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};
26 use super::ProjectionMode;
27 use super::TraitNotObjectSafe;
29 use super::SelectionResult;
30 use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure,
31 VtableFnPointer, VtableObject, VtableDefaultImpl};
32 use super::{VtableImplData, VtableObjectData, VtableBuiltinData,
33 VtableClosureData, VtableDefaultImplData, VtableFnPointerData};
36 use hir::def_id::DefId;
38 use infer::{InferCtxt, InferOk, TypeFreshener, TypeOrigin};
39 use ty::subst::{Subst, Substs, TypeSpace};
40 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
43 use ty::relate::TypeRelation;
45 use rustc_data_structures::snapshot_vec::{SnapshotVecDelegate, SnapshotVec};
46 use std::cell::RefCell;
48 use std::marker::PhantomData;
53 use util::nodemap::FnvHashMap;
55 struct InferredObligationsSnapshotVecDelegate<'tcx> {
56 phantom: PhantomData<&'tcx i32>,
58 impl<'tcx> SnapshotVecDelegate for InferredObligationsSnapshotVecDelegate<'tcx> {
59 type Value = PredicateObligation<'tcx>;
61 fn reverse(_: &mut Vec<Self::Value>, _: Self::Undo) {}
64 pub struct SelectionContext<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
65 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
67 /// Freshener used specifically for skolemizing entries on the
68 /// obligation stack. This ensures that all entries on the stack
69 /// at one time will have the same set of skolemized entries,
70 /// which is important for checking for trait bounds that
71 /// recursively require themselves.
72 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
74 /// If true, indicates that the evaluation should be conservative
75 /// and consider the possibility of types outside this crate.
76 /// This comes up primarily when resolving ambiguity. Imagine
77 /// there is some trait reference `$0 : Bar` where `$0` is an
78 /// inference variable. If `intercrate` is true, then we can never
79 /// say for sure that this reference is not implemented, even if
80 /// there are *no impls at all for `Bar`*, because `$0` could be
81 /// bound to some type that in a downstream crate that implements
82 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
83 /// though, we set this to false, because we are only interested
84 /// in types that the user could actually have written --- in
85 /// other words, we consider `$0 : Bar` to be unimplemented if
86 /// there is no type that the user could *actually name* that
87 /// would satisfy it. This avoids crippling inference, basically.
90 inferred_obligations: SnapshotVec<InferredObligationsSnapshotVecDelegate<'tcx>>,
93 // A stack that walks back up the stack frame.
94 struct TraitObligationStack<'prev, 'tcx: 'prev> {
95 obligation: &'prev TraitObligation<'tcx>,
97 /// Trait ref from `obligation` but skolemized with the
98 /// selection-context's freshener. Used to check for recursion.
99 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
101 previous: TraitObligationStackList<'prev, 'tcx>,
105 pub struct SelectionCache<'tcx> {
106 hashmap: RefCell<FnvHashMap<ty::TraitRef<'tcx>,
107 SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
110 pub enum MethodMatchResult {
111 MethodMatched(MethodMatchedData),
112 MethodAmbiguous(/* list of impls that could apply */ Vec<DefId>),
116 #[derive(Copy, Clone, Debug)]
117 pub enum MethodMatchedData {
118 // In the case of a precise match, we don't really need to store
119 // how the match was found. So don't.
122 // In the case of a coercion, we need to know the precise impl so
123 // that we can determine the type to which things were coerced.
124 CoerciveMethodMatch(/* impl we matched */ DefId)
127 /// The selection process begins by considering all impls, where
128 /// clauses, and so forth that might resolve an obligation. Sometimes
129 /// we'll be able to say definitively that (e.g.) an impl does not
130 /// apply to the obligation: perhaps it is defined for `usize` but the
131 /// obligation is for `int`. In that case, we drop the impl out of the
132 /// list. But the other cases are considered *candidates*.
134 /// For selection to succeed, there must be exactly one matching
135 /// candidate. If the obligation is fully known, this is guaranteed
136 /// by coherence. However, if the obligation contains type parameters
137 /// or variables, there may be multiple such impls.
139 /// It is not a real problem if multiple matching impls exist because
140 /// of type variables - it just means the obligation isn't sufficiently
141 /// elaborated. In that case we report an ambiguity, and the caller can
142 /// try again after more type information has been gathered or report a
143 /// "type annotations required" error.
145 /// However, with type parameters, this can be a real problem - type
146 /// parameters don't unify with regular types, but they *can* unify
147 /// with variables from blanket impls, and (unless we know its bounds
148 /// will always be satisfied) picking the blanket impl will be wrong
149 /// for at least *some* substitutions. To make this concrete, if we have
151 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
152 /// impl<T: fmt::Debug> AsDebug for T {
154 /// fn debug(self) -> fmt::Debug { self }
156 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
158 /// we can't just use the impl to resolve the <T as AsDebug> obligation
159 /// - a type from another crate (that doesn't implement fmt::Debug) could
160 /// implement AsDebug.
162 /// Because where-clauses match the type exactly, multiple clauses can
163 /// only match if there are unresolved variables, and we can mostly just
164 /// report this ambiguity in that case. This is still a problem - we can't
165 /// *do anything* with ambiguities that involve only regions. This is issue
168 /// If a single where-clause matches and there are no inference
169 /// variables left, then it definitely matches and we can just select
172 /// In fact, we even select the where-clause when the obligation contains
173 /// inference variables. The can lead to inference making "leaps of logic",
174 /// for example in this situation:
176 /// pub trait Foo<T> { fn foo(&self) -> T; }
177 /// impl<T> Foo<()> for T { fn foo(&self) { } }
178 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
180 /// pub fn foo<T>(t: T) where T: Foo<bool> {
181 /// println!("{:?}", <T as Foo<_>>::foo(&t));
183 /// fn main() { foo(false); }
185 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
186 /// impl and the where-clause. We select the where-clause and unify $0=bool,
187 /// so the program prints "false". However, if the where-clause is omitted,
188 /// the blanket impl is selected, we unify $0=(), and the program prints
191 /// Exactly the same issues apply to projection and object candidates, except
192 /// that we can have both a projection candidate and a where-clause candidate
193 /// for the same obligation. In that case either would do (except that
194 /// different "leaps of logic" would occur if inference variables are
195 /// present), and we just pick the where-clause. This is, for example,
196 /// required for associated types to work in default impls, as the bounds
197 /// are visible both as projection bounds and as where-clauses from the
198 /// parameter environment.
199 #[derive(PartialEq,Eq,Debug,Clone)]
200 enum SelectionCandidate<'tcx> {
201 BuiltinCandidate { has_nested: bool },
202 ParamCandidate(ty::PolyTraitRef<'tcx>),
203 ImplCandidate(DefId),
204 DefaultImplCandidate(DefId),
205 DefaultImplObjectCandidate(DefId),
207 /// This is a trait matching with a projected type as `Self`, and
208 /// we found an applicable bound in the trait definition.
211 /// Implementation of a `Fn`-family trait by one of the anonymous types
212 /// generated for a `||` expression. The ty::ClosureKind informs the
213 /// confirmation step what ClosureKind obligation to emit.
214 ClosureCandidate(/* closure */ DefId, ty::ClosureSubsts<'tcx>, ty::ClosureKind),
216 /// Implementation of a `Fn`-family trait by one of the anonymous
217 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
222 BuiltinObjectCandidate,
224 BuiltinUnsizeCandidate,
227 impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> {
228 type Lifted = SelectionCandidate<'tcx>;
229 fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
231 BuiltinCandidate { has_nested } => {
233 has_nested: has_nested
236 ImplCandidate(def_id) => ImplCandidate(def_id),
237 DefaultImplCandidate(def_id) => DefaultImplCandidate(def_id),
238 DefaultImplObjectCandidate(def_id) => {
239 DefaultImplObjectCandidate(def_id)
241 ProjectionCandidate => ProjectionCandidate,
242 FnPointerCandidate => FnPointerCandidate,
243 ObjectCandidate => ObjectCandidate,
244 BuiltinObjectCandidate => BuiltinObjectCandidate,
245 BuiltinUnsizeCandidate => BuiltinUnsizeCandidate,
247 ParamCandidate(ref trait_ref) => {
248 return tcx.lift(trait_ref).map(ParamCandidate);
250 ClosureCandidate(def_id, ref substs, kind) => {
251 return tcx.lift(substs).map(|substs| {
252 ClosureCandidate(def_id, substs, kind)
259 struct SelectionCandidateSet<'tcx> {
260 // a list of candidates that definitely apply to the current
261 // obligation (meaning: types unify).
262 vec: Vec<SelectionCandidate<'tcx>>,
264 // if this is true, then there were candidates that might or might
265 // not have applied, but we couldn't tell. This occurs when some
266 // of the input types are type variables, in which case there are
267 // various "builtin" rules that might or might not trigger.
271 #[derive(PartialEq,Eq,Debug,Clone)]
272 struct EvaluatedCandidate<'tcx> {
273 candidate: SelectionCandidate<'tcx>,
274 evaluation: EvaluationResult,
277 /// When does the builtin impl for `T: Trait` apply?
278 enum BuiltinImplConditions<'tcx> {
279 /// The impl is conditional on T1,T2,.. : Trait
280 Where(ty::Binder<Vec<Ty<'tcx>>>),
281 /// There is no built-in impl. There may be some other
282 /// candidate (a where-clause or user-defined impl).
284 /// There is *no* impl for this, builtin or not. Ignore
285 /// all where-clauses.
287 /// It is unknown whether there is an impl.
291 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
292 /// The result of trait evaluation. The order is important
293 /// here as the evaluation of a list is the maximum of the
295 enum EvaluationResult {
296 /// Evaluation successful
298 /// Evaluation failed because of recursion - treated as ambiguous
300 /// Evaluation is known to be ambiguous
302 /// Evaluation failed
307 pub struct EvaluationCache<'tcx> {
308 hashmap: RefCell<FnvHashMap<ty::PolyTraitRef<'tcx>, EvaluationResult>>
311 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
312 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
315 freshener: infcx.freshener(),
317 inferred_obligations: SnapshotVec::new(),
321 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
324 freshener: infcx.freshener(),
326 inferred_obligations: SnapshotVec::new(),
330 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
334 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
338 pub fn param_env(&self) -> &'cx ty::ParameterEnvironment<'tcx> {
339 self.infcx.param_env()
342 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
346 pub fn projection_mode(&self) -> ProjectionMode {
347 self.infcx.projection_mode()
350 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
352 fn in_snapshot<R, F>(&mut self, f: F) -> R
353 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
355 // The irrefutable nature of the operation means we don't need to snapshot the
356 // inferred_obligations vector.
357 self.infcx.in_snapshot(|snapshot| f(self, snapshot))
360 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
362 fn probe<R, F>(&mut self, f: F) -> R
363 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
365 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
366 let result = self.infcx.probe(|snapshot| f(self, snapshot));
367 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
371 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
372 /// the transaction fails and s.t. old obligations are retained.
373 fn commit_if_ok<T, E, F>(&mut self, f: F) -> Result<T, E> where
374 F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> Result<T, E>
376 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
377 match self.infcx.commit_if_ok(|snapshot| f(self, snapshot)) {
379 self.inferred_obligations.commit(inferred_obligations_snapshot);
383 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
390 ///////////////////////////////////////////////////////////////////////////
393 // The selection phase tries to identify *how* an obligation will
394 // be resolved. For example, it will identify which impl or
395 // parameter bound is to be used. The process can be inconclusive
396 // if the self type in the obligation is not fully inferred. Selection
397 // can result in an error in one of two ways:
399 // 1. If no applicable impl or parameter bound can be found.
400 // 2. If the output type parameters in the obligation do not match
401 // those specified by the impl/bound. For example, if the obligation
402 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
403 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
405 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
406 /// type environment by performing unification.
407 pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
408 -> SelectionResult<'tcx, Selection<'tcx>> {
409 debug!("select({:?})", obligation);
410 assert!(!obligation.predicate.has_escaping_regions());
412 let dep_node = obligation.predicate.dep_node();
413 let _task = self.tcx().dep_graph.in_task(dep_node);
415 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
416 match self.candidate_from_obligation(&stack)? {
419 let mut candidate = self.confirm_candidate(obligation, candidate)?;
420 // FIXME(#32730) remove this assertion once inferred obligations are propagated
422 assert!(self.inferred_obligations.len() == 0);
423 let inferred_obligations = (*self.inferred_obligations).into_iter().cloned();
424 candidate.nested_obligations_mut().extend(inferred_obligations);
430 ///////////////////////////////////////////////////////////////////////////
433 // Tests whether an obligation can be selected or whether an impl
434 // can be applied to particular types. It skips the "confirmation"
435 // step and hence completely ignores output type parameters.
437 // The result is "true" if the obligation *may* hold and "false" if
438 // we can be sure it does not.
440 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
441 pub fn evaluate_obligation(&mut self,
442 obligation: &PredicateObligation<'tcx>)
445 debug!("evaluate_obligation({:?})",
448 self.probe(|this, _| {
449 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
454 /// Evaluates whether the obligation `obligation` can be satisfied,
455 /// and returns `false` if not certain. However, this is not entirely
456 /// accurate if inference variables are involved.
457 pub fn evaluate_obligation_conservatively(&mut self,
458 obligation: &PredicateObligation<'tcx>)
461 debug!("evaluate_obligation_conservatively({:?})",
464 self.probe(|this, _| {
465 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
470 /// Evaluates the predicates in `predicates` recursively. Note that
471 /// this applies projections in the predicates, and therefore
472 /// is run within an inference probe.
473 fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
474 stack: TraitObligationStackList<'o, 'tcx>,
477 where I : Iterator<Item=&'a PredicateObligation<'tcx>>, 'tcx:'a
479 let mut result = EvaluatedToOk;
480 for obligation in predicates {
481 let eval = self.evaluate_predicate_recursively(stack, obligation);
482 debug!("evaluate_predicate_recursively({:?}) = {:?}",
485 EvaluatedToErr => { return EvaluatedToErr; }
486 EvaluatedToAmbig => { result = EvaluatedToAmbig; }
487 EvaluatedToUnknown => {
488 if result < EvaluatedToUnknown {
489 result = EvaluatedToUnknown;
498 fn evaluate_predicate_recursively<'o>(&mut self,
499 previous_stack: TraitObligationStackList<'o, 'tcx>,
500 obligation: &PredicateObligation<'tcx>)
503 debug!("evaluate_predicate_recursively({:?})",
506 // Check the cache from the tcx of predicates that we know
507 // have been proven elsewhere. This cache only contains
508 // predicates that are global in scope and hence unaffected by
509 // the current environment.
510 if self.tcx().fulfilled_predicates.borrow().check_duplicate(&obligation.predicate) {
511 return EvaluatedToOk;
514 match obligation.predicate {
515 ty::Predicate::Rfc1592(..) => EvaluatedToOk,
517 ty::Predicate::Trait(ref t) => {
518 assert!(!t.has_escaping_regions());
519 let obligation = obligation.with(t.clone());
520 self.evaluate_obligation_recursively(previous_stack, &obligation)
523 ty::Predicate::Equate(ref p) => {
524 // does this code ever run?
525 match self.infcx.equality_predicate(obligation.cause.span, p) {
526 Ok(InferOk { obligations, .. }) => {
527 self.inferred_obligations.extend(obligations);
530 Err(_) => EvaluatedToErr
534 ty::Predicate::WellFormed(ty) => {
535 match ty::wf::obligations(self.infcx, obligation.cause.body_id,
536 ty, obligation.cause.span) {
538 self.evaluate_predicates_recursively(previous_stack, obligations.iter()),
544 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
545 // we do not consider region relationships when
546 // evaluating trait matches
550 ty::Predicate::ObjectSafe(trait_def_id) => {
551 if self.tcx().is_object_safe(trait_def_id) {
558 ty::Predicate::Projection(ref data) => {
559 let project_obligation = obligation.with(data.clone());
560 match project::poly_project_and_unify_type(self, &project_obligation) {
561 Ok(Some(subobligations)) => {
562 self.evaluate_predicates_recursively(previous_stack,
563 subobligations.iter())
574 ty::Predicate::ClosureKind(closure_def_id, kind) => {
575 match self.infcx.closure_kind(closure_def_id) {
576 Some(closure_kind) => {
577 if closure_kind.extends(kind) {
591 fn evaluate_obligation_recursively<'o>(&mut self,
592 previous_stack: TraitObligationStackList<'o, 'tcx>,
593 obligation: &TraitObligation<'tcx>)
596 debug!("evaluate_obligation_recursively({:?})",
599 let stack = self.push_stack(previous_stack, obligation);
600 let fresh_trait_ref = stack.fresh_trait_ref;
601 if let Some(result) = self.check_evaluation_cache(fresh_trait_ref) {
602 debug!("CACHE HIT: EVAL({:?})={:?}",
608 let result = self.evaluate_stack(&stack);
610 debug!("CACHE MISS: EVAL({:?})={:?}",
613 self.insert_evaluation_cache(fresh_trait_ref, result);
618 fn evaluate_stack<'o>(&mut self,
619 stack: &TraitObligationStack<'o, 'tcx>)
622 // In intercrate mode, whenever any of the types are unbound,
623 // there can always be an impl. Even if there are no impls in
624 // this crate, perhaps the type would be unified with
625 // something from another crate that does provide an impl.
627 // In intra mode, we must still be conservative. The reason is
628 // that we want to avoid cycles. Imagine an impl like:
630 // impl<T:Eq> Eq for Vec<T>
632 // and a trait reference like `$0 : Eq` where `$0` is an
633 // unbound variable. When we evaluate this trait-reference, we
634 // will unify `$0` with `Vec<$1>` (for some fresh variable
635 // `$1`), on the condition that `$1 : Eq`. We will then wind
636 // up with many candidates (since that are other `Eq` impls
637 // that apply) and try to winnow things down. This results in
638 // a recursive evaluation that `$1 : Eq` -- as you can
639 // imagine, this is just where we started. To avoid that, we
640 // check for unbound variables and return an ambiguous (hence possible)
641 // match if we've seen this trait before.
643 // This suffices to allow chains like `FnMut` implemented in
644 // terms of `Fn` etc, but we could probably make this more
646 let input_types = stack.fresh_trait_ref.0.input_types();
647 let unbound_input_types = input_types.iter().any(|ty| ty.is_fresh());
648 if unbound_input_types && self.intercrate {
649 debug!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
650 stack.fresh_trait_ref);
651 return EvaluatedToAmbig;
653 if unbound_input_types &&
654 stack.iter().skip(1).any(
655 |prev| self.match_fresh_trait_refs(&stack.fresh_trait_ref,
656 &prev.fresh_trait_ref))
658 debug!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
659 stack.fresh_trait_ref);
660 return EvaluatedToUnknown;
663 // If there is any previous entry on the stack that precisely
664 // matches this obligation, then we can assume that the
665 // obligation is satisfied for now (still all other conditions
666 // must be met of course). One obvious case this comes up is
667 // marker traits like `Send`. Think of a linked list:
669 // struct List<T> { data: T, next: Option<Box<List<T>>> {
671 // `Box<List<T>>` will be `Send` if `T` is `Send` and
672 // `Option<Box<List<T>>>` is `Send`, and in turn
673 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
676 // Note that we do this comparison using the `fresh_trait_ref`
677 // fields. Because these have all been skolemized using
678 // `self.freshener`, we can be sure that (a) this will not
679 // affect the inferencer state and (b) that if we see two
680 // skolemized types with the same index, they refer to the
681 // same unbound type variable.
684 .skip(1) // skip top-most frame
685 .any(|prev| stack.fresh_trait_ref == prev.fresh_trait_ref)
687 debug!("evaluate_stack({:?}) --> recursive",
688 stack.fresh_trait_ref);
689 return EvaluatedToOk;
692 match self.candidate_from_obligation(stack) {
693 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
694 Ok(None) => EvaluatedToAmbig,
695 Err(..) => EvaluatedToErr
699 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
700 /// obligations are met. Returns true if `candidate` remains viable after this further
702 fn evaluate_candidate<'o>(&mut self,
703 stack: &TraitObligationStack<'o, 'tcx>,
704 candidate: &SelectionCandidate<'tcx>)
707 debug!("evaluate_candidate: depth={} candidate={:?}",
708 stack.obligation.recursion_depth, candidate);
709 let result = self.probe(|this, _| {
710 let candidate = (*candidate).clone();
711 match this.confirm_candidate(stack.obligation, candidate) {
713 this.evaluate_predicates_recursively(
715 selection.nested_obligations().iter())
717 Err(..) => EvaluatedToErr
720 debug!("evaluate_candidate: depth={} result={:?}",
721 stack.obligation.recursion_depth, result);
725 fn check_evaluation_cache(&self, trait_ref: ty::PolyTraitRef<'tcx>)
726 -> Option<EvaluationResult>
728 if self.can_use_global_caches() {
729 let cache = self.tcx().evaluation_cache.hashmap.borrow();
730 if let Some(cached) = cache.get(&trait_ref) {
731 return Some(cached.clone());
734 self.infcx.evaluation_cache.hashmap.borrow().get(&trait_ref).cloned()
737 fn insert_evaluation_cache(&mut self,
738 trait_ref: ty::PolyTraitRef<'tcx>,
739 result: EvaluationResult)
741 // Avoid caching results that depend on more than just the trait-ref:
742 // The stack can create EvaluatedToUnknown, and closure signatures
743 // being yet uninferred can create "spurious" EvaluatedToAmbig
744 // and EvaluatedToOk.
745 if result == EvaluatedToUnknown ||
746 ((result == EvaluatedToAmbig || result == EvaluatedToOk)
747 && trait_ref.has_closure_types())
752 if self.can_use_global_caches() {
753 let mut cache = self.tcx().evaluation_cache.hashmap.borrow_mut();
754 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
755 cache.insert(trait_ref, result);
760 self.infcx.evaluation_cache.hashmap.borrow_mut().insert(trait_ref, result);
763 ///////////////////////////////////////////////////////////////////////////
764 // CANDIDATE ASSEMBLY
766 // The selection process begins by examining all in-scope impls,
767 // caller obligations, and so forth and assembling a list of
768 // candidates. See `README.md` and the `Candidate` type for more
771 fn candidate_from_obligation<'o>(&mut self,
772 stack: &TraitObligationStack<'o, 'tcx>)
773 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
775 // Watch out for overflow. This intentionally bypasses (and does
776 // not update) the cache.
777 let recursion_limit = self.infcx.tcx.sess.recursion_limit.get();
778 if stack.obligation.recursion_depth >= recursion_limit {
779 self.infcx().report_overflow_error(&stack.obligation, true);
782 // Check the cache. Note that we skolemize the trait-ref
783 // separately rather than using `stack.fresh_trait_ref` -- this
784 // is because we want the unbound variables to be replaced
785 // with fresh skolemized types starting from index 0.
786 let cache_fresh_trait_pred =
787 self.infcx.freshen(stack.obligation.predicate.clone());
788 debug!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
789 cache_fresh_trait_pred,
791 assert!(!stack.obligation.predicate.has_escaping_regions());
793 match self.check_candidate_cache(&cache_fresh_trait_pred) {
795 debug!("CACHE HIT: SELECT({:?})={:?}",
796 cache_fresh_trait_pred,
803 // If no match, compute result and insert into cache.
804 let candidate = self.candidate_from_obligation_no_cache(stack);
806 if self.should_update_candidate_cache(&cache_fresh_trait_pred, &candidate) {
807 debug!("CACHE MISS: SELECT({:?})={:?}",
808 cache_fresh_trait_pred, candidate);
809 self.insert_candidate_cache(cache_fresh_trait_pred, candidate.clone());
815 // Treat negative impls as unimplemented
816 fn filter_negative_impls(&self, candidate: SelectionCandidate<'tcx>)
817 -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
818 if let ImplCandidate(def_id) = candidate {
819 if self.tcx().trait_impl_polarity(def_id) == Some(hir::ImplPolarity::Negative) {
820 return Err(Unimplemented)
826 fn candidate_from_obligation_no_cache<'o>(&mut self,
827 stack: &TraitObligationStack<'o, 'tcx>)
828 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
830 if stack.obligation.predicate.references_error() {
831 // If we encounter a `TyError`, we generally prefer the
832 // most "optimistic" result in response -- that is, the
833 // one least likely to report downstream errors. But
834 // because this routine is shared by coherence and by
835 // trait selection, there isn't an obvious "right" choice
836 // here in that respect, so we opt to just return
837 // ambiguity and let the upstream clients sort it out.
841 if !self.is_knowable(stack) {
842 debug!("coherence stage: not knowable");
846 let candidate_set = self.assemble_candidates(stack)?;
848 if candidate_set.ambiguous {
849 debug!("candidate set contains ambig");
853 let mut candidates = candidate_set.vec;
855 debug!("assembled {} candidates for {:?}: {:?}",
860 // At this point, we know that each of the entries in the
861 // candidate set is *individually* applicable. Now we have to
862 // figure out if they contain mutual incompatibilities. This
863 // frequently arises if we have an unconstrained input type --
864 // for example, we are looking for $0:Eq where $0 is some
865 // unconstrained type variable. In that case, we'll get a
866 // candidate which assumes $0 == int, one that assumes $0 ==
867 // usize, etc. This spells an ambiguity.
869 // If there is more than one candidate, first winnow them down
870 // by considering extra conditions (nested obligations and so
871 // forth). We don't winnow if there is exactly one
872 // candidate. This is a relatively minor distinction but it
873 // can lead to better inference and error-reporting. An
874 // example would be if there was an impl:
876 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
878 // and we were to see some code `foo.push_clone()` where `boo`
879 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
880 // we were to winnow, we'd wind up with zero candidates.
881 // Instead, we select the right impl now but report `Bar does
882 // not implement Clone`.
883 if candidates.len() == 1 {
884 return self.filter_negative_impls(candidates.pop().unwrap());
887 // Winnow, but record the exact outcome of evaluation, which
888 // is needed for specialization.
889 let mut candidates: Vec<_> = candidates.into_iter().filter_map(|c| {
890 let eval = self.evaluate_candidate(stack, &c);
891 if eval.may_apply() {
892 Some(EvaluatedCandidate {
901 // If there are STILL multiple candidate, we can further
902 // reduce the list by dropping duplicates -- including
903 // resolving specializations.
904 if candidates.len() > 1 {
906 while i < candidates.len() {
908 (0..candidates.len())
910 .any(|j| self.candidate_should_be_dropped_in_favor_of(&candidates[i],
913 debug!("Dropping candidate #{}/{}: {:?}",
914 i, candidates.len(), candidates[i]);
915 candidates.swap_remove(i);
917 debug!("Retaining candidate #{}/{}: {:?}",
918 i, candidates.len(), candidates[i]);
924 // If there are *STILL* multiple candidates, give up and
926 if candidates.len() > 1 {
927 debug!("multiple matches, ambig");
931 // If there are *NO* candidates, then there are no impls --
932 // that we know of, anyway. Note that in the case where there
933 // are unbound type variables within the obligation, it might
934 // be the case that you could still satisfy the obligation
935 // from another crate by instantiating the type variables with
936 // a type from another crate that does have an impl. This case
937 // is checked for in `evaluate_stack` (and hence users
938 // who might care about this case, like coherence, should use
940 if candidates.is_empty() {
941 return Err(Unimplemented);
944 // Just one candidate left.
945 self.filter_negative_impls(candidates.pop().unwrap().candidate)
948 fn is_knowable<'o>(&mut self,
949 stack: &TraitObligationStack<'o, 'tcx>)
952 debug!("is_knowable(intercrate={})", self.intercrate);
954 if !self.intercrate {
958 let obligation = &stack.obligation;
959 let predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
961 // ok to skip binder because of the nature of the
962 // trait-ref-is-knowable check, which does not care about
964 let trait_ref = &predicate.skip_binder().trait_ref;
966 coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
969 /// Returns true if the global caches can be used.
970 /// Do note that if the type itself is not in the
971 /// global tcx, the local caches will be used.
972 fn can_use_global_caches(&self) -> bool {
973 // If there are any where-clauses in scope, then we always use
974 // a cache local to this particular scope. Otherwise, we
975 // switch to a global cache. We used to try and draw
976 // finer-grained distinctions, but that led to a serious of
977 // annoying and weird bugs like #22019 and #18290. This simple
978 // rule seems to be pretty clearly safe and also still retains
979 // a very high hit rate (~95% when compiling rustc).
980 if !self.param_env().caller_bounds.is_empty() {
984 // Avoid using the master cache during coherence and just rely
985 // on the local cache. This effectively disables caching
986 // during coherence. It is really just a simplification to
987 // avoid us having to fear that coherence results "pollute"
988 // the master cache. Since coherence executes pretty quickly,
989 // it's not worth going to more trouble to increase the
990 // hit-rate I don't think.
995 // Otherwise, we can use the global cache.
999 fn check_candidate_cache(&mut self,
1000 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
1001 -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
1003 let trait_ref = &cache_fresh_trait_pred.0.trait_ref;
1004 if self.can_use_global_caches() {
1005 let cache = self.tcx().selection_cache.hashmap.borrow();
1006 if let Some(cached) = cache.get(&trait_ref) {
1007 return Some(cached.clone());
1010 self.infcx.selection_cache.hashmap.borrow().get(trait_ref).cloned()
1013 fn insert_candidate_cache(&mut self,
1014 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1015 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
1017 let trait_ref = cache_fresh_trait_pred.0.trait_ref;
1018 if self.can_use_global_caches() {
1019 let mut cache = self.tcx().selection_cache.hashmap.borrow_mut();
1020 if let Some(trait_ref) = self.tcx().lift_to_global(&trait_ref) {
1021 if let Some(candidate) = self.tcx().lift_to_global(&candidate) {
1022 cache.insert(trait_ref, candidate);
1028 self.infcx.selection_cache.hashmap.borrow_mut().insert(trait_ref, candidate);
1031 fn should_update_candidate_cache(&mut self,
1032 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>,
1033 candidate: &SelectionResult<'tcx, SelectionCandidate<'tcx>>)
1036 // In general, it's a good idea to cache results, even
1037 // ambiguous ones, to save us some trouble later. But we have
1038 // to be careful not to cache results that could be
1039 // invalidated later by advances in inference. Normally, this
1040 // is not an issue, because any inference variables whose
1041 // types are not yet bound are "freshened" in the cache key,
1042 // which means that if we later get the same request once that
1043 // type variable IS bound, we'll have a different cache key.
1044 // For example, if we have `Vec<_#0t> : Foo`, and `_#0t` is
1045 // not yet known, we may cache the result as `None`. But if
1046 // later `_#0t` is bound to `Bar`, then when we freshen we'll
1047 // have `Vec<Bar> : Foo` as the cache key.
1049 // HOWEVER, it CAN happen that we get an ambiguity result in
1050 // one particular case around closures where the cache key
1051 // would not change. That is when the precise types of the
1052 // upvars that a closure references have not yet been figured
1053 // out (i.e., because it is not yet known if they are captured
1054 // by ref, and if by ref, what kind of ref). In these cases,
1055 // when matching a builtin bound, we will yield back an
1056 // ambiguous result. But the *cache key* is just the closure type,
1057 // it doesn't capture the state of the upvar computation.
1059 // To avoid this trap, just don't cache ambiguous results if
1060 // the self-type contains no inference byproducts (that really
1061 // shouldn't happen in other circumstances anyway, given
1065 Ok(Some(_)) | Err(_) => true,
1067 cache_fresh_trait_pred.0.trait_ref.substs.types.has_infer_types()
1072 fn assemble_candidates<'o>(&mut self,
1073 stack: &TraitObligationStack<'o, 'tcx>)
1074 -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
1076 let TraitObligationStack { obligation, .. } = *stack;
1077 let ref obligation = Obligation {
1078 cause: obligation.cause.clone(),
1079 recursion_depth: obligation.recursion_depth,
1080 predicate: self.infcx().resolve_type_vars_if_possible(&obligation.predicate)
1083 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1084 // FIXME(#20297): Self is a type variable (e.g. `_: AsRef<str>`).
1086 // This is somewhat problematic, as the current scheme can't really
1087 // handle it turning to be a projection. This does end up as truly
1088 // ambiguous in most cases anyway.
1090 // Until this is fixed, take the fast path out - this also improves
1091 // performance by preventing assemble_candidates_from_impls from
1092 // matching every impl for this trait.
1093 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
1096 let mut candidates = SelectionCandidateSet {
1101 // Other bounds. Consider both in-scope bounds from fn decl
1102 // and applicable impls. There is a certain set of precedence rules here.
1104 match self.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) {
1105 Some(ty::BoundCopy) => {
1106 debug!("obligation self ty is {:?}",
1107 obligation.predicate.0.self_ty());
1109 // User-defined copy impls are permitted, but only for
1110 // structs and enums.
1111 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1113 // For other types, we'll use the builtin rules.
1114 let copy_conditions = self.copy_conditions(obligation);
1115 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1117 Some(ty::BoundSized) => {
1118 // Sized is never implementable by end-users, it is
1119 // always automatically computed.
1120 let sized_conditions = self.sized_conditions(obligation);
1121 self.assemble_builtin_bound_candidates(sized_conditions,
1125 None if self.tcx().lang_items.unsize_trait() ==
1126 Some(obligation.predicate.def_id()) => {
1127 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1130 Some(ty::BoundSend) |
1131 Some(ty::BoundSync) |
1133 self.assemble_closure_candidates(obligation, &mut candidates)?;
1134 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1135 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1136 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1140 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1141 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1142 // Default implementations have lower priority, so we only
1143 // consider triggering a default if there is no other impl that can apply.
1144 if candidates.vec.is_empty() {
1145 self.assemble_candidates_from_default_impls(obligation, &mut candidates)?;
1147 debug!("candidate list size: {}", candidates.vec.len());
1151 fn assemble_candidates_from_projected_tys(&mut self,
1152 obligation: &TraitObligation<'tcx>,
1153 candidates: &mut SelectionCandidateSet<'tcx>)
1155 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1157 // FIXME(#20297) -- just examining the self-type is very simplistic
1159 // before we go into the whole skolemization thing, just
1160 // quickly check if the self-type is a projection at all.
1161 let trait_def_id = match obligation.predicate.0.trait_ref.self_ty().sty {
1162 ty::TyProjection(ref data) => data.trait_ref.def_id,
1163 ty::TyInfer(ty::TyVar(_)) => {
1164 span_bug!(obligation.cause.span,
1165 "Self=_ should have been handled by assemble_candidates");
1170 debug!("assemble_candidates_for_projected_tys: trait_def_id={:?}",
1173 let result = self.probe(|this, snapshot| {
1174 this.match_projection_obligation_against_bounds_from_trait(obligation,
1179 candidates.vec.push(ProjectionCandidate);
1183 fn match_projection_obligation_against_bounds_from_trait(
1185 obligation: &TraitObligation<'tcx>,
1186 snapshot: &infer::CombinedSnapshot)
1189 let poly_trait_predicate =
1190 self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1191 let (skol_trait_predicate, skol_map) =
1192 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot);
1193 debug!("match_projection_obligation_against_bounds_from_trait: \
1194 skol_trait_predicate={:?} skol_map={:?}",
1195 skol_trait_predicate,
1198 let projection_trait_ref = match skol_trait_predicate.trait_ref.self_ty().sty {
1199 ty::TyProjection(ref data) => &data.trait_ref,
1202 obligation.cause.span,
1203 "match_projection_obligation_against_bounds_from_trait() called \
1204 but self-ty not a projection: {:?}",
1205 skol_trait_predicate.trait_ref.self_ty());
1208 debug!("match_projection_obligation_against_bounds_from_trait: \
1209 projection_trait_ref={:?}",
1210 projection_trait_ref);
1212 let trait_predicates = self.tcx().lookup_predicates(projection_trait_ref.def_id);
1213 let bounds = trait_predicates.instantiate(self.tcx(), projection_trait_ref.substs);
1214 debug!("match_projection_obligation_against_bounds_from_trait: \
1218 let matching_bound =
1219 util::elaborate_predicates(self.tcx(), bounds.predicates.into_vec())
1223 |this, _| this.match_projection(obligation,
1225 skol_trait_predicate.trait_ref.clone(),
1229 debug!("match_projection_obligation_against_bounds_from_trait: \
1230 matching_bound={:?}",
1232 match matching_bound {
1235 // Repeat the successful match, if any, this time outside of a probe.
1236 let result = self.match_projection(obligation,
1238 skol_trait_predicate.trait_ref.clone(),
1242 self.infcx.pop_skolemized(skol_map, snapshot);
1250 fn match_projection(&mut self,
1251 obligation: &TraitObligation<'tcx>,
1252 trait_bound: ty::PolyTraitRef<'tcx>,
1253 skol_trait_ref: ty::TraitRef<'tcx>,
1254 skol_map: &infer::SkolemizationMap,
1255 snapshot: &infer::CombinedSnapshot)
1258 assert!(!skol_trait_ref.has_escaping_regions());
1259 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
1260 match self.infcx.sub_poly_trait_refs(false,
1262 trait_bound.clone(),
1263 ty::Binder(skol_trait_ref.clone())) {
1264 Ok(InferOk { obligations, .. }) => {
1265 self.inferred_obligations.extend(obligations);
1267 Err(_) => { return false; }
1270 self.infcx.leak_check(false, obligation.cause.span, skol_map, snapshot).is_ok()
1273 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1274 /// supplied to find out whether it is listed among them.
1276 /// Never affects inference environment.
1277 fn assemble_candidates_from_caller_bounds<'o>(&mut self,
1278 stack: &TraitObligationStack<'o, 'tcx>,
1279 candidates: &mut SelectionCandidateSet<'tcx>)
1280 -> Result<(),SelectionError<'tcx>>
1282 debug!("assemble_candidates_from_caller_bounds({:?})",
1286 self.param_env().caller_bounds
1288 .filter_map(|o| o.to_opt_poly_trait_ref());
1290 let matching_bounds =
1292 |bound| self.evaluate_where_clause(stack, bound.clone()).may_apply());
1294 let param_candidates =
1295 matching_bounds.map(|bound| ParamCandidate(bound));
1297 candidates.vec.extend(param_candidates);
1302 fn evaluate_where_clause<'o>(&mut self,
1303 stack: &TraitObligationStack<'o, 'tcx>,
1304 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
1307 self.probe(move |this, _| {
1308 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1309 Ok(obligations) => {
1310 this.evaluate_predicates_recursively(stack.list(), obligations.iter())
1312 Err(()) => EvaluatedToErr
1317 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1318 /// FnMut<..>` where `X` is a closure type.
1320 /// Note: the type parameters on a closure candidate are modeled as *output* type
1321 /// parameters and hence do not affect whether this trait is a match or not. They will be
1322 /// unified during the confirmation step.
1323 fn assemble_closure_candidates(&mut self,
1324 obligation: &TraitObligation<'tcx>,
1325 candidates: &mut SelectionCandidateSet<'tcx>)
1326 -> Result<(),SelectionError<'tcx>>
1328 let kind = match self.tcx().lang_items.fn_trait_kind(obligation.predicate.0.def_id()) {
1330 None => { return Ok(()); }
1333 // ok to skip binder because the substs on closure types never
1334 // touch bound regions, they just capture the in-scope
1335 // type/region parameters
1336 let self_ty = *obligation.self_ty().skip_binder();
1337 let (closure_def_id, substs) = match self_ty.sty {
1338 ty::TyClosure(id, substs) => (id, substs),
1339 ty::TyInfer(ty::TyVar(_)) => {
1340 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1341 candidates.ambiguous = true;
1344 _ => { return Ok(()); }
1347 debug!("assemble_unboxed_candidates: self_ty={:?} kind={:?} obligation={:?}",
1352 match self.infcx.closure_kind(closure_def_id) {
1353 Some(closure_kind) => {
1354 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
1355 if closure_kind.extends(kind) {
1356 candidates.vec.push(ClosureCandidate(closure_def_id, substs, kind));
1360 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1361 candidates.vec.push(ClosureCandidate(closure_def_id, substs, kind));
1368 /// Implement one of the `Fn()` family for a fn pointer.
1369 fn assemble_fn_pointer_candidates(&mut self,
1370 obligation: &TraitObligation<'tcx>,
1371 candidates: &mut SelectionCandidateSet<'tcx>)
1372 -> Result<(),SelectionError<'tcx>>
1374 // We provide impl of all fn traits for fn pointers.
1375 if self.tcx().lang_items.fn_trait_kind(obligation.predicate.def_id()).is_none() {
1379 // ok to skip binder because what we are inspecting doesn't involve bound regions
1380 let self_ty = *obligation.self_ty().skip_binder();
1382 ty::TyInfer(ty::TyVar(_)) => {
1383 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1384 candidates.ambiguous = true; // could wind up being a fn() type
1387 // provide an impl, but only for suitable `fn` pointers
1388 ty::TyFnDef(_, _, &ty::BareFnTy {
1389 unsafety: hir::Unsafety::Normal,
1391 sig: ty::Binder(ty::FnSig {
1393 output: ty::FnConverging(_),
1397 ty::TyFnPtr(&ty::BareFnTy {
1398 unsafety: hir::Unsafety::Normal,
1400 sig: ty::Binder(ty::FnSig {
1402 output: ty::FnConverging(_),
1406 candidates.vec.push(FnPointerCandidate);
1415 /// Search for impls that might apply to `obligation`.
1416 fn assemble_candidates_from_impls(&mut self,
1417 obligation: &TraitObligation<'tcx>,
1418 candidates: &mut SelectionCandidateSet<'tcx>)
1419 -> Result<(), SelectionError<'tcx>>
1421 debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
1423 let def = self.tcx().lookup_trait_def(obligation.predicate.def_id());
1425 def.for_each_relevant_impl(
1427 obligation.predicate.0.trait_ref.self_ty(),
1429 self.probe(|this, snapshot| { /* [1] */
1430 match this.match_impl(impl_def_id, obligation, snapshot) {
1432 candidates.vec.push(ImplCandidate(impl_def_id));
1434 // NB: we can safely drop the skol map
1435 // since we are in a probe [1]
1436 mem::drop(skol_map);
1447 fn assemble_candidates_from_default_impls(&mut self,
1448 obligation: &TraitObligation<'tcx>,
1449 candidates: &mut SelectionCandidateSet<'tcx>)
1450 -> Result<(), SelectionError<'tcx>>
1452 // OK to skip binder here because the tests we do below do not involve bound regions
1453 let self_ty = *obligation.self_ty().skip_binder();
1454 debug!("assemble_candidates_from_default_impls(self_ty={:?})", self_ty);
1456 let def_id = obligation.predicate.def_id();
1458 if self.tcx().trait_has_default_impl(def_id) {
1460 ty::TyTrait(..) => {
1461 // For object types, we don't know what the closed
1462 // over types are. For most traits, this means we
1463 // conservatively say nothing; a candidate may be
1464 // added by `assemble_candidates_from_object_ty`.
1465 // However, for the kind of magic reflect trait,
1466 // we consider it to be implemented even for
1467 // object types, because it just lets you reflect
1468 // onto the object type, not into the object's
1470 if self.tcx().has_attr(def_id, "rustc_reflect_like") {
1471 candidates.vec.push(DefaultImplObjectCandidate(def_id));
1475 ty::TyProjection(..) => {
1476 // In these cases, we don't know what the actual
1477 // type is. Therefore, we cannot break it down
1478 // into its constituent types. So we don't
1479 // consider the `..` impl but instead just add no
1480 // candidates: this means that typeck will only
1481 // succeed if there is another reason to believe
1482 // that this obligation holds. That could be a
1483 // where-clause or, in the case of an object type,
1484 // it could be that the object type lists the
1485 // trait (e.g. `Foo+Send : Send`). See
1486 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1487 // for an example of a test case that exercises
1490 ty::TyInfer(ty::TyVar(_)) => {
1491 // the defaulted impl might apply, we don't know
1492 candidates.ambiguous = true;
1495 candidates.vec.push(DefaultImplCandidate(def_id.clone()))
1503 /// Search for impls that might apply to `obligation`.
1504 fn assemble_candidates_from_object_ty(&mut self,
1505 obligation: &TraitObligation<'tcx>,
1506 candidates: &mut SelectionCandidateSet<'tcx>)
1508 debug!("assemble_candidates_from_object_ty(self_ty={:?})",
1509 obligation.self_ty().skip_binder());
1511 // Object-safety candidates are only applicable to object-safe
1512 // traits. Including this check is useful because it helps
1513 // inference in cases of traits like `BorrowFrom`, which are
1514 // not object-safe, and which rely on being able to infer the
1515 // self-type from one of the other inputs. Without this check,
1516 // these cases wind up being considered ambiguous due to a
1517 // (spurious) ambiguity introduced here.
1518 let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
1519 if !self.tcx().is_object_safe(predicate_trait_ref.def_id()) {
1523 self.probe(|this, _snapshot| {
1524 // the code below doesn't care about regions, and the
1525 // self-ty here doesn't escape this probe, so just erase
1527 let self_ty = this.tcx().erase_late_bound_regions(&obligation.self_ty());
1528 let poly_trait_ref = match self_ty.sty {
1529 ty::TyTrait(ref data) => {
1530 match this.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) {
1531 Some(bound @ ty::BoundSend) | Some(bound @ ty::BoundSync) => {
1532 if data.bounds.builtin_bounds.contains(&bound) {
1533 debug!("assemble_candidates_from_object_ty: matched builtin bound, \
1534 pushing candidate");
1535 candidates.vec.push(BuiltinObjectCandidate);
1542 data.principal_trait_ref_with_self_ty(this.tcx(), self_ty)
1544 ty::TyInfer(ty::TyVar(_)) => {
1545 debug!("assemble_candidates_from_object_ty: ambiguous");
1546 candidates.ambiguous = true; // could wind up being an object type
1554 debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1557 // Count only those upcast versions that match the trait-ref
1558 // we are looking for. Specifically, do not only check for the
1559 // correct trait, but also the correct type parameters.
1560 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1561 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1562 let upcast_trait_refs =
1563 util::supertraits(this.tcx(), poly_trait_ref)
1564 .filter(|upcast_trait_ref| {
1565 this.probe(|this, _| {
1566 let upcast_trait_ref = upcast_trait_ref.clone();
1567 this.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok()
1572 if upcast_trait_refs > 1 {
1573 // can be upcast in many ways; need more type information
1574 candidates.ambiguous = true;
1575 } else if upcast_trait_refs == 1 {
1576 candidates.vec.push(ObjectCandidate);
1581 /// Search for unsizing that might apply to `obligation`.
1582 fn assemble_candidates_for_unsizing(&mut self,
1583 obligation: &TraitObligation<'tcx>,
1584 candidates: &mut SelectionCandidateSet<'tcx>) {
1585 // We currently never consider higher-ranked obligations e.g.
1586 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1587 // because they are a priori invalid, and we could potentially add support
1588 // for them later, it's just that there isn't really a strong need for it.
1589 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1590 // impl, and those are generally applied to concrete types.
1592 // That said, one might try to write a fn with a where clause like
1593 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1594 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1595 // Still, you'd be more likely to write that where clause as
1597 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1598 // obligation above. Should be possible to extend this in the future.
1599 let source = match self.tcx().no_late_bound_regions(&obligation.self_ty()) {
1602 // Don't add any candidates if there are bound regions.
1606 let target = obligation.predicate.0.input_types()[0];
1608 debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1611 let may_apply = match (&source.sty, &target.sty) {
1612 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1613 (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => {
1614 // Upcasts permit two things:
1616 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1617 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1619 // Note that neither of these changes requires any
1620 // change at runtime. Eventually this will be
1623 // We always upcast when we can because of reason
1624 // #2 (region bounds).
1625 data_a.principal.def_id() == data_a.principal.def_id() &&
1626 data_a.bounds.builtin_bounds.is_superset(&data_b.bounds.builtin_bounds)
1630 (_, &ty::TyTrait(_)) => true,
1632 // Ambiguous handling is below T -> Trait, because inference
1633 // variables can still implement Unsize<Trait> and nested
1634 // obligations will have the final say (likely deferred).
1635 (&ty::TyInfer(ty::TyVar(_)), _) |
1636 (_, &ty::TyInfer(ty::TyVar(_))) => {
1637 debug!("assemble_candidates_for_unsizing: ambiguous");
1638 candidates.ambiguous = true;
1643 (&ty::TyArray(_, _), &ty::TySlice(_)) => true,
1645 // Struct<T> -> Struct<U>.
1646 (&ty::TyStruct(def_id_a, _), &ty::TyStruct(def_id_b, _)) => {
1647 def_id_a == def_id_b
1654 candidates.vec.push(BuiltinUnsizeCandidate);
1658 ///////////////////////////////////////////////////////////////////////////
1661 // Winnowing is the process of attempting to resolve ambiguity by
1662 // probing further. During the winnowing process, we unify all
1663 // type variables (ignoring skolemization) and then we also
1664 // attempt to evaluate recursive bounds to see if they are
1667 /// Returns true if `candidate_i` should be dropped in favor of
1668 /// `candidate_j`. Generally speaking we will drop duplicate
1669 /// candidates and prefer where-clause candidates.
1670 /// Returns true if `victim` should be dropped in favor of
1671 /// `other`. Generally speaking we will drop duplicate
1672 /// candidates and prefer where-clause candidates.
1674 /// See the comment for "SelectionCandidate" for more details.
1675 fn candidate_should_be_dropped_in_favor_of<'o>(
1677 victim: &EvaluatedCandidate<'tcx>,
1678 other: &EvaluatedCandidate<'tcx>)
1681 if victim.candidate == other.candidate {
1685 match other.candidate {
1687 ParamCandidate(_) | ProjectionCandidate => match victim.candidate {
1688 DefaultImplCandidate(..) => {
1690 "default implementations shouldn't be recorded \
1691 when there are other valid candidates");
1694 ClosureCandidate(..) |
1695 FnPointerCandidate |
1696 BuiltinObjectCandidate |
1697 BuiltinUnsizeCandidate |
1698 DefaultImplObjectCandidate(..) |
1699 BuiltinCandidate { .. } => {
1700 // We have a where-clause so don't go around looking
1705 ProjectionCandidate => {
1706 // Arbitrarily give param candidates priority
1707 // over projection and object candidates.
1710 ParamCandidate(..) => false,
1712 ImplCandidate(other_def) => {
1713 // See if we can toss out `victim` based on specialization.
1714 // This requires us to know *for sure* that the `other` impl applies
1715 // i.e. EvaluatedToOk:
1716 if other.evaluation == EvaluatedToOk {
1717 if let ImplCandidate(victim_def) = victim.candidate {
1718 let tcx = self.tcx().global_tcx();
1719 return traits::specializes(tcx, other_def, victim_def);
1729 ///////////////////////////////////////////////////////////////////////////
1732 // These cover the traits that are built-in to the language
1733 // itself. This includes `Copy` and `Sized` for sure. For the
1734 // moment, it also includes `Send` / `Sync` and a few others, but
1735 // those will hopefully change to library-defined traits in the
1738 // HACK: if this returns an error, selection exits without considering
1740 fn assemble_builtin_bound_candidates<'o>(&mut self,
1741 conditions: BuiltinImplConditions<'tcx>,
1742 candidates: &mut SelectionCandidateSet<'tcx>)
1743 -> Result<(),SelectionError<'tcx>>
1746 BuiltinImplConditions::Where(nested) => {
1747 debug!("builtin_bound: nested={:?}", nested);
1748 candidates.vec.push(BuiltinCandidate {
1749 has_nested: nested.skip_binder().len() > 0
1753 BuiltinImplConditions::None => { Ok(()) }
1754 BuiltinImplConditions::Ambiguous => {
1755 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
1756 Ok(candidates.ambiguous = true)
1758 BuiltinImplConditions::Never => { Err(Unimplemented) }
1762 fn sized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
1763 -> BuiltinImplConditions<'tcx>
1765 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
1767 // NOTE: binder moved to (*)
1768 let self_ty = self.infcx.shallow_resolve(
1769 obligation.predicate.skip_binder().self_ty());
1772 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
1773 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
1774 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyRawPtr(..) |
1775 ty::TyChar | ty::TyBox(_) | ty::TyRef(..) |
1776 ty::TyArray(..) | ty::TyClosure(..) |
1778 // safe for everything
1779 Where(ty::Binder(Vec::new()))
1782 ty::TyStr | ty::TySlice(_) | ty::TyTrait(..) => Never,
1784 ty::TyTuple(tys) => {
1785 // FIXME(#33242) we only need to constrain the last field
1786 Where(ty::Binder(tys.to_vec()))
1789 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
1790 let sized_crit = def.sized_constraint(self.tcx());
1791 // (*) binder moved here
1792 Where(ty::Binder(match sized_crit.sty {
1793 ty::TyTuple(tys) => tys.to_vec().subst(self.tcx(), substs),
1794 ty::TyBool => vec![],
1795 _ => vec![sized_crit.subst(self.tcx(), substs)]
1799 ty::TyProjection(_) | ty::TyParam(_) => None,
1800 ty::TyInfer(ty::TyVar(_)) => Ambiguous,
1802 ty::TyInfer(ty::FreshTy(_))
1803 | ty::TyInfer(ty::FreshIntTy(_))
1804 | ty::TyInfer(ty::FreshFloatTy(_)) => {
1805 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
1811 fn copy_conditions(&mut self, obligation: &TraitObligation<'tcx>)
1812 -> BuiltinImplConditions<'tcx>
1814 // NOTE: binder moved to (*)
1815 let self_ty = self.infcx.shallow_resolve(
1816 obligation.predicate.skip_binder().self_ty());
1818 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
1821 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
1822 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
1823 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyChar |
1824 ty::TyRawPtr(..) | ty::TyError |
1825 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
1826 Where(ty::Binder(Vec::new()))
1829 ty::TyBox(_) | ty::TyTrait(..) | ty::TyStr | ty::TySlice(..) |
1831 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
1835 ty::TyArray(element_ty, _) => {
1836 // (*) binder moved here
1837 Where(ty::Binder(vec![element_ty]))
1840 ty::TyTuple(tys) => {
1841 // (*) binder moved here
1842 Where(ty::Binder(tys.to_vec()))
1845 ty::TyStruct(..) | ty::TyEnum(..) | ty::TyProjection(..) | ty::TyParam(..) => {
1846 // Fallback to whatever user-defined impls exist in this case.
1850 ty::TyInfer(ty::TyVar(_)) => {
1851 // Unbound type variable. Might or might not have
1852 // applicable impls and so forth, depending on what
1853 // those type variables wind up being bound to.
1857 ty::TyInfer(ty::FreshTy(_))
1858 | ty::TyInfer(ty::FreshIntTy(_))
1859 | ty::TyInfer(ty::FreshFloatTy(_)) => {
1860 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
1866 /// For default impls, we need to break apart a type into its
1867 /// "constituent types" -- meaning, the types that it contains.
1869 /// Here are some (simple) examples:
1872 /// (i32, u32) -> [i32, u32]
1873 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1874 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1875 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1877 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
1887 ty::TyInfer(ty::IntVar(_)) |
1888 ty::TyInfer(ty::FloatVar(_)) |
1895 ty::TyProjection(..) |
1896 ty::TyInfer(ty::TyVar(_)) |
1897 ty::TyInfer(ty::FreshTy(_)) |
1898 ty::TyInfer(ty::FreshIntTy(_)) |
1899 ty::TyInfer(ty::FreshFloatTy(_)) => {
1900 bug!("asked to assemble constituent types of unexpected type: {:?}",
1904 ty::TyBox(referent_ty) => { // Box<T>
1908 ty::TyRawPtr(ty::TypeAndMut { ty: element_ty, ..}) |
1909 ty::TyRef(_, ty::TypeAndMut { ty: element_ty, ..}) => {
1913 ty::TyArray(element_ty, _) | ty::TySlice(element_ty) => {
1917 ty::TyTuple(ref tys) => {
1918 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1922 ty::TyClosure(_, ref substs) => {
1923 // FIXME(#27086). We are invariant w/r/t our
1924 // substs.func_substs, but we don't see them as
1925 // constituent types; this seems RIGHT but also like
1926 // something that a normal type couldn't simulate. Is
1927 // this just a gap with the way that PhantomData and
1928 // OIBIT interact? That is, there is no way to say
1929 // "make me invariant with respect to this TYPE, but
1930 // do not act as though I can reach it"
1931 substs.upvar_tys.to_vec()
1934 // for `PhantomData<T>`, we pass `T`
1935 ty::TyStruct(def, substs) if def.is_phantom_data() => {
1936 substs.types.get_slice(TypeSpace).to_vec()
1939 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
1941 .map(|f| f.ty(self.tcx(), substs))
1947 fn collect_predicates_for_types(&mut self,
1948 cause: ObligationCause<'tcx>,
1949 recursion_depth: usize,
1950 trait_def_id: DefId,
1951 types: ty::Binder<Vec<Ty<'tcx>>>)
1952 -> Vec<PredicateObligation<'tcx>>
1954 // Because the types were potentially derived from
1955 // higher-ranked obligations they may reference late-bound
1956 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
1957 // yield a type like `for<'a> &'a int`. In general, we
1958 // maintain the invariant that we never manipulate bound
1959 // regions, so we have to process these bound regions somehow.
1961 // The strategy is to:
1963 // 1. Instantiate those regions to skolemized regions (e.g.,
1964 // `for<'a> &'a int` becomes `&0 int`.
1965 // 2. Produce something like `&'0 int : Copy`
1966 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
1968 types.skip_binder().into_iter().flat_map(|ty| { // binder moved -\
1969 let ty: ty::Binder<Ty<'tcx>> = ty::Binder(ty); // <----------/
1971 self.in_snapshot(|this, snapshot| {
1972 let (skol_ty, skol_map) =
1973 this.infcx().skolemize_late_bound_regions(&ty, snapshot);
1974 let Normalized { value: normalized_ty, mut obligations } =
1975 project::normalize_with_depth(this,
1979 let skol_obligation =
1980 this.tcx().predicate_for_trait_def(
1986 obligations.push(skol_obligation);
1987 this.infcx().plug_leaks(skol_map, snapshot, &obligations)
1992 ///////////////////////////////////////////////////////////////////////////
1995 // Confirmation unifies the output type parameters of the trait
1996 // with the values found in the obligation, possibly yielding a
1997 // type error. See `README.md` for more details.
1999 fn confirm_candidate(&mut self,
2000 obligation: &TraitObligation<'tcx>,
2001 candidate: SelectionCandidate<'tcx>)
2002 -> Result<Selection<'tcx>,SelectionError<'tcx>>
2004 debug!("confirm_candidate({:?}, {:?})",
2009 BuiltinCandidate { has_nested } => {
2011 self.confirm_builtin_candidate(obligation, has_nested)))
2014 ParamCandidate(param) => {
2015 let obligations = self.confirm_param_candidate(obligation, param);
2016 Ok(VtableParam(obligations))
2019 DefaultImplCandidate(trait_def_id) => {
2020 let data = self.confirm_default_impl_candidate(obligation, trait_def_id);
2021 Ok(VtableDefaultImpl(data))
2024 DefaultImplObjectCandidate(trait_def_id) => {
2025 let data = self.confirm_default_impl_object_candidate(obligation, trait_def_id);
2026 Ok(VtableDefaultImpl(data))
2029 ImplCandidate(impl_def_id) => {
2030 Ok(VtableImpl(self.confirm_impl_candidate(obligation, impl_def_id)))
2033 ClosureCandidate(closure_def_id, substs, kind) => {
2034 let vtable_closure =
2035 self.confirm_closure_candidate(obligation, closure_def_id, substs, kind)?;
2036 Ok(VtableClosure(vtable_closure))
2039 BuiltinObjectCandidate => {
2040 // This indicates something like `(Trait+Send) :
2041 // Send`. In this case, we know that this holds
2042 // because that's what the object type is telling us,
2043 // and there's really no additional obligations to
2044 // prove and no types in particular to unify etc.
2045 Ok(VtableParam(Vec::new()))
2048 ObjectCandidate => {
2049 let data = self.confirm_object_candidate(obligation);
2050 Ok(VtableObject(data))
2053 FnPointerCandidate => {
2055 self.confirm_fn_pointer_candidate(obligation)?;
2056 Ok(VtableFnPointer(data))
2059 ProjectionCandidate => {
2060 self.confirm_projection_candidate(obligation);
2061 Ok(VtableParam(Vec::new()))
2064 BuiltinUnsizeCandidate => {
2065 let data = self.confirm_builtin_unsize_candidate(obligation)?;
2066 Ok(VtableBuiltin(data))
2071 fn confirm_projection_candidate(&mut self,
2072 obligation: &TraitObligation<'tcx>)
2074 self.in_snapshot(|this, snapshot| {
2076 this.match_projection_obligation_against_bounds_from_trait(obligation,
2082 fn confirm_param_candidate(&mut self,
2083 obligation: &TraitObligation<'tcx>,
2084 param: ty::PolyTraitRef<'tcx>)
2085 -> Vec<PredicateObligation<'tcx>>
2087 debug!("confirm_param_candidate({:?},{:?})",
2091 // During evaluation, we already checked that this
2092 // where-clause trait-ref could be unified with the obligation
2093 // trait-ref. Repeat that unification now without any
2094 // transactional boundary; it should not fail.
2095 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2096 Ok(obligations) => obligations,
2098 bug!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2105 fn confirm_builtin_candidate(&mut self,
2106 obligation: &TraitObligation<'tcx>,
2108 -> VtableBuiltinData<PredicateObligation<'tcx>>
2110 debug!("confirm_builtin_candidate({:?}, {:?})",
2111 obligation, has_nested);
2113 let obligations = if has_nested {
2114 let trait_def = obligation.predicate.def_id();
2115 let conditions = match trait_def {
2116 _ if Some(trait_def) == self.tcx().lang_items.sized_trait() => {
2117 self.sized_conditions(obligation)
2119 _ if Some(trait_def) == self.tcx().lang_items.copy_trait() => {
2120 self.copy_conditions(obligation)
2122 _ => bug!("unexpected builtin trait {:?}", trait_def)
2124 let nested = match conditions {
2125 BuiltinImplConditions::Where(nested) => nested,
2126 _ => bug!("obligation {:?} had matched a builtin impl but now doesn't",
2130 let cause = self.derived_cause(obligation, BuiltinDerivedObligation);
2131 self.collect_predicates_for_types(cause,
2132 obligation.recursion_depth+1,
2139 debug!("confirm_builtin_candidate: obligations={:?}",
2141 VtableBuiltinData { nested: obligations }
2144 /// This handles the case where a `impl Foo for ..` impl is being used.
2145 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2147 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2148 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2149 fn confirm_default_impl_candidate(&mut self,
2150 obligation: &TraitObligation<'tcx>,
2151 trait_def_id: DefId)
2152 -> VtableDefaultImplData<PredicateObligation<'tcx>>
2154 debug!("confirm_default_impl_candidate({:?}, {:?})",
2158 // binder is moved below
2159 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2160 let types = self.constituent_types_for_ty(self_ty);
2161 self.vtable_default_impl(obligation, trait_def_id, ty::Binder(types))
2164 fn confirm_default_impl_object_candidate(&mut self,
2165 obligation: &TraitObligation<'tcx>,
2166 trait_def_id: DefId)
2167 -> VtableDefaultImplData<PredicateObligation<'tcx>>
2169 debug!("confirm_default_impl_object_candidate({:?}, {:?})",
2173 assert!(self.tcx().has_attr(trait_def_id, "rustc_reflect_like"));
2175 // OK to skip binder, it is reintroduced below
2176 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2178 ty::TyTrait(ref data) => {
2179 // OK to skip the binder, it is reintroduced below
2180 let input_types = data.principal.skip_binder().substs.types.get_slice(TypeSpace);
2181 let assoc_types = data.bounds.projection_bounds
2183 .map(|pb| pb.skip_binder().ty);
2184 let all_types: Vec<_> = input_types.iter().cloned()
2188 // reintroduce the two binding levels we skipped, then flatten into one
2189 let all_types = ty::Binder(ty::Binder(all_types));
2190 let all_types = self.tcx().flatten_late_bound_regions(&all_types);
2192 self.vtable_default_impl(obligation, trait_def_id, all_types)
2195 bug!("asked to confirm default object implementation for non-object type: {:?}",
2201 /// See `confirm_default_impl_candidate`
2202 fn vtable_default_impl(&mut self,
2203 obligation: &TraitObligation<'tcx>,
2204 trait_def_id: DefId,
2205 nested: ty::Binder<Vec<Ty<'tcx>>>)
2206 -> VtableDefaultImplData<PredicateObligation<'tcx>>
2208 debug!("vtable_default_impl: nested={:?}", nested);
2210 let cause = self.derived_cause(obligation, BuiltinDerivedObligation);
2211 let mut obligations = self.collect_predicates_for_types(
2213 obligation.recursion_depth+1,
2217 let trait_obligations = self.in_snapshot(|this, snapshot| {
2218 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2219 let (trait_ref, skol_map) =
2220 this.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot);
2221 let cause = this.derived_cause(obligation, ImplDerivedObligation);
2222 this.impl_or_trait_obligations(cause,
2223 obligation.recursion_depth + 1,
2230 obligations.extend(trait_obligations);
2232 debug!("vtable_default_impl: obligations={:?}", obligations);
2234 VtableDefaultImplData {
2235 trait_def_id: trait_def_id,
2240 fn confirm_impl_candidate(&mut self,
2241 obligation: &TraitObligation<'tcx>,
2243 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2245 debug!("confirm_impl_candidate({:?},{:?})",
2249 // First, create the substitutions by matching the impl again,
2250 // this time not in a probe.
2251 self.in_snapshot(|this, snapshot| {
2252 let (substs, skol_map) =
2253 this.rematch_impl(impl_def_id, obligation,
2255 debug!("confirm_impl_candidate substs={:?}", substs);
2256 let cause = this.derived_cause(obligation, ImplDerivedObligation);
2257 this.vtable_impl(impl_def_id, substs, cause,
2258 obligation.recursion_depth + 1,
2263 fn vtable_impl(&mut self,
2265 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2266 cause: ObligationCause<'tcx>,
2267 recursion_depth: usize,
2268 skol_map: infer::SkolemizationMap,
2269 snapshot: &infer::CombinedSnapshot)
2270 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2272 debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2278 let mut impl_obligations =
2279 self.impl_or_trait_obligations(cause,
2286 debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2290 // Because of RFC447, the impl-trait-ref and obligations
2291 // are sufficient to determine the impl substs, without
2292 // relying on projections in the impl-trait-ref.
2294 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2295 impl_obligations.append(&mut substs.obligations);
2297 VtableImplData { impl_def_id: impl_def_id,
2298 substs: substs.value,
2299 nested: impl_obligations }
2302 fn confirm_object_candidate(&mut self,
2303 obligation: &TraitObligation<'tcx>)
2304 -> VtableObjectData<'tcx, PredicateObligation<'tcx>>
2306 debug!("confirm_object_candidate({:?})",
2309 // FIXME skipping binder here seems wrong -- we should
2310 // probably flatten the binder from the obligation and the
2311 // binder from the object. Have to try to make a broken test
2312 // case that results. -nmatsakis
2313 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2314 let poly_trait_ref = match self_ty.sty {
2315 ty::TyTrait(ref data) => {
2316 data.principal_trait_ref_with_self_ty(self.tcx(), self_ty)
2319 span_bug!(obligation.cause.span,
2320 "object candidate with non-object");
2324 let mut upcast_trait_ref = None;
2328 let tcx = self.tcx();
2330 // We want to find the first supertrait in the list of
2331 // supertraits that we can unify with, and do that
2332 // unification. We know that there is exactly one in the list
2333 // where we can unify because otherwise select would have
2334 // reported an ambiguity. (When we do find a match, also
2335 // record it for later.)
2337 util::supertraits(tcx, poly_trait_ref)
2341 |this, _| this.match_poly_trait_ref(obligation, t))
2343 Ok(_) => { upcast_trait_ref = Some(t); false }
2348 // Additionally, for each of the nonmatching predicates that
2349 // we pass over, we sum up the set of number of vtable
2350 // entries, so that we can compute the offset for the selected
2353 nonmatching.map(|t| tcx.count_own_vtable_entries(t))
2359 upcast_trait_ref: upcast_trait_ref.unwrap(),
2360 vtable_base: vtable_base,
2365 fn confirm_fn_pointer_candidate(&mut self, obligation: &TraitObligation<'tcx>)
2366 -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
2368 debug!("confirm_fn_pointer_candidate({:?})",
2371 // ok to skip binder; it is reintroduced below
2372 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2373 let sig = self_ty.fn_sig();
2375 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2378 util::TupleArgumentsFlag::Yes)
2379 .map_bound(|(trait_ref, _)| trait_ref);
2381 self.confirm_poly_trait_refs(obligation.cause.clone(),
2382 obligation.predicate.to_poly_trait_ref(),
2384 Ok(VtableFnPointerData { fn_ty: self_ty, nested: vec![] })
2387 fn confirm_closure_candidate(&mut self,
2388 obligation: &TraitObligation<'tcx>,
2389 closure_def_id: DefId,
2390 substs: ty::ClosureSubsts<'tcx>,
2391 kind: ty::ClosureKind)
2392 -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>,
2393 SelectionError<'tcx>>
2395 debug!("confirm_closure_candidate({:?},{:?},{:?})",
2403 } = self.closure_trait_ref(obligation, closure_def_id, substs);
2405 debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2410 self.confirm_poly_trait_refs(obligation.cause.clone(),
2411 obligation.predicate.to_poly_trait_ref(),
2414 obligations.push(Obligation::new(
2415 obligation.cause.clone(),
2416 ty::Predicate::ClosureKind(closure_def_id, kind)));
2418 Ok(VtableClosureData {
2419 closure_def_id: closure_def_id,
2420 substs: substs.clone(),
2425 /// In the case of closure types and fn pointers,
2426 /// we currently treat the input type parameters on the trait as
2427 /// outputs. This means that when we have a match we have only
2428 /// considered the self type, so we have to go back and make sure
2429 /// to relate the argument types too. This is kind of wrong, but
2430 /// since we control the full set of impls, also not that wrong,
2431 /// and it DOES yield better error messages (since we don't report
2432 /// errors as if there is no applicable impl, but rather report
2433 /// errors are about mismatched argument types.
2435 /// Here is an example. Imagine we have a closure expression
2436 /// and we desugared it so that the type of the expression is
2437 /// `Closure`, and `Closure` expects an int as argument. Then it
2438 /// is "as if" the compiler generated this impl:
2440 /// impl Fn(int) for Closure { ... }
2442 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2443 /// we have matched the self-type `Closure`. At this point we'll
2444 /// compare the `int` to `usize` and generate an error.
2446 /// Note that this checking occurs *after* the impl has selected,
2447 /// because these output type parameters should not affect the
2448 /// selection of the impl. Therefore, if there is a mismatch, we
2449 /// report an error to the user.
2450 fn confirm_poly_trait_refs(&mut self,
2451 obligation_cause: ObligationCause,
2452 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
2453 expected_trait_ref: ty::PolyTraitRef<'tcx>)
2454 -> Result<(), SelectionError<'tcx>>
2456 let origin = TypeOrigin::RelateOutputImplTypes(obligation_cause.span);
2458 let obligation_trait_ref = obligation_trait_ref.clone();
2459 self.infcx.sub_poly_trait_refs(false,
2461 expected_trait_ref.clone(),
2462 obligation_trait_ref.clone())
2463 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
2464 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
2467 fn confirm_builtin_unsize_candidate(&mut self,
2468 obligation: &TraitObligation<'tcx>,)
2469 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>,
2470 SelectionError<'tcx>> {
2471 let tcx = self.tcx();
2473 // assemble_candidates_for_unsizing should ensure there are no late bound
2474 // regions here. See the comment there for more details.
2475 let source = self.infcx.shallow_resolve(
2476 tcx.no_late_bound_regions(&obligation.self_ty()).unwrap());
2477 let target = self.infcx.shallow_resolve(obligation.predicate.0.input_types()[0]);
2479 debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2482 let mut nested = vec![];
2483 match (&source.sty, &target.sty) {
2484 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2485 (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => {
2486 // See assemble_candidates_for_unsizing for more info.
2487 let bounds = ty::ExistentialBounds {
2488 region_bound: data_b.bounds.region_bound,
2489 builtin_bounds: data_b.bounds.builtin_bounds,
2490 projection_bounds: data_a.bounds.projection_bounds.clone(),
2493 let new_trait = tcx.mk_trait(data_a.principal.clone(), bounds);
2494 let origin = TypeOrigin::Misc(obligation.cause.span);
2495 let InferOk { obligations, .. } =
2496 self.infcx.sub_types(false, origin, new_trait, target)
2497 .map_err(|_| Unimplemented)?;
2498 self.inferred_obligations.extend(obligations);
2500 // Register one obligation for 'a: 'b.
2501 let cause = ObligationCause::new(obligation.cause.span,
2502 obligation.cause.body_id,
2503 ObjectCastObligation(target));
2504 let outlives = ty::OutlivesPredicate(data_a.bounds.region_bound,
2505 data_b.bounds.region_bound);
2506 nested.push(Obligation::with_depth(cause,
2507 obligation.recursion_depth + 1,
2508 ty::Binder(outlives).to_predicate()));
2512 (_, &ty::TyTrait(ref data)) => {
2513 let mut object_dids = Some(data.principal_def_id()).into_iter();
2515 // data.bounds.builtin_bounds.iter().flat_map(|bound| {
2516 // tcx.lang_items.from_builtin_kind(bound).ok()
2518 // .chain(Some(data.principal_def_id()));
2519 if let Some(did) = object_dids.find(|did| {
2520 !tcx.is_object_safe(*did)
2522 return Err(TraitNotObjectSafe(did))
2525 let cause = ObligationCause::new(obligation.cause.span,
2526 obligation.cause.body_id,
2527 ObjectCastObligation(target));
2528 let mut push = |predicate| {
2529 nested.push(Obligation::with_depth(cause.clone(),
2530 obligation.recursion_depth + 1,
2534 // Create the obligation for casting from T to Trait.
2535 push(data.principal_trait_ref_with_self_ty(tcx, source).to_predicate());
2537 // We can only make objects from sized types.
2538 let mut builtin_bounds = data.bounds.builtin_bounds;
2539 builtin_bounds.insert(ty::BoundSized);
2541 // Create additional obligations for all the various builtin
2542 // bounds attached to the object cast. (In other words, if the
2543 // object type is Foo+Send, this would create an obligation
2544 // for the Send check.)
2545 for bound in &builtin_bounds {
2546 if let Ok(tr) = tcx.trait_ref_for_builtin_bound(bound, source) {
2547 push(tr.to_predicate());
2549 return Err(Unimplemented);
2553 // Create obligations for the projection predicates.
2554 for bound in data.projection_bounds_with_self_ty(tcx, source) {
2555 push(bound.to_predicate());
2558 // If the type is `Foo+'a`, ensures that the type
2559 // being cast to `Foo+'a` outlives `'a`:
2560 let outlives = ty::OutlivesPredicate(source,
2561 data.bounds.region_bound);
2562 push(ty::Binder(outlives).to_predicate());
2566 (&ty::TyArray(a, _), &ty::TySlice(b)) => {
2567 let origin = TypeOrigin::Misc(obligation.cause.span);
2568 let InferOk { obligations, .. } =
2569 self.infcx.sub_types(false, origin, a, b)
2570 .map_err(|_| Unimplemented)?;
2571 self.inferred_obligations.extend(obligations);
2574 // Struct<T> -> Struct<U>.
2575 (&ty::TyStruct(def, substs_a), &ty::TyStruct(_, substs_b)) => {
2578 .map(|f| f.unsubst_ty())
2579 .collect::<Vec<_>>();
2581 // The last field of the structure has to exist and contain type parameters.
2582 let field = if let Some(&field) = fields.last() {
2585 return Err(Unimplemented);
2587 let mut ty_params = vec![];
2588 for ty in field.walk() {
2589 if let ty::TyParam(p) = ty.sty {
2590 assert!(p.space == TypeSpace);
2591 let idx = p.idx as usize;
2592 if !ty_params.contains(&idx) {
2593 ty_params.push(idx);
2597 if ty_params.is_empty() {
2598 return Err(Unimplemented);
2601 // Replace type parameters used in unsizing with
2602 // TyError and ensure they do not affect any other fields.
2603 // This could be checked after type collection for any struct
2604 // with a potentially unsized trailing field.
2605 let mut new_substs = substs_a.clone();
2606 for &i in &ty_params {
2607 new_substs.types.get_mut_slice(TypeSpace)[i] = tcx.types.err;
2609 for &ty in fields.split_last().unwrap().1 {
2610 if ty.subst(tcx, &new_substs).references_error() {
2611 return Err(Unimplemented);
2615 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2616 let inner_source = field.subst(tcx, substs_a);
2617 let inner_target = field.subst(tcx, substs_b);
2619 // Check that the source structure with the target's
2620 // type parameters is a subtype of the target.
2621 for &i in &ty_params {
2622 let param_b = *substs_b.types.get(TypeSpace, i);
2623 new_substs.types.get_mut_slice(TypeSpace)[i] = param_b;
2625 let new_struct = tcx.mk_struct(def, tcx.mk_substs(new_substs));
2626 let origin = TypeOrigin::Misc(obligation.cause.span);
2627 let InferOk { obligations, .. } =
2628 self.infcx.sub_types(false, origin, new_struct, target)
2629 .map_err(|_| Unimplemented)?;
2630 self.inferred_obligations.extend(obligations);
2632 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2633 nested.push(tcx.predicate_for_trait_def(
2634 obligation.cause.clone(),
2635 obligation.predicate.def_id(),
2636 obligation.recursion_depth + 1,
2638 vec![inner_target]));
2644 Ok(VtableBuiltinData { nested: nested })
2647 ///////////////////////////////////////////////////////////////////////////
2650 // Matching is a common path used for both evaluation and
2651 // confirmation. It basically unifies types that appear in impls
2652 // and traits. This does affect the surrounding environment;
2653 // therefore, when used during evaluation, match routines must be
2654 // run inside of a `probe()` so that their side-effects are
2657 fn rematch_impl(&mut self,
2659 obligation: &TraitObligation<'tcx>,
2660 snapshot: &infer::CombinedSnapshot)
2661 -> (Normalized<'tcx, &'tcx Substs<'tcx>>, infer::SkolemizationMap)
2663 match self.match_impl(impl_def_id, obligation, snapshot) {
2664 Ok((substs, skol_map)) => (substs, skol_map),
2666 bug!("Impl {:?} was matchable against {:?} but now is not",
2673 fn match_impl(&mut self,
2675 obligation: &TraitObligation<'tcx>,
2676 snapshot: &infer::CombinedSnapshot)
2677 -> Result<(Normalized<'tcx, &'tcx Substs<'tcx>>,
2678 infer::SkolemizationMap), ()>
2680 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
2682 // Before we create the substitutions and everything, first
2683 // consider a "quick reject". This avoids creating more types
2684 // and so forth that we need to.
2685 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
2689 let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions(
2690 &obligation.predicate,
2692 let skol_obligation_trait_ref = skol_obligation.trait_ref;
2694 let impl_substs = util::fresh_type_vars_for_impl(self.infcx,
2695 obligation.cause.span,
2698 let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
2701 let impl_trait_ref =
2702 project::normalize_with_depth(self,
2703 obligation.cause.clone(),
2704 obligation.recursion_depth + 1,
2707 debug!("match_impl(impl_def_id={:?}, obligation={:?}, \
2708 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
2712 skol_obligation_trait_ref);
2714 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
2715 let InferOk { obligations, .. } =
2716 self.infcx.eq_trait_refs(false,
2718 impl_trait_ref.value.clone(),
2719 skol_obligation_trait_ref)
2721 debug!("match_impl: failed eq_trait_refs due to `{}`", e);
2724 self.inferred_obligations.extend(obligations);
2726 if let Err(e) = self.infcx.leak_check(false,
2727 obligation.cause.span,
2730 debug!("match_impl: failed leak check due to `{}`", e);
2734 debug!("match_impl: success impl_substs={:?}", impl_substs);
2737 obligations: impl_trait_ref.obligations
2741 fn fast_reject_trait_refs(&mut self,
2742 obligation: &TraitObligation,
2743 impl_trait_ref: &ty::TraitRef)
2746 // We can avoid creating type variables and doing the full
2747 // substitution if we find that any of the input types, when
2748 // simplified, do not match.
2750 obligation.predicate.0.input_types().iter()
2751 .zip(impl_trait_ref.input_types())
2752 .any(|(&obligation_ty, &impl_ty)| {
2753 let simplified_obligation_ty =
2754 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
2755 let simplified_impl_ty =
2756 fast_reject::simplify_type(self.tcx(), impl_ty, false);
2758 simplified_obligation_ty.is_some() &&
2759 simplified_impl_ty.is_some() &&
2760 simplified_obligation_ty != simplified_impl_ty
2764 /// Normalize `where_clause_trait_ref` and try to match it against
2765 /// `obligation`. If successful, return any predicates that
2766 /// result from the normalization. Normalization is necessary
2767 /// because where-clauses are stored in the parameter environment
2769 fn match_where_clause_trait_ref(&mut self,
2770 obligation: &TraitObligation<'tcx>,
2771 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
2772 -> Result<Vec<PredicateObligation<'tcx>>,()>
2774 self.match_poly_trait_ref(obligation, where_clause_trait_ref)?;
2778 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2779 /// obligation is satisfied.
2780 fn match_poly_trait_ref(&mut self,
2781 obligation: &TraitObligation<'tcx>,
2782 poly_trait_ref: ty::PolyTraitRef<'tcx>)
2785 debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
2789 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
2790 self.infcx.sub_poly_trait_refs(false,
2793 obligation.predicate.to_poly_trait_ref())
2794 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
2798 ///////////////////////////////////////////////////////////////////////////
2801 fn match_fresh_trait_refs(&self,
2802 previous: &ty::PolyTraitRef<'tcx>,
2803 current: &ty::PolyTraitRef<'tcx>)
2806 let mut matcher = ty::_match::Match::new(self.tcx());
2807 matcher.relate(previous, current).is_ok()
2810 fn push_stack<'o,'s:'o>(&mut self,
2811 previous_stack: TraitObligationStackList<'s, 'tcx>,
2812 obligation: &'o TraitObligation<'tcx>)
2813 -> TraitObligationStack<'o, 'tcx>
2815 let fresh_trait_ref =
2816 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
2818 TraitObligationStack {
2819 obligation: obligation,
2820 fresh_trait_ref: fresh_trait_ref,
2821 previous: previous_stack,
2825 fn closure_trait_ref_unnormalized(&mut self,
2826 obligation: &TraitObligation<'tcx>,
2827 closure_def_id: DefId,
2828 substs: ty::ClosureSubsts<'tcx>)
2829 -> ty::PolyTraitRef<'tcx>
2831 let closure_type = self.infcx.closure_type(closure_def_id, substs);
2832 let ty::Binder((trait_ref, _)) =
2833 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2834 obligation.predicate.0.self_ty(), // (1)
2836 util::TupleArgumentsFlag::No);
2837 // (1) Feels icky to skip the binder here, but OTOH we know
2838 // that the self-type is an unboxed closure type and hence is
2839 // in fact unparameterized (or at least does not reference any
2840 // regions bound in the obligation). Still probably some
2841 // refactoring could make this nicer.
2843 ty::Binder(trait_ref)
2846 fn closure_trait_ref(&mut self,
2847 obligation: &TraitObligation<'tcx>,
2848 closure_def_id: DefId,
2849 substs: ty::ClosureSubsts<'tcx>)
2850 -> Normalized<'tcx, ty::PolyTraitRef<'tcx>>
2852 let trait_ref = self.closure_trait_ref_unnormalized(
2853 obligation, closure_def_id, substs);
2855 // A closure signature can contain associated types which
2856 // must be normalized.
2857 normalize_with_depth(self,
2858 obligation.cause.clone(),
2859 obligation.recursion_depth+1,
2863 /// Returns the obligations that are implied by instantiating an
2864 /// impl or trait. The obligations are substituted and fully
2865 /// normalized. This is used when confirming an impl or default
2867 fn impl_or_trait_obligations(&mut self,
2868 cause: ObligationCause<'tcx>,
2869 recursion_depth: usize,
2870 def_id: DefId, // of impl or trait
2871 substs: &Substs<'tcx>, // for impl or trait
2872 skol_map: infer::SkolemizationMap,
2873 snapshot: &infer::CombinedSnapshot)
2874 -> Vec<PredicateObligation<'tcx>>
2876 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
2877 let tcx = self.tcx();
2879 // To allow for one-pass evaluation of the nested obligation,
2880 // each predicate must be preceded by the obligations required
2882 // for example, if we have:
2883 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
2884 // the impl will have the following predicates:
2885 // <V as Iterator>::Item = U,
2886 // U: Iterator, U: Sized,
2887 // V: Iterator, V: Sized,
2888 // <U as Iterator>::Item: Copy
2889 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2890 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2891 // `$1: Copy`, so we must ensure the obligations are emitted in
2893 let predicates = tcx
2894 .lookup_predicates(def_id)
2896 .flat_map(|predicate| {
2898 normalize_with_depth(self, cause.clone(), recursion_depth,
2899 &predicate.subst(tcx, substs));
2900 predicate.obligations.into_iter().chain(
2902 cause: cause.clone(),
2903 recursion_depth: recursion_depth,
2904 predicate: predicate.value
2907 self.infcx().plug_leaks(skol_map, snapshot, &predicates)
2910 #[allow(unused_comparisons)]
2911 fn derived_cause(&self,
2912 obligation: &TraitObligation<'tcx>,
2913 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
2914 -> ObligationCause<'tcx>
2917 * Creates a cause for obligations that are derived from
2918 * `obligation` by a recursive search (e.g., for a builtin
2919 * bound, or eventually a `impl Foo for ..`). If `obligation`
2920 * is itself a derived obligation, this is just a clone, but
2921 * otherwise we create a "derived obligation" cause so as to
2922 * keep track of the original root obligation for error
2926 // NOTE(flaper87): As of now, it keeps track of the whole error
2927 // chain. Ideally, we should have a way to configure this either
2928 // by using -Z verbose or just a CLI argument.
2929 if obligation.recursion_depth >= 0 {
2930 let derived_cause = DerivedObligationCause {
2931 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
2932 parent_code: Rc::new(obligation.cause.code.clone())
2934 let derived_code = variant(derived_cause);
2935 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
2937 obligation.cause.clone()
2942 impl<'tcx> SelectionCache<'tcx> {
2943 pub fn new() -> SelectionCache<'tcx> {
2945 hashmap: RefCell::new(FnvHashMap())
2950 impl<'tcx> EvaluationCache<'tcx> {
2951 pub fn new() -> EvaluationCache<'tcx> {
2953 hashmap: RefCell::new(FnvHashMap())
2958 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {
2959 fn list(&'o self) -> TraitObligationStackList<'o,'tcx> {
2960 TraitObligationStackList::with(self)
2963 fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> {
2968 #[derive(Copy, Clone)]
2969 struct TraitObligationStackList<'o,'tcx:'o> {
2970 head: Option<&'o TraitObligationStack<'o,'tcx>>
2973 impl<'o,'tcx> TraitObligationStackList<'o,'tcx> {
2974 fn empty() -> TraitObligationStackList<'o,'tcx> {
2975 TraitObligationStackList { head: None }
2978 fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> {
2979 TraitObligationStackList { head: Some(r) }
2983 impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{
2984 type Item = &'o TraitObligationStack<'o,'tcx>;
2986 fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> {
2997 impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> {
2998 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
2999 write!(f, "TraitObligationStack({:?})", self.obligation)
3003 impl EvaluationResult {
3004 fn may_apply(&self) -> bool {
3008 EvaluatedToUnknown => true,
3010 EvaluatedToErr => false
3015 impl MethodMatchResult {
3016 pub fn may_apply(&self) -> bool {
3018 MethodMatched(_) => true,
3019 MethodAmbiguous(_) => true,
3020 MethodDidNotMatch => false,