576 if closure_kind
.extends(kind
) {
590 fn evaluate_obligation_recursively
<'o
>(&mut self,
591 previous_stack
: TraitObligationStackList
<'o
, 'tcx
>,
592 obligation
: &TraitObligation
<'tcx
>)
595 debug
!("evaluate_obligation_recursively({:?})",
598 let stack
= self.push_stack(previous_stack
, obligation
);
599 let fresh_trait_ref
= stack
.fresh_trait_ref
;
600 if let Some(result
) = self.check_evaluation_cache(fresh_trait_ref
) {
601 debug
!("CACHE HIT: EVAL({:?})={:?}",
607 let result
= self.evaluate_stack(&stack
);
609 debug
!("CACHE MISS: EVAL({:?})={:?}",
612 self.insert_evaluation_cache(fresh_trait_ref
, result
);
617 fn evaluate_stack
<'o
>(&mut self,
618 stack
: &TraitObligationStack
<'o
, 'tcx
>)
621 // In intercrate mode, whenever any of the types are unbound,
622 // there can always be an impl. Even if there are no impls in
623 // this crate, perhaps the type would be unified with
624 // something from another crate that does provide an impl.
626 // In intra mode, we must still be conservative. The reason is
627 // that we want to avoid cycles. Imagine an impl like:
629 // impl<T:Eq> Eq for Vec<T>
631 // and a trait reference like `$0 : Eq` where `$0` is an
632 // unbound variable. When we evaluate this trait-reference, we
633 // will unify `$0` with `Vec<$1>` (for some fresh variable
634 // `$1`), on the condition that `$1 : Eq`. We will then wind
635 // up with many candidates (since that are other `Eq` impls
636 // that apply) and try to winnow things down. This results in
637 // a recursive evaluation that `$1 : Eq` -- as you can
638 // imagine, this is just where we started. To avoid that, we
639 // check for unbound variables and return an ambiguous (hence possible)
640 // match if we've seen this trait before.
642 // This suffices to allow chains like `FnMut` implemented in
643 // terms of `Fn` etc, but we could probably make this more
645 let unbound_input_types
= stack
.fresh_trait_ref
.input_types().any(|ty
| ty
.is_fresh());
646 if unbound_input_types
&& self.intercrate
{
647 debug
!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
648 stack
.fresh_trait_ref
);
649 return EvaluatedToAmbig
;
651 if unbound_input_types
&&
652 stack
.iter().skip(1).any(
653 |prev
| self.match_fresh_trait_refs(&stack
.fresh_trait_ref
,
654 &prev
.fresh_trait_ref
))
656 debug
!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
657 stack
.fresh_trait_ref
);
658 return EvaluatedToUnknown
;
661 // If there is any previous entry on the stack that precisely
662 // matches this obligation, then we can assume that the
663 // obligation is satisfied for now (still all other conditions
664 // must be met of course). One obvious case this comes up is
665 // marker traits like `Send`. Think of a linked list:
667 // struct List<T> { data: T, next: Option<Box<List<T>>> {
669 // `Box<List<T>>` will be `Send` if `T` is `Send` and
670 // `Option<Box<List<T>>>` is `Send`, and in turn
671 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
674 // Note that we do this comparison using the `fresh_trait_ref`
675 // fields. Because these have all been skolemized using
676 // `self.freshener`, we can be sure that (a) this will not
677 // affect the inferencer state and (b) that if we see two
678 // skolemized types with the same index, they refer to the
679 // same unbound type variable.
682 .skip(1) // skip top-most frame
683 .any(|prev
| stack
.fresh_trait_ref
== prev
.fresh_trait_ref
)
685 debug
!("evaluate_stack({:?}) --> recursive",
686 stack
.fresh_trait_ref
);
687 return EvaluatedToOk
;
690 match self.candidate_from_obligation(stack
) {
691 Ok(Some(c
)) => self.evaluate_candidate(stack
, &c
),
692 Ok(None
) => EvaluatedToAmbig
,
693 Err(..) => EvaluatedToErr
697 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
698 /// obligations are met. Returns true if `candidate` remains viable after this further
700 fn evaluate_candidate
<'o
>(&mut self,
701 stack
: &TraitObligationStack
<'o
, 'tcx
>,
702 candidate
: &SelectionCandidate
<'tcx
>)
705 debug
!("evaluate_candidate: depth={} candidate={:?}",
706 stack
.obligation
.recursion_depth
, candidate
);
707 let result
= self.probe(|this
, _
| {
708 let candidate
= (*candidate
).clone();
709 match this
.confirm_candidate(stack
.obligation
, candidate
) {
711 this
.evaluate_predicates_recursively(
713 selection
.nested_obligations().iter())
715 Err(..) => EvaluatedToErr
718 debug
!("evaluate_candidate: depth={} result={:?}",
719 stack
.obligation
.recursion_depth
, result
);
723 fn check_evaluation_cache(&self, trait_ref
: ty
::PolyTraitRef
<'tcx
>)
724 -> Option
<EvaluationResult
>
726 if self.can_use_global_caches() {
727 let cache
= self.tcx().evaluation_cache
.hashmap
.borrow();
728 if let Some(cached
) = cache
.get(&trait_ref
) {
729 return Some(cached
.clone());
732 self.infcx
.evaluation_cache
.hashmap
.borrow().get(&trait_ref
).cloned()
735 fn insert_evaluation_cache(&mut self,
736 trait_ref
: ty
::PolyTraitRef
<'tcx
>,
737 result
: EvaluationResult
)
739 // Avoid caching results that depend on more than just the trait-ref:
740 // The stack can create EvaluatedToUnknown, and closure signatures
741 // being yet uninferred can create "spurious" EvaluatedToAmbig
742 // and EvaluatedToOk.
743 if result
== EvaluatedToUnknown
||
744 ((result
== EvaluatedToAmbig
|| result
== EvaluatedToOk
)
745 && trait_ref
.has_closure_types())
750 if self.can_use_global_caches() {
751 let mut cache
= self.tcx().evaluation_cache
.hashmap
.borrow_mut();
752 if let Some(trait_ref
) = self.tcx().lift_to_global(&trait_ref
) {
753 cache
.insert(trait_ref
, result
);
758 self.infcx
.evaluation_cache
.hashmap
.borrow_mut().insert(trait_ref
, result
);
761 ///////////////////////////////////////////////////////////////////////////
762 // CANDIDATE ASSEMBLY
764 // The selection process begins by examining all in-scope impls,
765 // caller obligations, and so forth and assembling a list of
766 // candidates. See `README.md` and the `Candidate` type for more
769 fn candidate_from_obligation
<'o
>(&mut self,
770 stack
: &TraitObligationStack
<'o
, 'tcx
>)
771 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>
773 // Watch out for overflow. This intentionally bypasses (and does
774 // not update) the cache.
775 let recursion_limit
= self.infcx
.tcx
.sess
.recursion_limit
.get();
776 if stack
.obligation
.recursion_depth
>= recursion_limit
{
777 self.infcx().report_overflow_error(&stack
.obligation
, true);
780 // Check the cache. Note that we skolemize the trait-ref
781 // separately rather than using `stack.fresh_trait_ref` -- this
782 // is because we want the unbound variables to be replaced
783 // with fresh skolemized types starting from index 0.
784 let cache_fresh_trait_pred
=
785 self.infcx
.freshen(stack
.obligation
.predicate
.clone());
786 debug
!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
787 cache_fresh_trait_pred
,
789 assert
!(!stack
.obligation
.predicate
.has_escaping_regions());
791 match self.check_candidate_cache(&cache_fresh_trait_pred
) {
793 debug
!("CACHE HIT: SELECT({:?})={:?}",
794 cache_fresh_trait_pred
,
801 // If no match, compute result and insert into cache.
802 let candidate
= self.candidate_from_obligation_no_cache(stack
);
804 if self.should_update_candidate_cache(&cache_fresh_trait_pred
, &candidate
) {
805 debug
!("CACHE MISS: SELECT({:?})={:?}",
806 cache_fresh_trait_pred
, candidate
);
807 self.insert_candidate_cache(cache_fresh_trait_pred
, candidate
.clone());
813 // Treat negative impls as unimplemented
814 fn filter_negative_impls(&self, candidate
: SelectionCandidate
<'tcx
>)
815 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>> {
816 if let ImplCandidate(def_id
) = candidate
{
817 if self.tcx().trait_impl_polarity(def_id
) == hir
::ImplPolarity
::Negative
{
818 return Err(Unimplemented
)
824 fn candidate_from_obligation_no_cache
<'o
>(&mut self,
825 stack
: &TraitObligationStack
<'o
, 'tcx
>)
826 -> SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>
828 if stack
.obligation
.predicate
.references_error() {
829 // If we encounter a `TyError`, we generally prefer the
830 // most "optimistic" result in response -- that is, the
831 // one least likely to report downstream errors. But
832 // because this routine is shared by coherence and by
833 // trait selection, there isn't an obvious "right" choice
834 // here in that respect, so we opt to just return
835 // ambiguity and let the upstream clients sort it out.
839 if !self.is_knowable(stack
) {
840 debug
!("coherence stage: not knowable");
844 let candidate_set
= self.assemble_candidates(stack
)?
;
846 if candidate_set
.ambiguous
{
847 debug
!("candidate set contains ambig");
851 let mut candidates
= candidate_set
.vec
;
853 debug
!("assembled {} candidates for {:?}: {:?}",
858 // At this point, we know that each of the entries in the
859 // candidate set is *individually* applicable. Now we have to
860 // figure out if they contain mutual incompatibilities. This
861 // frequently arises if we have an unconstrained input type --
862 // for example, we are looking for $0:Eq where $0 is some
863 // unconstrained type variable. In that case, we'll get a
864 // candidate which assumes $0 == int, one that assumes $0 ==
865 // usize, etc. This spells an ambiguity.
867 // If there is more than one candidate, first winnow them down
868 // by considering extra conditions (nested obligations and so
869 // forth). We don't winnow if there is exactly one
870 // candidate. This is a relatively minor distinction but it
871 // can lead to better inference and error-reporting. An
872 // example would be if there was an impl:
874 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
876 // and we were to see some code `foo.push_clone()` where `boo`
877 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
878 // we were to winnow, we'd wind up with zero candidates.
879 // Instead, we select the right impl now but report `Bar does
880 // not implement Clone`.
881 if candidates
.len() == 1 {
882 return self.filter_negative_impls(candidates
.pop().unwrap());
885 // Winnow, but record the exact outcome of evaluation, which
886 // is needed for specialization.
887 let mut candidates
: Vec
<_
> = candidates
.into_iter().filter_map(|c
| {
888 let eval
= self.evaluate_candidate(stack
, &c
);
889 if eval
.may_apply() {
890 Some(EvaluatedCandidate
{
899 // If there are STILL multiple candidate, we can further
900 // reduce the list by dropping duplicates -- including
901 // resolving specializations.
902 if candidates
.len() > 1 {
904 while i
< candidates
.len() {
906 (0..candidates
.len())
908 .any(|j
| self.candidate_should_be_dropped_in_favor_of(&candidates
[i
],
911 debug
!("Dropping candidate #{}/{}: {:?}",
912 i
, candidates
.len(), candidates
[i
]);
913 candidates
.swap_remove(i
);
915 debug
!("Retaining candidate #{}/{}: {:?}",
916 i
, candidates
.len(), candidates
[i
]);
922 // If there are *STILL* multiple candidates, give up and
924 if candidates
.len() > 1 {
925 debug
!("multiple matches, ambig");
929 // If there are *NO* candidates, then there are no impls --
930 // that we know of, anyway. Note that in the case where there
931 // are unbound type variables within the obligation, it might
932 // be the case that you could still satisfy the obligation
933 // from another crate by instantiating the type variables with
934 // a type from another crate that does have an impl. This case
935 // is checked for in `evaluate_stack` (and hence users
936 // who might care about this case, like coherence, should use
938 if candidates
.is_empty() {
939 return Err(Unimplemented
);
942 // Just one candidate left.
943 self.filter_negative_impls(candidates
.pop().unwrap().candidate
)
946 fn is_knowable
<'o
>(&mut self,
947 stack
: &TraitObligationStack
<'o
, 'tcx
>)
950 debug
!("is_knowable(intercrate={})", self.intercrate
);
952 if !self.intercrate
{
956 let obligation
= &stack
.obligation
;
957 let predicate
= self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
);
959 // ok to skip binder because of the nature of the
960 // trait-ref-is-knowable check, which does not care about
962 let trait_ref
= &predicate
.skip_binder().trait_ref
;
964 coherence
::trait_ref_is_knowable(self.tcx(), trait_ref
)
967 /// Returns true if the global caches can be used.
968 /// Do note that if the type itself is not in the
969 /// global tcx, the local caches will be used.
970 fn can_use_global_caches(&self) -> bool
{
971 // If there are any where-clauses in scope, then we always use
972 // a cache local to this particular scope. Otherwise, we
973 // switch to a global cache. We used to try and draw
974 // finer-grained distinctions, but that led to a serious of
975 // annoying and weird bugs like #22019 and #18290. This simple
976 // rule seems to be pretty clearly safe and also still retains
977 // a very high hit rate (~95% when compiling rustc).
978 if !self.param_env().caller_bounds
.is_empty() {
982 // Avoid using the master cache during coherence and just rely
983 // on the local cache. This effectively disables caching
984 // during coherence. It is really just a simplification to
985 // avoid us having to fear that coherence results "pollute"
986 // the master cache. Since coherence executes pretty quickly,
987 // it's not worth going to more trouble to increase the
988 // hit-rate I don't think.
993 // Otherwise, we can use the global cache.
997 fn check_candidate_cache(&mut self,
998 cache_fresh_trait_pred
: &ty
::PolyTraitPredicate
<'tcx
>)
999 -> Option
<SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>>
1001 let trait_ref
= &cache_fresh_trait_pred
.0.trait_ref
;
1002 if self.can_use_global_caches() {
1003 let cache
= self.tcx().selection_cache
.hashmap
.borrow();
1004 if let Some(cached
) = cache
.get(&trait_ref
) {
1005 return Some(cached
.clone());
1008 self.infcx
.selection_cache
.hashmap
.borrow().get(trait_ref
).cloned()
1011 fn insert_candidate_cache(&mut self,
1012 cache_fresh_trait_pred
: ty
::PolyTraitPredicate
<'tcx
>,
1013 candidate
: SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>)
1015 let trait_ref
= cache_fresh_trait_pred
.0.trait_ref
;
1016 if self.can_use_global_caches() {
1017 let mut cache
= self.tcx().selection_cache
.hashmap
.borrow_mut();
1018 if let Some(trait_ref
) = self.tcx().lift_to_global(&trait_ref
) {
1019 if let Some(candidate
) = self.tcx().lift_to_global(&candidate
) {
1020 cache
.insert(trait_ref
, candidate
);
1026 self.infcx
.selection_cache
.hashmap
.borrow_mut().insert(trait_ref
, candidate
);
1029 fn should_update_candidate_cache(&mut self,
1030 cache_fresh_trait_pred
: &ty
::PolyTraitPredicate
<'tcx
>,
1031 candidate
: &SelectionResult
<'tcx
, SelectionCandidate
<'tcx
>>)
1034 // In general, it's a good idea to cache results, even
1035 // ambiguous ones, to save us some trouble later. But we have
1036 // to be careful not to cache results that could be
1037 // invalidated later by advances in inference. Normally, this
1038 // is not an issue, because any inference variables whose
1039 // types are not yet bound are "freshened" in the cache key,
1040 // which means that if we later get the same request once that
1041 // type variable IS bound, we'll have a different cache key.
1042 // For example, if we have `Vec<_#0t> : Foo`, and `_#0t` is
1043 // not yet known, we may cache the result as `None`. But if
1044 // later `_#0t` is bound to `Bar`, then when we freshen we'll
1045 // have `Vec<Bar> : Foo` as the cache key.
1047 // HOWEVER, it CAN happen that we get an ambiguity result in
1048 // one particular case around closures where the cache key
1049 // would not change. That is when the precise types of the
1050 // upvars that a closure references have not yet been figured
1051 // out (i.e., because it is not yet known if they are captured
1052 // by ref, and if by ref, what kind of ref). In these cases,
1053 // when matching a builtin bound, we will yield back an
1054 // ambiguous result. But the *cache key* is just the closure type,
1055 // it doesn't capture the state of the upvar computation.
1057 // To avoid this trap, just don't cache ambiguous results if
1058 // the self-type contains no inference byproducts (that really
1059 // shouldn't happen in other circumstances anyway, given
1063 Ok(Some(_
)) | Err(_
) => true,
1064 Ok(None
) => cache_fresh_trait_pred
.has_infer_types()
1068 fn assemble_candidates
<'o
>(&mut self,
1069 stack
: &TraitObligationStack
<'o
, 'tcx
>)
1070 -> Result
<SelectionCandidateSet
<'tcx
>, SelectionError
<'tcx
>>
1072 let TraitObligationStack { obligation, .. }
= *stack
;
1073 let ref obligation
= Obligation
{
1074 cause
: obligation
.cause
.clone(),
1075 recursion_depth
: obligation
.recursion_depth
,
1076 predicate
: self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
)
1079 if obligation
.predicate
.skip_binder().self_ty().is_ty_var() {
1080 // FIXME(#20297): Self is a type variable (e.g. `_: AsRef<str>`).
1082 // This is somewhat problematic, as the current scheme can't really
1083 // handle it turning to be a projection. This does end up as truly
1084 // ambiguous in most cases anyway.
1086 // Until this is fixed, take the fast path out - this also improves
1087 // performance by preventing assemble_candidates_from_impls from
1088 // matching every impl for this trait.
1089 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true }
);
1092 let mut candidates
= SelectionCandidateSet
{
1097 // Other bounds. Consider both in-scope bounds from fn decl
1098 // and applicable impls. There is a certain set of precedence rules here.
1100 match self.tcx().lang_items
.to_builtin_kind(obligation
.predicate
.def_id()) {
1101 Some(ty
::BoundCopy
) => {
1102 debug
!("obligation self ty is {:?}",
1103 obligation
.predicate
.0.self_ty());
1105 // User-defined copy impls are permitted, but only for
1106 // structs and enums.
1107 self.assemble_candidates_from_impls(obligation
, &mut candidates
)?
;
1109 // For other types, we'll use the builtin rules.
1110 let copy_conditions
= self.copy_conditions(obligation
);
1111 self.assemble_builtin_bound_candidates(copy_conditions
, &mut candidates
)?
;
1113 Some(ty
::BoundSized
) => {
1114 // Sized is never implementable by end-users, it is
1115 // always automatically computed.
1116 let sized_conditions
= self.sized_conditions(obligation
);
1117 self.assemble_builtin_bound_candidates(sized_conditions
,
1121 None
if self.tcx().lang_items
.unsize_trait() ==
1122 Some(obligation
.predicate
.def_id()) => {
1123 self.assemble_candidates_for_unsizing(obligation
, &mut candidates
);
1126 Some(ty
::BoundSend
) |
1127 Some(ty
::BoundSync
) |
1129 self.assemble_closure_candidates(obligation
, &mut candidates
)?
;
1130 self.assemble_fn_pointer_candidates(obligation
, &mut candidates
)?
;
1131 self.assemble_candidates_from_impls(obligation
, &mut candidates
)?
;
1132 self.assemble_candidates_from_object_ty(obligation
, &mut candidates
);
1136 self.assemble_candidates_from_projected_tys(obligation
, &mut candidates
);
1137 self.assemble_candidates_from_caller_bounds(stack
, &mut candidates
)?
;
1138 // Default implementations have lower priority, so we only
1139 // consider triggering a default if there is no other impl that can apply.
1140 if candidates
.vec
.is_empty() {
1141 self.assemble_candidates_from_default_impls(obligation
, &mut candidates
)?
;
1143 debug
!("candidate list size: {}", candidates
.vec
.len());
1147 fn assemble_candidates_from_projected_tys(&mut self,
1148 obligation
: &TraitObligation
<'tcx
>,
1149 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1151 debug
!("assemble_candidates_for_projected_tys({:?})", obligation
);
1153 // FIXME(#20297) -- just examining the self-type is very simplistic
1155 // before we go into the whole skolemization thing, just
1156 // quickly check if the self-type is a projection at all.
1157 match obligation
.predicate
.0.trait_ref
.self_ty().sty
{
1158 ty
::TyProjection(_
) | ty
::TyAnon(..) => {}
1159 ty
::TyInfer(ty
::TyVar(_
)) => {
1160 span_bug
!(obligation
.cause
.span
,
1161 "Self=_ should have been handled by assemble_candidates");
1166 let result
= self.probe(|this
, snapshot
| {
1167 this
.match_projection_obligation_against_definition_bounds(obligation
,
1172 candidates
.vec
.push(ProjectionCandidate
);
1176 fn match_projection_obligation_against_definition_bounds(
1178 obligation
: &TraitObligation
<'tcx
>,
1179 snapshot
: &infer
::CombinedSnapshot
)
1182 let poly_trait_predicate
=
1183 self.infcx().resolve_type_vars_if_possible(&obligation
.predicate
);
1184 let (skol_trait_predicate
, skol_map
) =
1185 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate
, snapshot
);
1186 debug
!("match_projection_obligation_against_definition_bounds: \
1187 skol_trait_predicate={:?} skol_map={:?}",
1188 skol_trait_predicate
,
1191 let (def_id
, substs
) = match skol_trait_predicate
.trait_ref
.self_ty().sty
{
1192 ty
::TyProjection(ref data
) => (data
.trait_ref
.def_id
, data
.trait_ref
.substs
),
1193 ty
::TyAnon(def_id
, substs
) => (def_id
, substs
),
1196 obligation
.cause
.span
,
1197 "match_projection_obligation_against_definition_bounds() called \
1198 but self-ty not a projection: {:?}",
1199 skol_trait_predicate
.trait_ref
.self_ty());
1202 debug
!("match_projection_obligation_against_definition_bounds: \
1203 def_id={:?}, substs={:?}",
1206 let item_predicates
= self.tcx().lookup_predicates(def_id
);
1207 let bounds
= item_predicates
.instantiate(self.tcx(), substs
);
1208 debug
!("match_projection_obligation_against_definition_bounds: \
1212 let matching_bound
=
1213 util
::elaborate_predicates(self.tcx(), bounds
.predicates
)
1217 |this
, _
| this
.match_projection(obligation
,
1219 skol_trait_predicate
.trait_ref
.clone(),
1223 debug
!("match_projection_obligation_against_definition_bounds: \
1224 matching_bound={:?}",
1226 match matching_bound
{
1229 // Repeat the successful match, if any, this time outside of a probe.
1230 let result
= self.match_projection(obligation
,
1232 skol_trait_predicate
.trait_ref
.clone(),
1236 self.infcx
.pop_skolemized(skol_map
, snapshot
);
1244 fn match_projection(&mut self,
1245 obligation
: &TraitObligation
<'tcx
>,
1246 trait_bound
: ty
::PolyTraitRef
<'tcx
>,
1247 skol_trait_ref
: ty
::TraitRef
<'tcx
>,
1248 skol_map
: &infer
::SkolemizationMap
<'tcx
>,
1249 snapshot
: &infer
::CombinedSnapshot
)
1252 assert
!(!skol_trait_ref
.has_escaping_regions());
1253 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation
.cause
.span
);
1254 match self.infcx
.sub_poly_trait_refs(false,
1256 trait_bound
.clone(),
1257 ty
::Binder(skol_trait_ref
.clone())) {
1258 Ok(InferOk { obligations, .. }
) => {
1259 self.inferred_obligations
.extend(obligations
);
1261 Err(_
) => { return false; }
1264 self.infcx
.leak_check(false, obligation
.cause
.span
, skol_map
, snapshot
).is_ok()
1267 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1268 /// supplied to find out whether it is listed among them.
1270 /// Never affects inference environment.
1271 fn assemble_candidates_from_caller_bounds
<'o
>(&mut self,
1272 stack
: &TraitObligationStack
<'o
, 'tcx
>,
1273 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1274 -> Result
<(),SelectionError
<'tcx
>>
1276 debug
!("assemble_candidates_from_caller_bounds({:?})",
1280 self.param_env().caller_bounds
1282 .filter_map(|o
| o
.to_opt_poly_trait_ref());
1284 let matching_bounds
=
1286 |bound
| self.evaluate_where_clause(stack
, bound
.clone()).may_apply());
1288 let param_candidates
=
1289 matching_bounds
.map(|bound
| ParamCandidate(bound
));
1291 candidates
.vec
.extend(param_candidates
);
1296 fn evaluate_where_clause
<'o
>(&mut self,
1297 stack
: &TraitObligationStack
<'o
, 'tcx
>,
1298 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
1301 self.probe(move |this
, _
| {
1302 match this
.match_where_clause_trait_ref(stack
.obligation
, where_clause_trait_ref
) {
1303 Ok(obligations
) => {
1304 this
.evaluate_predicates_recursively(stack
.list(), obligations
.iter())
1306 Err(()) => EvaluatedToErr
1311 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1312 /// FnMut<..>` where `X` is a closure type.
1314 /// Note: the type parameters on a closure candidate are modeled as *output* type
1315 /// parameters and hence do not affect whether this trait is a match or not. They will be
1316 /// unified during the confirmation step.
1317 fn assemble_closure_candidates(&mut self,
1318 obligation
: &TraitObligation
<'tcx
>,
1319 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1320 -> Result
<(),SelectionError
<'tcx
>>
1322 let kind
= match self.tcx().lang_items
.fn_trait_kind(obligation
.predicate
.0.def_id()) {
1324 None
=> { return Ok(()); }
1327 // ok to skip binder because the substs on closure types never
1328 // touch bound regions, they just capture the in-scope
1329 // type/region parameters
1330 let self_ty
= *obligation
.self_ty().skip_binder();
1331 let (closure_def_id
, substs
) = match self_ty
.sty
{
1332 ty
::TyClosure(id
, substs
) => (id
, substs
),
1333 ty
::TyInfer(ty
::TyVar(_
)) => {
1334 debug
!("assemble_unboxed_closure_candidates: ambiguous self-type");
1335 candidates
.ambiguous
= true;
1338 _
=> { return Ok(()); }
1341 debug
!("assemble_unboxed_candidates: self_ty={:?} kind={:?} obligation={:?}",
1346 match self.infcx
.closure_kind(closure_def_id
) {
1347 Some(closure_kind
) => {
1348 debug
!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind
);
1349 if closure_kind
.extends(kind
) {
1350 candidates
.vec
.push(ClosureCandidate(closure_def_id
, substs
, kind
));
1354 debug
!("assemble_unboxed_candidates: closure_kind not yet known");
1355 candidates
.vec
.push(ClosureCandidate(closure_def_id
, substs
, kind
));
1362 /// Implement one of the `Fn()` family for a fn pointer.
1363 fn assemble_fn_pointer_candidates(&mut self,
1364 obligation
: &TraitObligation
<'tcx
>,
1365 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1366 -> Result
<(),SelectionError
<'tcx
>>
1368 // We provide impl of all fn traits for fn pointers.
1369 if self.tcx().lang_items
.fn_trait_kind(obligation
.predicate
.def_id()).is_none() {
1373 // ok to skip binder because what we are inspecting doesn't involve bound regions
1374 let self_ty
= *obligation
.self_ty().skip_binder();
1376 ty
::TyInfer(ty
::TyVar(_
)) => {
1377 debug
!("assemble_fn_pointer_candidates: ambiguous self-type");
1378 candidates
.ambiguous
= true; // could wind up being a fn() type
1381 // provide an impl, but only for suitable `fn` pointers
1382 ty
::TyFnDef(.., &ty
::BareFnTy
{
1383 unsafety
: hir
::Unsafety
::Normal
,
1385 sig
: ty
::Binder(ty
::FnSig
{
1391 ty
::TyFnPtr(&ty
::BareFnTy
{
1392 unsafety
: hir
::Unsafety
::Normal
,
1394 sig
: ty
::Binder(ty
::FnSig
{
1400 candidates
.vec
.push(FnPointerCandidate
);
1409 /// Search for impls that might apply to `obligation`.
1410 fn assemble_candidates_from_impls(&mut self,
1411 obligation
: &TraitObligation
<'tcx
>,
1412 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1413 -> Result
<(), SelectionError
<'tcx
>>
1415 debug
!("assemble_candidates_from_impls(obligation={:?})", obligation
);
1417 let def
= self.tcx().lookup_trait_def(obligation
.predicate
.def_id());
1419 def
.for_each_relevant_impl(
1421 obligation
.predicate
.0.trait_ref
.self_ty(),
1423 self.probe(|this
, snapshot
| { /* [1] */
1424 match this
.match_impl(impl_def_id
, obligation
, snapshot
) {
1426 candidates
.vec
.push(ImplCandidate(impl_def_id
));
1428 // NB: we can safely drop the skol map
1429 // since we are in a probe [1]
1430 mem
::drop(skol_map
);
1441 fn assemble_candidates_from_default_impls(&mut self,
1442 obligation
: &TraitObligation
<'tcx
>,
1443 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1444 -> Result
<(), SelectionError
<'tcx
>>
1446 // OK to skip binder here because the tests we do below do not involve bound regions
1447 let self_ty
= *obligation
.self_ty().skip_binder();
1448 debug
!("assemble_candidates_from_default_impls(self_ty={:?})", self_ty
);
1450 let def_id
= obligation
.predicate
.def_id();
1452 if self.tcx().trait_has_default_impl(def_id
) {
1454 ty
::TyTrait(..) => {
1455 // For object types, we don't know what the closed
1456 // over types are. For most traits, this means we
1457 // conservatively say nothing; a candidate may be
1458 // added by `assemble_candidates_from_object_ty`.
1459 // However, for the kind of magic reflect trait,
1460 // we consider it to be implemented even for
1461 // object types, because it just lets you reflect
1462 // onto the object type, not into the object's
1464 if self.tcx().has_attr(def_id
, "rustc_reflect_like") {
1465 candidates
.vec
.push(DefaultImplObjectCandidate(def_id
));
1469 ty
::TyProjection(..) |
1471 // In these cases, we don't know what the actual
1472 // type is. Therefore, we cannot break it down
1473 // into its constituent types. So we don't
1474 // consider the `..` impl but instead just add no
1475 // candidates: this means that typeck will only
1476 // succeed if there is another reason to believe
1477 // that this obligation holds. That could be a
1478 // where-clause or, in the case of an object type,
1479 // it could be that the object type lists the
1480 // trait (e.g. `Foo+Send : Send`). See
1481 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1482 // for an example of a test case that exercises
1485 ty
::TyInfer(ty
::TyVar(_
)) => {
1486 // the defaulted impl might apply, we don't know
1487 candidates
.ambiguous
= true;
1490 candidates
.vec
.push(DefaultImplCandidate(def_id
.clone()))
1498 /// Search for impls that might apply to `obligation`.
1499 fn assemble_candidates_from_object_ty(&mut self,
1500 obligation
: &TraitObligation
<'tcx
>,
1501 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1503 debug
!("assemble_candidates_from_object_ty(self_ty={:?})",
1504 obligation
.self_ty().skip_binder());
1506 // Object-safety candidates are only applicable to object-safe
1507 // traits. Including this check is useful because it helps
1508 // inference in cases of traits like `BorrowFrom`, which are
1509 // not object-safe, and which rely on being able to infer the
1510 // self-type from one of the other inputs. Without this check,
1511 // these cases wind up being considered ambiguous due to a
1512 // (spurious) ambiguity introduced here.
1513 let predicate_trait_ref
= obligation
.predicate
.to_poly_trait_ref();
1514 if !self.tcx().is_object_safe(predicate_trait_ref
.def_id()) {
1518 self.probe(|this
, _snapshot
| {
1519 // the code below doesn't care about regions, and the
1520 // self-ty here doesn't escape this probe, so just erase
1522 let self_ty
= this
.tcx().erase_late_bound_regions(&obligation
.self_ty());
1523 let poly_trait_ref
= match self_ty
.sty
{
1524 ty
::TyTrait(ref data
) => {
1525 match this
.tcx().lang_items
.to_builtin_kind(obligation
.predicate
.def_id()) {
1526 Some(bound @ ty
::BoundSend
) | Some(bound @ ty
::BoundSync
) => {
1527 if data
.builtin_bounds
.contains(&bound
) {
1528 debug
!("assemble_candidates_from_object_ty: matched builtin bound, \
1529 pushing candidate");
1530 candidates
.vec
.push(BuiltinObjectCandidate
);
1537 data
.principal
.with_self_ty(this
.tcx(), self_ty
)
1539 ty
::TyInfer(ty
::TyVar(_
)) => {
1540 debug
!("assemble_candidates_from_object_ty: ambiguous");
1541 candidates
.ambiguous
= true; // could wind up being an object type
1549 debug
!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1552 // Count only those upcast versions that match the trait-ref
1553 // we are looking for. Specifically, do not only check for the
1554 // correct trait, but also the correct type parameters.
1555 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1556 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1557 let upcast_trait_refs
=
1558 util
::supertraits(this
.tcx(), poly_trait_ref
)
1559 .filter(|upcast_trait_ref
| {
1560 this
.probe(|this
, _
| {
1561 let upcast_trait_ref
= upcast_trait_ref
.clone();
1562 this
.match_poly_trait_ref(obligation
, upcast_trait_ref
).is_ok()
1567 if upcast_trait_refs
> 1 {
1568 // can be upcast in many ways; need more type information
1569 candidates
.ambiguous
= true;
1570 } else if upcast_trait_refs
== 1 {
1571 candidates
.vec
.push(ObjectCandidate
);
1576 /// Search for unsizing that might apply to `obligation`.
1577 fn assemble_candidates_for_unsizing(&mut self,
1578 obligation
: &TraitObligation
<'tcx
>,
1579 candidates
: &mut SelectionCandidateSet
<'tcx
>) {
1580 // We currently never consider higher-ranked obligations e.g.
1581 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1582 // because they are a priori invalid, and we could potentially add support
1583 // for them later, it's just that there isn't really a strong need for it.
1584 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1585 // impl, and those are generally applied to concrete types.
1587 // That said, one might try to write a fn with a where clause like
1588 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1589 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1590 // Still, you'd be more likely to write that where clause as
1592 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1593 // obligation above. Should be possible to extend this in the future.
1594 let source
= match self.tcx().no_late_bound_regions(&obligation
.self_ty()) {
1597 // Don't add any candidates if there are bound regions.
1601 let target
= obligation
.predicate
.skip_binder().trait_ref
.substs
.type_at(1);
1603 debug
!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1606 let may_apply
= match (&source
.sty
, &target
.sty
) {
1607 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1608 (&ty
::TyTrait(ref data_a
), &ty
::TyTrait(ref data_b
)) => {
1609 // Upcasts permit two things:
1611 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1612 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1614 // Note that neither of these changes requires any
1615 // change at runtime. Eventually this will be
1618 // We always upcast when we can because of reason
1619 // #2 (region bounds).
1620 data_a
.principal
.def_id() == data_a
.principal
.def_id() &&
1621 data_a
.builtin_bounds
.is_superset(&data_b
.builtin_bounds
)
1625 (_
, &ty
::TyTrait(_
)) => true,
1627 // Ambiguous handling is below T -> Trait, because inference
1628 // variables can still implement Unsize<Trait> and nested
1629 // obligations will have the final say (likely deferred).
1630 (&ty
::TyInfer(ty
::TyVar(_
)), _
) |
1631 (_
, &ty
::TyInfer(ty
::TyVar(_
))) => {
1632 debug
!("assemble_candidates_for_unsizing: ambiguous");
1633 candidates
.ambiguous
= true;
1638 (&ty
::TyArray(..), &ty
::TySlice(_
)) => true,
1640 // Struct<T> -> Struct<U>.
1641 (&ty
::TyAdt(def_id_a
, _
), &ty
::TyAdt(def_id_b
, _
)) if def_id_a
.is_struct() => {
1642 def_id_a
== def_id_b
1649 candidates
.vec
.push(BuiltinUnsizeCandidate
);
1653 ///////////////////////////////////////////////////////////////////////////
1656 // Winnowing is the process of attempting to resolve ambiguity by
1657 // probing further. During the winnowing process, we unify all
1658 // type variables (ignoring skolemization) and then we also
1659 // attempt to evaluate recursive bounds to see if they are
1662 /// Returns true if `candidate_i` should be dropped in favor of
1663 /// `candidate_j`. Generally speaking we will drop duplicate
1664 /// candidates and prefer where-clause candidates.
1665 /// Returns true if `victim` should be dropped in favor of
1666 /// `other`. Generally speaking we will drop duplicate
1667 /// candidates and prefer where-clause candidates.
1669 /// See the comment for "SelectionCandidate" for more details.
1670 fn candidate_should_be_dropped_in_favor_of
<'o
>(
1672 victim
: &EvaluatedCandidate
<'tcx
>,
1673 other
: &EvaluatedCandidate
<'tcx
>)
1676 if victim
.candidate
== other
.candidate
{
1680 match other
.candidate
{
1682 ParamCandidate(_
) | ProjectionCandidate
=> match victim
.candidate
{
1683 DefaultImplCandidate(..) => {
1685 "default implementations shouldn't be recorded \
1686 when there are other valid candidates");
1689 ClosureCandidate(..) |
1690 FnPointerCandidate
|
1691 BuiltinObjectCandidate
|
1692 BuiltinUnsizeCandidate
|
1693 DefaultImplObjectCandidate(..) |
1694 BuiltinCandidate { .. }
=> {
1695 // We have a where-clause so don't go around looking
1700 ProjectionCandidate
=> {
1701 // Arbitrarily give param candidates priority
1702 // over projection and object candidates.
1705 ParamCandidate(..) => false,
1707 ImplCandidate(other_def
) => {
1708 // See if we can toss out `victim` based on specialization.
1709 // This requires us to know *for sure* that the `other` impl applies
1710 // i.e. EvaluatedToOk:
1711 if other
.evaluation
== EvaluatedToOk
{
1712 if let ImplCandidate(victim_def
) = victim
.candidate
{
1713 let tcx
= self.tcx().global_tcx();
1714 return traits
::specializes(tcx
, other_def
, victim_def
);
1724 ///////////////////////////////////////////////////////////////////////////
1727 // These cover the traits that are built-in to the language
1728 // itself. This includes `Copy` and `Sized` for sure. For the
1729 // moment, it also includes `Send` / `Sync` and a few others, but
1730 // those will hopefully change to library-defined traits in the
1733 // HACK: if this returns an error, selection exits without considering
1735 fn assemble_builtin_bound_candidates
<'o
>(&mut self,
1736 conditions
: BuiltinImplConditions
<'tcx
>,
1737 candidates
: &mut SelectionCandidateSet
<'tcx
>)
1738 -> Result
<(),SelectionError
<'tcx
>>
1741 BuiltinImplConditions
::Where(nested
) => {
1742 debug
!("builtin_bound: nested={:?}", nested
);
1743 candidates
.vec
.push(BuiltinCandidate
{
1744 has_nested
: nested
.skip_binder().len() > 0
1748 BuiltinImplConditions
::None
=> { Ok(()) }
1749 BuiltinImplConditions
::Ambiguous
=> {
1750 debug
!("assemble_builtin_bound_candidates: ambiguous builtin");
1751 Ok(candidates
.ambiguous
= true)
1753 BuiltinImplConditions
::Never
=> { Err(Unimplemented) }
1757 fn sized_conditions(&mut self, obligation
: &TraitObligation
<'tcx
>)
1758 -> BuiltinImplConditions
<'tcx
>
1760 use self::BuiltinImplConditions
::{Ambiguous, None, Never, Where}
;
1762 // NOTE: binder moved to (*)
1763 let self_ty
= self.infcx
.shallow_resolve(
1764 obligation
.predicate
.skip_binder().self_ty());
1767 ty
::TyInfer(ty
::IntVar(_
)) | ty
::TyInfer(ty
::FloatVar(_
)) |
1768 ty
::TyUint(_
) | ty
::TyInt(_
) | ty
::TyBool
| ty
::TyFloat(_
) |
1769 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) | ty
::TyRawPtr(..) |
1770 ty
::TyChar
| ty
::TyBox(_
) | ty
::TyRef(..) |
1771 ty
::TyArray(..) | ty
::TyClosure(..) | ty
::TyNever
|
1773 // safe for everything
1774 Where(ty
::Binder(Vec
::new()))
1777 ty
::TyStr
| ty
::TySlice(_
) | ty
::TyTrait(..) => Never
,
1779 ty
::TyTuple(tys
) => {
1780 Where(ty
::Binder(tys
.last().into_iter().cloned().collect()))
1783 ty
::TyAdt(def
, substs
) => {
1784 let sized_crit
= def
.sized_constraint(self.tcx());
1785 // (*) binder moved here
1786 Where(ty
::Binder(match sized_crit
.sty
{
1787 ty
::TyTuple(tys
) => tys
.to_vec().subst(self.tcx(), substs
),
1788 ty
::TyBool
=> vec
![],
1789 _
=> vec
![sized_crit
.subst(self.tcx(), substs
)]
1793 ty
::TyProjection(_
) | ty
::TyParam(_
) | ty
::TyAnon(..) => None
,
1794 ty
::TyInfer(ty
::TyVar(_
)) => Ambiguous
,
1796 ty
::TyInfer(ty
::FreshTy(_
))
1797 | ty
::TyInfer(ty
::FreshIntTy(_
))
1798 | ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
1799 bug
!("asked to assemble builtin bounds of unexpected type: {:?}",
1805 fn copy_conditions(&mut self, obligation
: &TraitObligation
<'tcx
>)
1806 -> BuiltinImplConditions
<'tcx
>
1808 // NOTE: binder moved to (*)
1809 let self_ty
= self.infcx
.shallow_resolve(
1810 obligation
.predicate
.skip_binder().self_ty());
1812 use self::BuiltinImplConditions
::{Ambiguous, None, Never, Where}
;
1815 ty
::TyInfer(ty
::IntVar(_
)) | ty
::TyInfer(ty
::FloatVar(_
)) |
1816 ty
::TyUint(_
) | ty
::TyInt(_
) | ty
::TyBool
| ty
::TyFloat(_
) |
1817 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) | ty
::TyChar
|
1818 ty
::TyRawPtr(..) | ty
::TyError
| ty
::TyNever
|
1819 ty
::TyRef(_
, ty
::TypeAndMut { ty: _, mutbl: hir::MutImmutable }
) => {
1820 Where(ty
::Binder(Vec
::new()))
1823 ty
::TyBox(_
) | ty
::TyTrait(..) | ty
::TyStr
| ty
::TySlice(..) |
1825 ty
::TyRef(_
, ty
::TypeAndMut { ty: _, mutbl: hir::MutMutable }
) => {
1829 ty
::TyArray(element_ty
, _
) => {
1830 // (*) binder moved here
1831 Where(ty
::Binder(vec
![element_ty
]))
1834 ty
::TyTuple(tys
) => {
1835 // (*) binder moved here
1836 Where(ty
::Binder(tys
.to_vec()))
1839 ty
::TyAdt(..) | ty
::TyProjection(..) | ty
::TyParam(..) | ty
::TyAnon(..) => {
1840 // Fallback to whatever user-defined impls exist in this case.
1844 ty
::TyInfer(ty
::TyVar(_
)) => {
1845 // Unbound type variable. Might or might not have
1846 // applicable impls and so forth, depending on what
1847 // those type variables wind up being bound to.
1851 ty
::TyInfer(ty
::FreshTy(_
))
1852 | ty
::TyInfer(ty
::FreshIntTy(_
))
1853 | ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
1854 bug
!("asked to assemble builtin bounds of unexpected type: {:?}",
1860 /// For default impls, we need to break apart a type into its
1861 /// "constituent types" -- meaning, the types that it contains.
1863 /// Here are some (simple) examples:
1866 /// (i32, u32) -> [i32, u32]
1867 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1868 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1869 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1871 fn constituent_types_for_ty(&self, t
: Ty
<'tcx
>) -> Vec
<Ty
<'tcx
>> {
1881 ty
::TyInfer(ty
::IntVar(_
)) |
1882 ty
::TyInfer(ty
::FloatVar(_
)) |
1890 ty
::TyProjection(..) |
1892 ty
::TyInfer(ty
::TyVar(_
)) |
1893 ty
::TyInfer(ty
::FreshTy(_
)) |
1894 ty
::TyInfer(ty
::FreshIntTy(_
)) |
1895 ty
::TyInfer(ty
::FreshFloatTy(_
)) => {
1896 bug
!("asked to assemble constituent types of unexpected type: {:?}",
1900 ty
::TyBox(referent_ty
) => { // Box<T>
1904 ty
::TyRawPtr(ty
::TypeAndMut { ty: element_ty, ..}
) |
1905 ty
::TyRef(_
, ty
::TypeAndMut { ty: element_ty, ..}
) => {
1909 ty
::TyArray(element_ty
, _
) | ty
::TySlice(element_ty
) => {
1913 ty
::TyTuple(ref tys
) => {
1914 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1918 ty
::TyClosure(_
, ref substs
) => {
1919 // FIXME(#27086). We are invariant w/r/t our
1920 // substs.func_substs, but we don't see them as
1921 // constituent types; this seems RIGHT but also like
1922 // something that a normal type couldn't simulate. Is
1923 // this just a gap with the way that PhantomData and
1924 // OIBIT interact? That is, there is no way to say
1925 // "make me invariant with respect to this TYPE, but
1926 // do not act as though I can reach it"
1927 substs
.upvar_tys
.to_vec()
1930 // for `PhantomData<T>`, we pass `T`
1931 ty
::TyAdt(def
, substs
) if def
.is_phantom_data() => {
1932 substs
.types().collect()
1935 ty
::TyAdt(def
, substs
) => {
1937 .map(|f
| f
.ty(self.tcx(), substs
))
1943 fn collect_predicates_for_types(&mut self,
1944 cause
: ObligationCause
<'tcx
>,
1945 recursion_depth
: usize,
1946 trait_def_id
: DefId
,
1947 types
: ty
::Binder
<Vec
<Ty
<'tcx
>>>)
1948 -> Vec
<PredicateObligation
<'tcx
>>
1950 // Because the types were potentially derived from
1951 // higher-ranked obligations they may reference late-bound
1952 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
1953 // yield a type like `for<'a> &'a int`. In general, we
1954 // maintain the invariant that we never manipulate bound
1955 // regions, so we have to process these bound regions somehow.
1957 // The strategy is to:
1959 // 1. Instantiate those regions to skolemized regions (e.g.,
1960 // `for<'a> &'a int` becomes `&0 int`.
1961 // 2. Produce something like `&'0 int : Copy`
1962 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
1964 types
.skip_binder().into_iter().flat_map(|ty
| { // binder moved -\
1965 let ty
: ty
::Binder
<Ty
<'tcx
>> = ty
::Binder(ty
); // <----------/
1967 self.in_snapshot(|this
, snapshot
| {
1968 let (skol_ty
, skol_map
) =
1969 this
.infcx().skolemize_late_bound_regions(&ty
, snapshot
);
1970 let Normalized { value: normalized_ty, mut obligations }
=
1971 project
::normalize_with_depth(this
,
1975 let skol_obligation
=
1976 this
.tcx().predicate_for_trait_def(
1982 obligations
.push(skol_obligation
);
1983 this
.infcx().plug_leaks(skol_map
, snapshot
, &obligations
)
1988 ///////////////////////////////////////////////////////////////////////////
1991 // Confirmation unifies the output type parameters of the trait
1992 // with the values found in the obligation, possibly yielding a
1993 // type error. See `README.md` for more details.
1995 fn confirm_candidate(&mut self,
1996 obligation
: &TraitObligation
<'tcx
>,
1997 candidate
: SelectionCandidate
<'tcx
>)
1998 -> Result
<Selection
<'tcx
>,SelectionError
<'tcx
>>
2000 debug
!("confirm_candidate({:?}, {:?})",
2005 BuiltinCandidate { has_nested }
=> {
2007 self.confirm_builtin_candidate(obligation
, has_nested
)))
2010 ParamCandidate(param
) => {
2011 let obligations
= self.confirm_param_candidate(obligation
, param
);
2012 Ok(VtableParam(obligations
))
2015 DefaultImplCandidate(trait_def_id
) => {
2016 let data
= self.confirm_default_impl_candidate(obligation
, trait_def_id
);
2017 Ok(VtableDefaultImpl(data
))
2020 DefaultImplObjectCandidate(trait_def_id
) => {
2021 let data
= self.confirm_default_impl_object_candidate(obligation
, trait_def_id
);
2022 Ok(VtableDefaultImpl(data
))
2025 ImplCandidate(impl_def_id
) => {
2026 Ok(VtableImpl(self.confirm_impl_candidate(obligation
, impl_def_id
)))
2029 ClosureCandidate(closure_def_id
, substs
, kind
) => {
2030 let vtable_closure
=
2031 self.confirm_closure_candidate(obligation
, closure_def_id
, substs
, kind
)?
;
2032 Ok(VtableClosure(vtable_closure
))
2035 BuiltinObjectCandidate
=> {
2036 // This indicates something like `(Trait+Send) :
2037 // Send`. In this case, we know that this holds
2038 // because that's what the object type is telling us,
2039 // and there's really no additional obligations to
2040 // prove and no types in particular to unify etc.
2041 Ok(VtableParam(Vec
::new()))
2044 ObjectCandidate
=> {
2045 let data
= self.confirm_object_candidate(obligation
);
2046 Ok(VtableObject(data
))
2049 FnPointerCandidate
=> {
2051 self.confirm_fn_pointer_candidate(obligation
)?
;
2052 Ok(VtableFnPointer(data
))
2055 ProjectionCandidate
=> {
2056 self.confirm_projection_candidate(obligation
);
2057 Ok(VtableParam(Vec
::new()))
2060 BuiltinUnsizeCandidate
=> {
2061 let data
= self.confirm_builtin_unsize_candidate(obligation
)?
;
2062 Ok(VtableBuiltin(data
))
2067 fn confirm_projection_candidate(&mut self,
2068 obligation
: &TraitObligation
<'tcx
>)
2070 self.in_snapshot(|this
, snapshot
| {
2072 this
.match_projection_obligation_against_definition_bounds(obligation
,
2078 fn confirm_param_candidate(&mut self,
2079 obligation
: &TraitObligation
<'tcx
>,
2080 param
: ty
::PolyTraitRef
<'tcx
>)
2081 -> Vec
<PredicateObligation
<'tcx
>>
2083 debug
!("confirm_param_candidate({:?},{:?})",
2087 // During evaluation, we already checked that this
2088 // where-clause trait-ref could be unified with the obligation
2089 // trait-ref. Repeat that unification now without any
2090 // transactional boundary; it should not fail.
2091 match self.match_where_clause_trait_ref(obligation
, param
.clone()) {
2092 Ok(obligations
) => obligations
,
2094 bug
!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2101 fn confirm_builtin_candidate(&mut self,
2102 obligation
: &TraitObligation
<'tcx
>,
2104 -> VtableBuiltinData
<PredicateObligation
<'tcx
>>
2106 debug
!("confirm_builtin_candidate({:?}, {:?})",
2107 obligation
, has_nested
);
2109 let obligations
= if has_nested
{
2110 let trait_def
= obligation
.predicate
.def_id();
2111 let conditions
= match trait_def
{
2112 _
if Some(trait_def
) == self.tcx().lang_items
.sized_trait() => {
2113 self.sized_conditions(obligation
)
2115 _
if Some(trait_def
) == self.tcx().lang_items
.copy_trait() => {
2116 self.copy_conditions(obligation
)
2118 _
=> bug
!("unexpected builtin trait {:?}", trait_def
)
2120 let nested
= match conditions
{
2121 BuiltinImplConditions
::Where(nested
) => nested
,
2122 _
=> bug
!("obligation {:?} had matched a builtin impl but now doesn't",
2126 let cause
= obligation
.derived_cause(BuiltinDerivedObligation
);
2127 self.collect_predicates_for_types(cause
,
2128 obligation
.recursion_depth
+1,
2135 debug
!("confirm_builtin_candidate: obligations={:?}",
2137 VtableBuiltinData { nested: obligations }
2140 /// This handles the case where a `impl Foo for ..` impl is being used.
2141 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2143 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2144 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2145 fn confirm_default_impl_candidate(&mut self,
2146 obligation
: &TraitObligation
<'tcx
>,
2147 trait_def_id
: DefId
)
2148 -> VtableDefaultImplData
<PredicateObligation
<'tcx
>>
2150 debug
!("confirm_default_impl_candidate({:?}, {:?})",
2154 // binder is moved below
2155 let self_ty
= self.infcx
.shallow_resolve(obligation
.predicate
.skip_binder().self_ty());
2156 let types
= self.constituent_types_for_ty(self_ty
);
2157 self.vtable_default_impl(obligation
, trait_def_id
, ty
::Binder(types
))
2160 fn confirm_default_impl_object_candidate(&mut self,
2161 obligation
: &TraitObligation
<'tcx
>,
2162 trait_def_id
: DefId
)
2163 -> VtableDefaultImplData
<PredicateObligation
<'tcx
>>
2165 debug
!("confirm_default_impl_object_candidate({:?}, {:?})",
2169 assert
!(self.tcx().has_attr(trait_def_id
, "rustc_reflect_like"));
2171 // OK to skip binder, it is reintroduced below
2172 let self_ty
= self.infcx
.shallow_resolve(obligation
.predicate
.skip_binder().self_ty());
2174 ty
::TyTrait(ref data
) => {
2175 // OK to skip the binder, it is reintroduced below
2176 let input_types
= data
.principal
.input_types();
2177 let assoc_types
= data
.projection_bounds
.iter()
2178 .map(|pb
| pb
.skip_binder().ty
);
2179 let all_types
: Vec
<_
> = input_types
.chain(assoc_types
)
2182 // reintroduce the two binding levels we skipped, then flatten into one
2183 let all_types
= ty
::Binder(ty
::Binder(all_types
));
2184 let all_types
= self.tcx().flatten_late_bound_regions(&all_types
);
2186 self.vtable_default_impl(obligation
, trait_def_id
, all_types
)
2189 bug
!("asked to confirm default object implementation for non-object type: {:?}",
2195 /// See `confirm_default_impl_candidate`
2196 fn vtable_default_impl(&mut self,
2197 obligation
: &TraitObligation
<'tcx
>,
2198 trait_def_id
: DefId
,
2199 nested
: ty
::Binder
<Vec
<Ty
<'tcx
>>>)
2200 -> VtableDefaultImplData
<PredicateObligation
<'tcx
>>
2202 debug
!("vtable_default_impl: nested={:?}", nested
);
2204 let cause
= obligation
.derived_cause(BuiltinDerivedObligation
);
2205 let mut obligations
= self.collect_predicates_for_types(
2207 obligation
.recursion_depth
+1,
2211 let trait_obligations
= self.in_snapshot(|this
, snapshot
| {
2212 let poly_trait_ref
= obligation
.predicate
.to_poly_trait_ref();
2213 let (trait_ref
, skol_map
) =
2214 this
.infcx().skolemize_late_bound_regions(&poly_trait_ref
, snapshot
);
2215 let cause
= obligation
.derived_cause(ImplDerivedObligation
);
2216 this
.impl_or_trait_obligations(cause
,
2217 obligation
.recursion_depth
+ 1,
2224 obligations
.extend(trait_obligations
);
2226 debug
!("vtable_default_impl: obligations={:?}", obligations
);
2228 VtableDefaultImplData
{
2229 trait_def_id
: trait_def_id
,
2234 fn confirm_impl_candidate(&mut self,
2235 obligation
: &TraitObligation
<'tcx
>,
2237 -> VtableImplData
<'tcx
, PredicateObligation
<'tcx
>>
2239 debug
!("confirm_impl_candidate({:?},{:?})",
2243 // First, create the substitutions by matching the impl again,
2244 // this time not in a probe.
2245 self.in_snapshot(|this
, snapshot
| {
2246 let (substs
, skol_map
) =
2247 this
.rematch_impl(impl_def_id
, obligation
,
2249 debug
!("confirm_impl_candidate substs={:?}", substs
);
2250 let cause
= obligation
.derived_cause(ImplDerivedObligation
);
2251 this
.vtable_impl(impl_def_id
, substs
, cause
,
2252 obligation
.recursion_depth
+ 1,
2257 fn vtable_impl(&mut self,
2259 mut substs
: Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
2260 cause
: ObligationCause
<'tcx
>,
2261 recursion_depth
: usize,
2262 skol_map
: infer
::SkolemizationMap
<'tcx
>,
2263 snapshot
: &infer
::CombinedSnapshot
)
2264 -> VtableImplData
<'tcx
, PredicateObligation
<'tcx
>>
2266 debug
!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2272 let mut impl_obligations
=
2273 self.impl_or_trait_obligations(cause
,
2280 debug
!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2284 // Because of RFC447, the impl-trait-ref and obligations
2285 // are sufficient to determine the impl substs, without
2286 // relying on projections in the impl-trait-ref.
2288 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2289 impl_obligations
.append(&mut substs
.obligations
);
2291 VtableImplData
{ impl_def_id
: impl_def_id
,
2292 substs
: substs
.value
,
2293 nested
: impl_obligations
}
2296 fn confirm_object_candidate(&mut self,
2297 obligation
: &TraitObligation
<'tcx
>)
2298 -> VtableObjectData
<'tcx
, PredicateObligation
<'tcx
>>
2300 debug
!("confirm_object_candidate({:?})",
2303 // FIXME skipping binder here seems wrong -- we should
2304 // probably flatten the binder from the obligation and the
2305 // binder from the object. Have to try to make a broken test
2306 // case that results. -nmatsakis
2307 let self_ty
= self.infcx
.shallow_resolve(*obligation
.self_ty().skip_binder());
2308 let poly_trait_ref
= match self_ty
.sty
{
2309 ty
::TyTrait(ref data
) => {
2310 data
.principal
.with_self_ty(self.tcx(), self_ty
)
2313 span_bug
!(obligation
.cause
.span
,
2314 "object candidate with non-object");
2318 let mut upcast_trait_ref
= None
;
2322 let tcx
= self.tcx();
2324 // We want to find the first supertrait in the list of
2325 // supertraits that we can unify with, and do that
2326 // unification. We know that there is exactly one in the list
2327 // where we can unify because otherwise select would have
2328 // reported an ambiguity. (When we do find a match, also
2329 // record it for later.)
2331 util
::supertraits(tcx
, poly_trait_ref
)
2335 |this
, _
| this
.match_poly_trait_ref(obligation
, t
))
2337 Ok(_
) => { upcast_trait_ref = Some(t); false }
2342 // Additionally, for each of the nonmatching predicates that
2343 // we pass over, we sum up the set of number of vtable
2344 // entries, so that we can compute the offset for the selected
2347 nonmatching
.map(|t
| tcx
.count_own_vtable_entries(t
))
2353 upcast_trait_ref
: upcast_trait_ref
.unwrap(),
2354 vtable_base
: vtable_base
,
2359 fn confirm_fn_pointer_candidate(&mut self, obligation
: &TraitObligation
<'tcx
>)
2360 -> Result
<VtableFnPointerData
<'tcx
, PredicateObligation
<'tcx
>>, SelectionError
<'tcx
>>
2362 debug
!("confirm_fn_pointer_candidate({:?})",
2365 // ok to skip binder; it is reintroduced below
2366 let self_ty
= self.infcx
.shallow_resolve(*obligation
.self_ty().skip_binder());
2367 let sig
= self_ty
.fn_sig();
2369 self.tcx().closure_trait_ref_and_return_type(obligation
.predicate
.def_id(),
2372 util
::TupleArgumentsFlag
::Yes
)
2373 .map_bound(|(trait_ref
, _
)| trait_ref
);
2375 self.confirm_poly_trait_refs(obligation
.cause
.clone(),
2376 obligation
.predicate
.to_poly_trait_ref(),
2378 Ok(VtableFnPointerData { fn_ty: self_ty, nested: vec![] }
)
2381 fn confirm_closure_candidate(&mut self,
2382 obligation
: &TraitObligation
<'tcx
>,
2383 closure_def_id
: DefId
,
2384 substs
: ty
::ClosureSubsts
<'tcx
>,
2385 kind
: ty
::ClosureKind
)
2386 -> Result
<VtableClosureData
<'tcx
, PredicateObligation
<'tcx
>>,
2387 SelectionError
<'tcx
>>
2389 debug
!("confirm_closure_candidate({:?},{:?},{:?})",
2397 } = self.closure_trait_ref(obligation
, closure_def_id
, substs
);
2399 debug
!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2404 self.confirm_poly_trait_refs(obligation
.cause
.clone(),
2405 obligation
.predicate
.to_poly_trait_ref(),
2408 obligations
.push(Obligation
::new(
2409 obligation
.cause
.clone(),
2410 ty
::Predicate
::ClosureKind(closure_def_id
, kind
)));
2412 Ok(VtableClosureData
{
2413 closure_def_id
: closure_def_id
,
2414 substs
: substs
.clone(),
2419 /// In the case of closure types and fn pointers,
2420 /// we currently treat the input type parameters on the trait as
2421 /// outputs. This means that when we have a match we have only
2422 /// considered the self type, so we have to go back and make sure
2423 /// to relate the argument types too. This is kind of wrong, but
2424 /// since we control the full set of impls, also not that wrong,
2425 /// and it DOES yield better error messages (since we don't report
2426 /// errors as if there is no applicable impl, but rather report
2427 /// errors are about mismatched argument types.
2429 /// Here is an example. Imagine we have a closure expression
2430 /// and we desugared it so that the type of the expression is
2431 /// `Closure`, and `Closure` expects an int as argument. Then it
2432 /// is "as if" the compiler generated this impl:
2434 /// impl Fn(int) for Closure { ... }
2436 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2437 /// we have matched the self-type `Closure`. At this point we'll
2438 /// compare the `int` to `usize` and generate an error.
2440 /// Note that this checking occurs *after* the impl has selected,
2441 /// because these output type parameters should not affect the
2442 /// selection of the impl. Therefore, if there is a mismatch, we
2443 /// report an error to the user.
2444 fn confirm_poly_trait_refs(&mut self,
2445 obligation_cause
: ObligationCause
,
2446 obligation_trait_ref
: ty
::PolyTraitRef
<'tcx
>,
2447 expected_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
2448 -> Result
<(), SelectionError
<'tcx
>>
2450 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation_cause
.span
);
2452 let obligation_trait_ref
= obligation_trait_ref
.clone();
2453 self.infcx
.sub_poly_trait_refs(false,
2455 expected_trait_ref
.clone(),
2456 obligation_trait_ref
.clone())
2457 .map(|InferOk { obligations, .. }
| self.inferred_obligations
.extend(obligations
))
2458 .map_err(|e
| OutputTypeParameterMismatch(expected_trait_ref
, obligation_trait_ref
, e
))
2461 fn confirm_builtin_unsize_candidate(&mut self,
2462 obligation
: &TraitObligation
<'tcx
>,)
2463 -> Result
<VtableBuiltinData
<PredicateObligation
<'tcx
>>,
2464 SelectionError
<'tcx
>> {
2465 let tcx
= self.tcx();
2467 // assemble_candidates_for_unsizing should ensure there are no late bound
2468 // regions here. See the comment there for more details.
2469 let source
= self.infcx
.shallow_resolve(
2470 tcx
.no_late_bound_regions(&obligation
.self_ty()).unwrap());
2471 let target
= obligation
.predicate
.skip_binder().trait_ref
.substs
.type_at(1);
2472 let target
= self.infcx
.shallow_resolve(target
);
2474 debug
!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2477 let mut nested
= vec
![];
2478 match (&source
.sty
, &target
.sty
) {
2479 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2480 (&ty
::TyTrait(ref data_a
), &ty
::TyTrait(ref data_b
)) => {
2481 // See assemble_candidates_for_unsizing for more info.
2482 let new_trait
= tcx
.mk_trait(ty
::TraitObject
{
2483 principal
: data_a
.principal
,
2484 region_bound
: data_b
.region_bound
,
2485 builtin_bounds
: data_b
.builtin_bounds
,
2486 projection_bounds
: data_a
.projection_bounds
.clone(),
2488 let origin
= TypeOrigin
::Misc(obligation
.cause
.span
);
2489 let InferOk { obligations, .. }
=
2490 self.infcx
.sub_types(false, origin
, new_trait
, target
)
2491 .map_err(|_
| Unimplemented
)?
;
2492 self.inferred_obligations
.extend(obligations
);
2494 // Register one obligation for 'a: 'b.
2495 let cause
= ObligationCause
::new(obligation
.cause
.span
,
2496 obligation
.cause
.body_id
,
2497 ObjectCastObligation(target
));
2498 let outlives
= ty
::OutlivesPredicate(data_a
.region_bound
,
2499 data_b
.region_bound
);
2500 nested
.push(Obligation
::with_depth(cause
,
2501 obligation
.recursion_depth
+ 1,
2502 ty
::Binder(outlives
).to_predicate()));
2506 (_
, &ty
::TyTrait(ref data
)) => {
2507 let mut object_dids
=
2508 data
.builtin_bounds
.iter().flat_map(|bound
| {
2509 tcx
.lang_items
.from_builtin_kind(bound
).ok()
2511 .chain(Some(data
.principal
.def_id()));
2512 if let Some(did
) = object_dids
.find(|did
| {
2513 !tcx
.is_object_safe(*did
)
2515 return Err(TraitNotObjectSafe(did
))
2518 let cause
= ObligationCause
::new(obligation
.cause
.span
,
2519 obligation
.cause
.body_id
,
2520 ObjectCastObligation(target
));
2521 let mut push
= |predicate
| {
2522 nested
.push(Obligation
::with_depth(cause
.clone(),
2523 obligation
.recursion_depth
+ 1,
2527 // Create the obligation for casting from T to Trait.
2528 push(data
.principal
.with_self_ty(tcx
, source
).to_predicate());
2530 // We can only make objects from sized types.
2531 let mut builtin_bounds
= data
.builtin_bounds
;
2532 builtin_bounds
.insert(ty
::BoundSized
);
2534 // Create additional obligations for all the various builtin
2535 // bounds attached to the object cast. (In other words, if the
2536 // object type is Foo+Send, this would create an obligation
2537 // for the Send check.)
2538 for bound
in &builtin_bounds
{
2539 if let Ok(tr
) = tcx
.trait_ref_for_builtin_bound(bound
, source
) {
2540 push(tr
.to_predicate());
2542 return Err(Unimplemented
);
2546 // Create obligations for the projection predicates.
2547 for bound
in &data
.projection_bounds
{
2548 push(bound
.with_self_ty(tcx
, source
).to_predicate());
2551 // If the type is `Foo+'a`, ensures that the type
2552 // being cast to `Foo+'a` outlives `'a`:
2553 let outlives
= ty
::OutlivesPredicate(source
, data
.region_bound
);
2554 push(ty
::Binder(outlives
).to_predicate());
2558 (&ty
::TyArray(a
, _
), &ty
::TySlice(b
)) => {
2559 let origin
= TypeOrigin
::Misc(obligation
.cause
.span
);
2560 let InferOk { obligations, .. }
=
2561 self.infcx
.sub_types(false, origin
, a
, b
)
2562 .map_err(|_
| Unimplemented
)?
;
2563 self.inferred_obligations
.extend(obligations
);
2566 // Struct<T> -> Struct<U>.
2567 (&ty
::TyAdt(def
, substs_a
), &ty
::TyAdt(_
, substs_b
)) => {
2570 .map(|f
| f
.unsubst_ty())
2571 .collect
::<Vec
<_
>>();
2573 // The last field of the structure has to exist and contain type parameters.
2574 let field
= if let Some(&field
) = fields
.last() {
2577 return Err(Unimplemented
);
2579 let mut ty_params
= BitVector
::new(substs_a
.types().count());
2580 let mut found
= false;
2581 for ty
in field
.walk() {
2582 if let ty
::TyParam(p
) = ty
.sty
{
2583 ty_params
.insert(p
.idx
as usize);
2588 return Err(Unimplemented
);
2591 // Replace type parameters used in unsizing with
2592 // TyError and ensure they do not affect any other fields.
2593 // This could be checked after type collection for any struct
2594 // with a potentially unsized trailing field.
2595 let params
= substs_a
.params().iter().enumerate().map(|(i
, &k
)| {
2596 if ty_params
.contains(i
) {
2597 Kind
::from(tcx
.types
.err
)
2602 let substs
= Substs
::new(tcx
, params
);
2603 for &ty
in fields
.split_last().unwrap().1 {
2604 if ty
.subst(tcx
, substs
).references_error() {
2605 return Err(Unimplemented
);
2609 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2610 let inner_source
= field
.subst(tcx
, substs_a
);
2611 let inner_target
= field
.subst(tcx
, substs_b
);
2613 // Check that the source structure with the target's
2614 // type parameters is a subtype of the target.
2615 let params
= substs_a
.params().iter().enumerate().map(|(i
, &k
)| {
2616 if ty_params
.contains(i
) {
2617 Kind
::from(substs_b
.type_at(i
))
2622 let new_struct
= tcx
.mk_adt(def
, Substs
::new(tcx
, params
));
2623 let origin
= TypeOrigin
::Misc(obligation
.cause
.span
);
2624 let InferOk { obligations, .. }
=
2625 self.infcx
.sub_types(false, origin
, new_struct
, target
)
2626 .map_err(|_
| Unimplemented
)?
;
2627 self.inferred_obligations
.extend(obligations
);
2629 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2630 nested
.push(tcx
.predicate_for_trait_def(
2631 obligation
.cause
.clone(),
2632 obligation
.predicate
.def_id(),
2633 obligation
.recursion_depth
+ 1,
2641 Ok(VtableBuiltinData { nested: nested }
)
2644 ///////////////////////////////////////////////////////////////////////////
2647 // Matching is a common path used for both evaluation and
2648 // confirmation. It basically unifies types that appear in impls
2649 // and traits. This does affect the surrounding environment;
2650 // therefore, when used during evaluation, match routines must be
2651 // run inside of a `probe()` so that their side-effects are
2654 fn rematch_impl(&mut self,
2656 obligation
: &TraitObligation
<'tcx
>,
2657 snapshot
: &infer
::CombinedSnapshot
)
2658 -> (Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
2659 infer
::SkolemizationMap
<'tcx
>)
2661 match self.match_impl(impl_def_id
, obligation
, snapshot
) {
2662 Ok((substs
, skol_map
)) => (substs
, skol_map
),
2664 bug
!("Impl {:?} was matchable against {:?} but now is not",
2671 fn match_impl(&mut self,
2673 obligation
: &TraitObligation
<'tcx
>,
2674 snapshot
: &infer
::CombinedSnapshot
)
2675 -> Result
<(Normalized
<'tcx
, &'tcx Substs
<'tcx
>>,
2676 infer
::SkolemizationMap
<'tcx
>), ()>
2678 let impl_trait_ref
= self.tcx().impl_trait_ref(impl_def_id
).unwrap();
2680 // Before we create the substitutions and everything, first
2681 // consider a "quick reject". This avoids creating more types
2682 // and so forth that we need to.
2683 if self.fast_reject_trait_refs(obligation
, &impl_trait_ref
) {
2687 let (skol_obligation
, skol_map
) = self.infcx().skolemize_late_bound_regions(
2688 &obligation
.predicate
,
2690 let skol_obligation_trait_ref
= skol_obligation
.trait_ref
;
2692 let impl_substs
= self.infcx
.fresh_substs_for_item(obligation
.cause
.span
,
2695 let impl_trait_ref
= impl_trait_ref
.subst(self.tcx(),
2698 let impl_trait_ref
=
2699 project
::normalize_with_depth(self,
2700 obligation
.cause
.clone(),
2701 obligation
.recursion_depth
+ 1,
2704 debug
!("match_impl(impl_def_id={:?}, obligation={:?}, \
2705 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
2709 skol_obligation_trait_ref
);
2711 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation
.cause
.span
);
2712 let InferOk { obligations, .. }
=
2713 self.infcx
.eq_trait_refs(false,
2715 impl_trait_ref
.value
.clone(),
2716 skol_obligation_trait_ref
)
2718 debug
!("match_impl: failed eq_trait_refs due to `{}`", e
);
2721 self.inferred_obligations
.extend(obligations
);
2723 if let Err(e
) = self.infcx
.leak_check(false,
2724 obligation
.cause
.span
,
2727 debug
!("match_impl: failed leak check due to `{}`", e
);
2731 debug
!("match_impl: success impl_substs={:?}", impl_substs
);
2734 obligations
: impl_trait_ref
.obligations
2738 fn fast_reject_trait_refs(&mut self,
2739 obligation
: &TraitObligation
,
2740 impl_trait_ref
: &ty
::TraitRef
)
2743 // We can avoid creating type variables and doing the full
2744 // substitution if we find that any of the input types, when
2745 // simplified, do not match.
2747 obligation
.predicate
.skip_binder().input_types()
2748 .zip(impl_trait_ref
.input_types())
2749 .any(|(obligation_ty
, impl_ty
)| {
2750 let simplified_obligation_ty
=
2751 fast_reject
::simplify_type(self.tcx(), obligation_ty
, true);
2752 let simplified_impl_ty
=
2753 fast_reject
::simplify_type(self.tcx(), impl_ty
, false);
2755 simplified_obligation_ty
.is_some() &&
2756 simplified_impl_ty
.is_some() &&
2757 simplified_obligation_ty
!= simplified_impl_ty
2761 /// Normalize `where_clause_trait_ref` and try to match it against
2762 /// `obligation`. If successful, return any predicates that
2763 /// result from the normalization. Normalization is necessary
2764 /// because where-clauses are stored in the parameter environment
2766 fn match_where_clause_trait_ref(&mut self,
2767 obligation
: &TraitObligation
<'tcx
>,
2768 where_clause_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
2769 -> Result
<Vec
<PredicateObligation
<'tcx
>>,()>
2771 self.match_poly_trait_ref(obligation
, where_clause_trait_ref
)?
;
2775 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2776 /// obligation is satisfied.
2777 fn match_poly_trait_ref(&mut self,
2778 obligation
: &TraitObligation
<'tcx
>,
2779 poly_trait_ref
: ty
::PolyTraitRef
<'tcx
>)
2782 debug
!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
2786 let origin
= TypeOrigin
::RelateOutputImplTypes(obligation
.cause
.span
);
2787 self.infcx
.sub_poly_trait_refs(false,
2790 obligation
.predicate
.to_poly_trait_ref())
2791 .map(|InferOk { obligations, .. }
| self.inferred_obligations
.extend(obligations
))
2795 ///////////////////////////////////////////////////////////////////////////
2798 fn match_fresh_trait_refs(&self,
2799 previous
: &ty
::PolyTraitRef
<'tcx
>,
2800 current
: &ty
::PolyTraitRef
<'tcx
>)
2803 let mut matcher
= ty
::_match
::Match
::new(self.tcx());
2804 matcher
.relate(previous
, current
).is_ok()
2807 fn push_stack
<'o
,'s
:'o
>(&mut self,
2808 previous_stack
: TraitObligationStackList
<'s
, 'tcx
>,
2809 obligation
: &'o TraitObligation
<'tcx
>)
2810 -> TraitObligationStack
<'o
, 'tcx
>
2812 let fresh_trait_ref
=
2813 obligation
.predicate
.to_poly_trait_ref().fold_with(&mut self.freshener
);
2815 TraitObligationStack
{
2816 obligation
: obligation
,
2817 fresh_trait_ref
: fresh_trait_ref
,
2818 previous
: previous_stack
,
2822 fn closure_trait_ref_unnormalized(&mut self,
2823 obligation
: &TraitObligation
<'tcx
>,
2824 closure_def_id
: DefId
,
2825 substs
: ty
::ClosureSubsts
<'tcx
>)
2826 -> ty
::PolyTraitRef
<'tcx
>
2828 let closure_type
= self.infcx
.closure_type(closure_def_id
, substs
);
2829 let ty
::Binder((trait_ref
, _
)) =
2830 self.tcx().closure_trait_ref_and_return_type(obligation
.predicate
.def_id(),
2831 obligation
.predicate
.0.self_ty(), // (1)
2833 util
::TupleArgumentsFlag
::No
);
2834 // (1) Feels icky to skip the binder here, but OTOH we know
2835 // that the self-type is an unboxed closure type and hence is
2836 // in fact unparameterized (or at least does not reference any
2837 // regions bound in the obligation). Still probably some
2838 // refactoring could make this nicer.
2840 ty
::Binder(trait_ref
)
2843 fn closure_trait_ref(&mut self,
2844 obligation
: &TraitObligation
<'tcx
>,
2845 closure_def_id
: DefId
,
2846 substs
: ty
::ClosureSubsts
<'tcx
>)
2847 -> Normalized
<'tcx
, ty
::PolyTraitRef
<'tcx
>>
2849 let trait_ref
= self.closure_trait_ref_unnormalized(
2850 obligation
, closure_def_id
, substs
);
2852 // A closure signature can contain associated types which
2853 // must be normalized.
2854 normalize_with_depth(self,
2855 obligation
.cause
.clone(),
2856 obligation
.recursion_depth
+1,
2860 /// Returns the obligations that are implied by instantiating an
2861 /// impl or trait. The obligations are substituted and fully
2862 /// normalized. This is used when confirming an impl or default
2864 fn impl_or_trait_obligations(&mut self,
2865 cause
: ObligationCause
<'tcx
>,
2866 recursion_depth
: usize,
2867 def_id
: DefId
, // of impl or trait
2868 substs
: &Substs
<'tcx
>, // for impl or trait
2869 skol_map
: infer
::SkolemizationMap
<'tcx
>,
2870 snapshot
: &infer
::CombinedSnapshot
)
2871 -> Vec
<PredicateObligation
<'tcx
>>
2873 debug
!("impl_or_trait_obligations(def_id={:?})", def_id
);
2874 let tcx
= self.tcx();
2876 // To allow for one-pass evaluation of the nested obligation,
2877 // each predicate must be preceded by the obligations required
2879 // for example, if we have:
2880 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
2881 // the impl will have the following predicates:
2882 // <V as Iterator>::Item = U,
2883 // U: Iterator, U: Sized,
2884 // V: Iterator, V: Sized,
2885 // <U as Iterator>::Item: Copy
2886 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2887 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2888 // `$1: Copy`, so we must ensure the obligations are emitted in
2890 let predicates
= tcx
.lookup_predicates(def_id
);
2891 assert_eq
!(predicates
.parent
, None
);
2892 let predicates
= predicates
.predicates
.iter().flat_map(|predicate
| {
2893 let predicate
= normalize_with_depth(self, cause
.clone(), recursion_depth
,
2894 &predicate
.subst(tcx
, substs
));
2895 predicate
.obligations
.into_iter().chain(
2897 cause
: cause
.clone(),
2898 recursion_depth
: recursion_depth
,
2899 predicate
: predicate
.value
2902 self.infcx().plug_leaks(skol_map
, snapshot
, &predicates
)
2906 impl<'tcx
> TraitObligation
<'tcx
> {
2907 #[allow(unused_comparisons)]
2908 pub fn derived_cause(&self,
2909 variant
: fn(DerivedObligationCause
<'tcx
>) -> ObligationCauseCode
<'tcx
>)
2910 -> ObligationCause
<'tcx
>
2913 * Creates a cause for obligations that are derived from
2914 * `obligation` by a recursive search (e.g., for a builtin
2915 * bound, or eventually a `impl Foo for ..`). If `obligation`
2916 * is itself a derived obligation, this is just a clone, but
2917 * otherwise we create a "derived obligation" cause so as to
2918 * keep track of the original root obligation for error
2922 let obligation
= self;
2924 // NOTE(flaper87): As of now, it keeps track of the whole error
2925 // chain. Ideally, we should have a way to configure this either
2926 // by using -Z verbose or just a CLI argument.
2927 if obligation
.recursion_depth
>= 0 {
2928 let derived_cause
= DerivedObligationCause
{
2929 parent_trait_ref
: obligation
.predicate
.to_poly_trait_ref(),
2930 parent_code
: Rc
::new(obligation
.cause
.code
.clone())
2932 let derived_code
= variant(derived_cause
);
2933 ObligationCause
::new(obligation
.cause
.span
, obligation
.cause
.body_id
, derived_code
)
2935 obligation
.cause
.clone()
2940 impl<'tcx
> SelectionCache
<'tcx
> {
2941 pub fn new() -> SelectionCache
<'tcx
> {
2943 hashmap
: RefCell
::new(FnvHashMap())
2948 impl<'tcx
> EvaluationCache
<'tcx
> {
2949 pub fn new() -> EvaluationCache
<'tcx
> {
2951 hashmap
: RefCell
::new(FnvHashMap())
2956 impl<'o
,'tcx
> TraitObligationStack
<'o
,'tcx
> {
2957 fn list(&'o
self) -> TraitObligationStackList
<'o
,'tcx
> {
2958 TraitObligationStackList
::with(self)
2961 fn iter(&'o
self) -> TraitObligationStackList
<'o
,'tcx
> {
2966 #[derive(Copy, Clone)]
2967 struct TraitObligationStackList
<'o
,'tcx
:'o
> {
2968 head
: Option
<&'o TraitObligationStack
<'o
,'tcx
>>
2971 impl<'o
,'tcx
> TraitObligationStackList
<'o
,'tcx
> {
2972 fn empty() -> TraitObligationStackList
<'o
,'tcx
> {
2973 TraitObligationStackList { head: None }
2976 fn with(r
: &'o TraitObligationStack
<'o
,'tcx
>) -> TraitObligationStackList
<'o
,'tcx
> {
2977 TraitObligationStackList { head: Some(r) }
2981 impl<'o
,'tcx
> Iterator
for TraitObligationStackList
<'o
,'tcx
>{
2982 type Item
= &'o TraitObligationStack
<'o
,'tcx
>;
2984 fn next(&mut self) -> Option
<&'o TraitObligationStack
<'o
,'tcx
>> {
2995 impl<'o
,'tcx
> fmt
::Debug
for TraitObligationStack
<'o
,'tcx
> {
2996 fn fmt(&self, f
: &mut fmt
::Formatter
) -> fmt
::Result
{
2997 write
!(f
, "TraitObligationStack({:?})", self.obligation
)
3001 impl EvaluationResult
{
3002 fn may_apply(&self) -> bool
{
3006 EvaluatedToUnknown
=> true,
3008 EvaluatedToErr
=> false
3013 impl MethodMatchResult
{
3014 pub fn may_apply(&self) -> bool
{
3016 MethodMatched(_
) => true,
3017 MethodAmbiguous(_
) => true,
3018 MethodDidNotMatch
=> false,