1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! See `README.md` for high-level documentation
13 pub use self::MethodMatchResult::*;
14 pub use self::MethodMatchedData::*;
15 use self::SelectionCandidate::*;
16 use self::EvaluationResult::*;
19 use super::DerivedObligationCause;
21 use super::project::{normalize_with_depth, Normalized};
22 use super::{PredicateObligation, TraitObligation, ObligationCause};
23 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation};
24 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch};
25 use super::{ObjectCastObligation, Obligation};
27 use super::TraitNotObjectSafe;
29 use super::SelectionResult;
30 use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure,
31 VtableFnPointer, VtableObject, VtableDefaultImpl};
32 use super::{VtableImplData, VtableObjectData, VtableBuiltinData,
33 VtableClosureData, VtableDefaultImplData, VtableFnPointerData};
36 use hir::def_id::DefId;
38 use infer::{InferCtxt, InferOk, TypeFreshener, TypeOrigin};
39 use ty::subst::{Kind, Subst, Substs};
40 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
43 use ty::relate::TypeRelation;
45 use rustc_data_structures::bitvec::BitVector;
46 use rustc_data_structures::snapshot_vec::{SnapshotVecDelegate, SnapshotVec};
47 use std::cell::RefCell;
49 use std::marker::PhantomData;
54 use util::nodemap::FnvHashMap;
56 struct InferredObligationsSnapshotVecDelegate<'tcx> {
57 phantom: PhantomData<&'tcx i32>,
59 impl<'tcx> SnapshotVecDelegate for InferredObligationsSnapshotVecDelegate<'tcx> {
60 type Value = PredicateObligation<'tcx>;
62 fn reverse(_: &mut Vec<Self::Value>, _: Self::Undo) {}
65 pub struct SelectionContext<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
66 infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>,
68 /// Freshener used specifically for skolemizing entries on the
69 /// obligation stack. This ensures that all entries on the stack
70 /// at one time will have the same set of skolemized entries,
71 /// which is important for checking for trait bounds that
72 /// recursively require themselves.
73 freshener: TypeFreshener<'cx, 'gcx, 'tcx>,
75 /// If true, indicates that the evaluation should be conservative
76 /// and consider the possibility of types outside this crate.
77 /// This comes up primarily when resolving ambiguity. Imagine
78 /// there is some trait reference `$0 : Bar` where `$0` is an
79 /// inference variable. If `intercrate` is true, then we can never
80 /// say for sure that this reference is not implemented, even if
81 /// there are *no impls at all for `Bar`*, because `$0` could be
82 /// bound to some type that in a downstream crate that implements
83 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
84 /// though, we set this to false, because we are only interested
85 /// in types that the user could actually have written --- in
86 /// other words, we consider `$0 : Bar` to be unimplemented if
87 /// there is no type that the user could *actually name* that
88 /// would satisfy it. This avoids crippling inference, basically.
91 inferred_obligations: SnapshotVec<InferredObligationsSnapshotVecDelegate<'tcx>>,
94 // A stack that walks back up the stack frame.
95 struct TraitObligationStack<'prev, 'tcx: 'prev> {
96 obligation: &'prev TraitObligation<'tcx>,
98 /// Trait ref from `obligation` but skolemized with the
99 /// selection-context's freshener. Used to check for recursion.
100 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
102 previous: TraitObligationStackList<'prev, 'tcx>,
106 pub struct SelectionCache<'tcx> {
107 hashmap: RefCell<FnvHashMap<ty::TraitRef<'tcx>,
108 SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
111 pub enum MethodMatchResult {
112 MethodMatched(MethodMatchedData),
113 MethodAmbiguous(/* list of impls that could apply */ Vec<DefId>),
117 #[derive(Copy, Clone, Debug)]
118 pub enum MethodMatchedData {
119 // In the case of a precise match, we don't really need to store
120 // how the match was found. So don't.
123 // In the case of a coercion, we need to know the precise impl so
124 // that we can determine the type to which things were coerced.
125 CoerciveMethodMatch(/* impl we matched */ DefId)
128 /// The selection process begins by considering all impls, where
129 /// clauses, and so forth that might resolve an obligation. Sometimes
130 /// we'll be able to say definitively that (e.g.) an impl does not
131 /// apply to the obligation: perhaps it is defined for `usize` but the
132 /// obligation is for `int`. In that case, we drop the impl out of the
133 /// list. But the other cases are considered *candidates*.
135 /// For selection to succeed, there must be exactly one matching
136 /// candidate. If the obligation is fully known, this is guaranteed
137 /// by coherence. However, if the obligation contains type parameters
138 /// or variables, there may be multiple such impls.
140 /// It is not a real problem if multiple matching impls exist because
141 /// of type variables - it just means the obligation isn't sufficiently
142 /// elaborated. In that case we report an ambiguity, and the caller can
143 /// try again after more type information has been gathered or report a
144 /// "type annotations required" error.
146 /// However, with type parameters, this can be a real problem - type
147 /// parameters don't unify with regular types, but they *can* unify
148 /// with variables from blanket impls, and (unless we know its bounds
149 /// will always be satisfied) picking the blanket impl will be wrong
150 /// for at least *some* substitutions. To make this concrete, if we have
152 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
153 /// impl<T: fmt::Debug> AsDebug for T {
155 /// fn debug(self) -> fmt::Debug { self }
157 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
159 /// we can't just use the impl to resolve the <T as AsDebug> obligation
160 /// - a type from another crate (that doesn't implement fmt::Debug) could
161 /// implement AsDebug.
163 /// Because where-clauses match the type exactly, multiple clauses can
164 /// only match if there are unresolved variables, and we can mostly just
165 /// report this ambiguity in that case. This is still a problem - we can't
166 /// *do anything* with ambiguities that involve only regions. This is issue
169 /// If a single where-clause matches and there are no inference
170 /// variables left, then it definitely matches and we can just select
173 /// In fact, we even select the where-clause when the obligation contains
174 /// inference variables. The can lead to inference making "leaps of logic",
175 /// for example in this situation:
177 /// pub trait Foo<T> { fn foo(&self) -> T; }
178 /// impl<T> Foo<()> for T { fn foo(&self) { } }
179 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
181 /// pub fn foo<T>(t: T) where T: Foo<bool> {
182 /// println!("{:?}", <T as Foo<_>>::foo(&t));
184 /// fn main() { foo(false); }
186 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
187 /// impl and the where-clause. We select the where-clause and unify $0=bool,
188 /// so the program prints "false". However, if the where-clause is omitted,
189 /// the blanket impl is selected, we unify $0=(), and the program prints
192 /// Exactly the same issues apply to projection and object candidates, except
193 /// that we can have both a projection candidate and a where-clause candidate
194 /// for the same obligation. In that case either would do (except that
195 /// different "leaps of logic" would occur if inference variables are
196 /// present), and we just pick the where-clause. This is, for example,
197 /// required for associated types to work in default impls, as the bounds
198 /// are visible both as projection bounds and as where-clauses from the
199 /// parameter environment.
200 #[derive(PartialEq,Eq,Debug,Clone)]
201 enum SelectionCandidate<'tcx> {
202 BuiltinCandidate { has_nested: bool },
203 ParamCandidate(ty::PolyTraitRef<'tcx>),
204 ImplCandidate(DefId),
205 DefaultImplCandidate(DefId),
206 DefaultImplObjectCandidate(DefId),
208 /// This is a trait matching with a projected type as `Self`, and
209 /// we found an applicable bound in the trait definition.
212 /// Implementation of a `Fn`-family trait by one of the anonymous types
213 /// generated for a `||` expression. The ty::ClosureKind informs the
214 /// confirmation step what ClosureKind obligation to emit.
215 ClosureCandidate(/* closure */ DefId, ty::ClosureSubsts<'tcx>, ty::ClosureKind),
217 /// Implementation of a `Fn`-family trait by one of the anonymous
218 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
223 BuiltinObjectCandidate,
225 BuiltinUnsizeCandidate,
228 impl<'a, 'tcx> ty::Lift<'tcx> for SelectionCandidate<'a> {
229 type Lifted = SelectionCandidate<'tcx>;
230 fn lift_to_tcx<'b, 'gcx>(&self, tcx: TyCtxt<'b, 'gcx, 'tcx>) -> Option<Self::Lifted> {
232 BuiltinCandidate { has_nested } => {
234 has_nested: has_nested
237 ImplCandidate(def_id) => ImplCandidate(def_id),
238 DefaultImplCandidate(def_id) => DefaultImplCandidate(def_id),
239 DefaultImplObjectCandidate(def_id) => {
240 DefaultImplObjectCandidate(def_id)
242 ProjectionCandidate => ProjectionCandidate,
243 FnPointerCandidate => FnPointerCandidate,
244 ObjectCandidate => ObjectCandidate,
245 BuiltinObjectCandidate => BuiltinObjectCandidate,
246 BuiltinUnsizeCandidate => BuiltinUnsizeCandidate,
248 ParamCandidate(ref trait_ref) => {
249 return tcx.lift(trait_ref).map(ParamCandidate);
251 ClosureCandidate(def_id, ref substs, kind) => {
252 return tcx.lift(substs).map(|substs| {
253 ClosureCandidate(def_id, substs, kind)
260 struct SelectionCandidateSet<'tcx> {
261 // a list of candidates that definitely apply to the current
262 // obligation (meaning: types unify).
263 vec: Vec<SelectionCandidate<'tcx>>,
265 // if this is true, then there were candidates that might or might
266 // not have applied, but we couldn't tell. This occurs when some
267 // of the input types are type variables, in which case there are
268 // various "builtin" rules that might or might not trigger.
272 #[derive(PartialEq,Eq,Debug,Clone)]
273 struct EvaluatedCandidate<'tcx> {
274 candidate: SelectionCandidate<'tcx>,
275 evaluation: EvaluationResult,
278 /// When does the builtin impl for `T: Trait` apply?
279 enum BuiltinImplConditions<'tcx> {
280 /// The impl is conditional on T1,T2,.. : Trait
281 Where(ty::Binder<Vec<Ty<'tcx>>>),
282 /// There is no built-in impl. There may be some other
283 /// candidate (a where-clause or user-defined impl).
285 /// There is *no* impl for this, builtin or not. Ignore
286 /// all where-clauses.
288 /// It is unknown whether there is an impl.
292 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
293 /// The result of trait evaluation. The order is important
294 /// here as the evaluation of a list is the maximum of the
296 enum EvaluationResult {
297 /// Evaluation successful
299 /// Evaluation failed because of recursion - treated as ambiguous
301 /// Evaluation is known to be ambiguous
303 /// Evaluation failed
308 pub struct EvaluationCache<'tcx> {
309 hashmap: RefCell<FnvHashMap<ty::PolyTraitRef<'tcx>, EvaluationResult>>
312 impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
313 pub fn new(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
316 freshener: infcx.freshener(),
318 inferred_obligations: SnapshotVec::new(),
322 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'gcx, 'tcx>) -> SelectionContext<'cx, 'gcx, 'tcx> {
325 freshener: infcx.freshener(),
327 inferred_obligations: SnapshotVec::new(),
331 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
335 pub fn tcx(&self) -> TyCtxt<'cx, 'gcx, 'tcx> {
339 pub fn param_env(&self) -> &'cx ty::ParameterEnvironment<'tcx> {
340 self.infcx.param_env()
343 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'gcx, 'tcx> {
347 pub fn projection_mode(&self) -> Reveal {
348 self.infcx.projection_mode()
351 /// Wraps the inference context's in_snapshot s.t. snapshot handling is only from the selection
353 fn in_snapshot<R, F>(&mut self, f: F) -> R
354 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
356 // The irrefutable nature of the operation means we don't need to snapshot the
357 // inferred_obligations vector.
358 self.infcx.in_snapshot(|snapshot| f(self, snapshot))
361 /// Wraps a probe s.t. obligations collected during it are ignored and old obligations are
363 fn probe<R, F>(&mut self, f: F) -> R
364 where F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> R
366 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
367 let result = self.infcx.probe(|snapshot| f(self, snapshot));
368 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
372 /// Wraps a commit_if_ok s.t. obligations collected during it are not returned in selection if
373 /// the transaction fails and s.t. old obligations are retained.
374 fn commit_if_ok<T, E, F>(&mut self, f: F) -> Result<T, E> where
375 F: FnOnce(&mut Self, &infer::CombinedSnapshot) -> Result<T, E>
377 let inferred_obligations_snapshot = self.inferred_obligations.start_snapshot();
378 match self.infcx.commit_if_ok(|snapshot| f(self, snapshot)) {
380 self.inferred_obligations.commit(inferred_obligations_snapshot);
384 self.inferred_obligations.rollback_to(inferred_obligations_snapshot);
391 ///////////////////////////////////////////////////////////////////////////
394 // The selection phase tries to identify *how* an obligation will
395 // be resolved. For example, it will identify which impl or
396 // parameter bound is to be used. The process can be inconclusive
397 // if the self type in the obligation is not fully inferred. Selection
398 // can result in an error in one of two ways:
400 // 1. If no applicable impl or parameter bound can be found.
401 // 2. If the output type parameters in the obligation do not match
402 // those specified by the impl/bound. For example, if the obligation
403 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
404 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
406 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
407 /// type environment by performing unification.
408 pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
409 -> SelectionResult<'tcx, Selection<'tcx>> {
410 debug!("select({:?})", obligation);
411 assert!(!obligation.predicate.has_escaping_regions());
413 let dep_node = obligation.predicate.dep_node();
414 let _task = self.tcx().dep_graph.in_task(dep_node);
416 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
417 match self.candidate_from_obligation(&stack)? {
420 let mut candidate = self.confirm_candidate(obligation, candidate)?;
421 // FIXME(#32730) remove this assertion once inferred obligations are propagated
423 assert!(self.inferred_obligations.len() == 0);
424 let inferred_obligations = (*self.inferred_obligations).into_iter().cloned();
425 candidate.nested_obligations_mut().extend(inferred_obligations);
431 ///////////////////////////////////////////////////////////////////////////
434 // Tests whether an obligation can be selected or whether an impl
435 // can be applied to particular types. It skips the "confirmation"
436 // step and hence completely ignores output type parameters.
438 // The result is "true" if the obligation *may* hold and "false" if
439 // we can be sure it does not.
441 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
442 pub fn evaluate_obligation(&mut self,
443 obligation: &PredicateObligation<'tcx>)
446 debug!("evaluate_obligation({:?})",
449 self.probe(|this, _| {
450 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
455 /// Evaluates whether the obligation `obligation` can be satisfied,
456 /// and returns `false` if not certain. However, this is not entirely
457 /// accurate if inference variables are involved.
458 pub fn evaluate_obligation_conservatively(&mut self,
459 obligation: &PredicateObligation<'tcx>)
462 debug!("evaluate_obligation_conservatively({:?})",
465 self.probe(|this, _| {
466 this.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
471 /// Evaluates the predicates in `predicates` recursively. Note that
472 /// this applies projections in the predicates, and therefore
473 /// is run within an inference probe.
474 fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
475 stack: TraitObligationStackList<'o, 'tcx>,
478 where I : Iterator<Item=&'a PredicateObligation<'tcx>>, 'tcx:'a
480 let mut result = EvaluatedToOk;
481 for obligation in predicates {
482 let eval = self.evaluate_predicate_recursively(stack, obligation);
483 debug!("evaluate_predicate_recursively({:?}) = {:?}",
486 EvaluatedToErr => { return EvaluatedToErr; }
487 EvaluatedToAmbig => { result = EvaluatedToAmbig; }
488 EvaluatedToUnknown => {
489 if result < EvaluatedToUnknown {
490 result = EvaluatedToUnknown;
499 fn evaluate_predicate_recursively<'o>(&mut self,
500 previous_stack: TraitObligationStackList<'o, 'tcx>,
501 obligation: &PredicateObligation<'tcx>)
504 debug!("evaluate_predicate_recursively({:?})",
507 // Check the cache from the tcx of predicates that we know
508 // have been proven elsewhere. This cache only contains
509 // predicates that are global in scope and hence unaffected by
510 // the current environment.
511 if self.tcx().fulfilled_predicates.borrow().check_duplicate(&obligation.predicate) {
512 return EvaluatedToOk;
515 match obligation.predicate {
516 ty::Predicate::Rfc1592(..) => EvaluatedToOk,
518 ty::Predicate::Trait(ref t) => {
519 assert!(!t.has_escaping_regions());
520 let obligation = obligation.with(t.clone());
521 self.evaluate_obligation_recursively(previous_stack, &obligation)
524 ty::Predicate::Equate(ref p) => {
525 // does this code ever run?
526 match self.infcx.equality_predicate(obligation.cause.span, p) {
527 Ok(InferOk { obligations, .. }) => {
528 self.inferred_obligations.extend(obligations);
531 Err(_) => EvaluatedToErr
535 ty::Predicate::WellFormed(ty) => {
536 match ty::wf::obligations(self.infcx, obligation.cause.body_id,
537 ty, obligation.cause.span) {
539 self.evaluate_predicates_recursively(previous_stack, obligations.iter()),
545 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
546 // we do not consider region relationships when
547 // evaluating trait matches
551 ty::Predicate::ObjectSafe(trait_def_id) => {
552 if self.tcx().is_object_safe(trait_def_id) {
559 ty::Predicate::Projection(ref data) => {
560 let project_obligation = obligation.with(data.clone());
561 match project::poly_project_and_unify_type(self, &project_obligation) {
562 Ok(Some(subobligations)) => {
563 self.evaluate_predicates_recursively(previous_stack,
564 subobligations.iter())
575 ty::Predicate::ClosureKind(closure_def_id, kind) => {
576 match self.infcx.closure_kind(closure_def_id) {
577 Some(closure_kind) => {
578 if closure_kind.extends(kind) {
592 fn evaluate_obligation_recursively<'o>(&mut self,
593 previous_stack: TraitObligationStackList<'o, 'tcx>,
594 obligation: &TraitObligation<'tcx>)
597 debug!("evaluate_obligation_recursively({:?})",
600 let stack = self.push_stack(previous_stack, obligation);
601 let fresh_trait_ref = stack.fresh_trait_ref;
602 if let Some(result) = self.check_evaluation_cache(fresh_trait_ref) {
603 debug!("CACHE HIT: EVAL({:?})={:?}",
609 let result = self.evaluate_stack(&stack);
611 debug!("CACHE MISS: EVAL({:?})={:?}",
614 self.insert_evaluation_cache(fresh_trait_ref, result);
619 fn evaluate_stack<'o>(&mut self,
620 stack: &TraitObligationStack<'o, 'tcx>)
623 // In intercrate mode, whenever any of the types are unbound,
624 // there can always be an impl. Even if there are no impls in
625 // this crate, perhaps the type would be unified with
626 // something from another crate that does provide an impl.
628 // In intra mode, we must still be conservative. The reason is
629 // that we want to avoid cycles. Imagine an impl like:
631 // impl<T:Eq> Eq for Vec<T>
633 // and a trait reference like `$0 : Eq` where `$0` is an
634 // unbound variable. When we evaluate this trait-reference, we
635 // will unify `$0` with `Vec<$1>` (for some fresh variable
636 // `$1`), on the condition that `$1 : Eq`. We will then wind
637 // up with many candidates (since that are other `Eq` impls
638 // that apply) and try to winnow things down. This results in
639 // a recursive evaluation that `$1 : Eq` -- as you can
640 // imagine, this is just where we started. To avoid that, we
641 // check for unbound variables and return an ambiguous (hence possible)
642 // match if we've seen this trait before.
644 // This suffices to allow chains like `FnMut` implemented in
645 // terms of `Fn` etc, but we could probably make this more
647 let unbound_input_types = stack.fresh_trait_ref.input_types().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,
1066 Ok(None) => cache_fresh_trait_pred.has_infer_types()
1070 fn assemble_candidates<'o>(&mut self,
1071 stack: &TraitObligationStack<'o, 'tcx>)
1072 -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
1074 let TraitObligationStack { obligation, .. } = *stack;
1075 let ref obligation = Obligation {
1076 cause: obligation.cause.clone(),
1077 recursion_depth: obligation.recursion_depth,
1078 predicate: self.infcx().resolve_type_vars_if_possible(&obligation.predicate)
1081 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
1082 // FIXME(#20297): Self is a type variable (e.g. `_: AsRef<str>`).
1084 // This is somewhat problematic, as the current scheme can't really
1085 // handle it turning to be a projection. This does end up as truly
1086 // ambiguous in most cases anyway.
1088 // Until this is fixed, take the fast path out - this also improves
1089 // performance by preventing assemble_candidates_from_impls from
1090 // matching every impl for this trait.
1091 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
1094 let mut candidates = SelectionCandidateSet {
1099 // Other bounds. Consider both in-scope bounds from fn decl
1100 // and applicable impls. There is a certain set of precedence rules here.
1102 match self.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) {
1103 Some(ty::BoundCopy) => {
1104 debug!("obligation self ty is {:?}",
1105 obligation.predicate.0.self_ty());
1107 // User-defined copy impls are permitted, but only for
1108 // structs and enums.
1109 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1111 // For other types, we'll use the builtin rules.
1112 let copy_conditions = self.copy_conditions(obligation);
1113 self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
1115 Some(ty::BoundSized) => {
1116 // Sized is never implementable by end-users, it is
1117 // always automatically computed.
1118 let sized_conditions = self.sized_conditions(obligation);
1119 self.assemble_builtin_bound_candidates(sized_conditions,
1123 None if self.tcx().lang_items.unsize_trait() ==
1124 Some(obligation.predicate.def_id()) => {
1125 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1128 Some(ty::BoundSend) |
1129 Some(ty::BoundSync) |
1131 self.assemble_closure_candidates(obligation, &mut candidates)?;
1132 self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
1133 self.assemble_candidates_from_impls(obligation, &mut candidates)?;
1134 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1138 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1139 self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
1140 // Default implementations have lower priority, so we only
1141 // consider triggering a default if there is no other impl that can apply.
1142 if candidates.vec.is_empty() {
1143 self.assemble_candidates_from_default_impls(obligation, &mut candidates)?;
1145 debug!("candidate list size: {}", candidates.vec.len());
1149 fn assemble_candidates_from_projected_tys(&mut self,
1150 obligation: &TraitObligation<'tcx>,
1151 candidates: &mut SelectionCandidateSet<'tcx>)
1153 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1155 // FIXME(#20297) -- just examining the self-type is very simplistic
1157 // before we go into the whole skolemization thing, just
1158 // quickly check if the self-type is a projection at all.
1159 match obligation.predicate.0.trait_ref.self_ty().sty {
1160 ty::TyProjection(_) | ty::TyAnon(..) => {}
1161 ty::TyInfer(ty::TyVar(_)) => {
1162 span_bug!(obligation.cause.span,
1163 "Self=_ should have been handled by assemble_candidates");
1168 let result = self.probe(|this, snapshot| {
1169 this.match_projection_obligation_against_definition_bounds(obligation,
1174 candidates.vec.push(ProjectionCandidate);
1178 fn match_projection_obligation_against_definition_bounds(
1180 obligation: &TraitObligation<'tcx>,
1181 snapshot: &infer::CombinedSnapshot)
1184 let poly_trait_predicate =
1185 self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1186 let (skol_trait_predicate, skol_map) =
1187 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot);
1188 debug!("match_projection_obligation_against_definition_bounds: \
1189 skol_trait_predicate={:?} skol_map={:?}",
1190 skol_trait_predicate,
1193 let (def_id, substs) = match skol_trait_predicate.trait_ref.self_ty().sty {
1194 ty::TyProjection(ref data) => (data.trait_ref.def_id, data.trait_ref.substs),
1195 ty::TyAnon(def_id, substs) => (def_id, substs),
1198 obligation.cause.span,
1199 "match_projection_obligation_against_definition_bounds() called \
1200 but self-ty not a projection: {:?}",
1201 skol_trait_predicate.trait_ref.self_ty());
1204 debug!("match_projection_obligation_against_definition_bounds: \
1205 def_id={:?}, substs={:?}",
1208 let item_predicates = self.tcx().lookup_predicates(def_id);
1209 let bounds = item_predicates.instantiate(self.tcx(), substs);
1210 debug!("match_projection_obligation_against_definition_bounds: \
1214 let matching_bound =
1215 util::elaborate_predicates(self.tcx(), bounds.predicates)
1219 |this, _| this.match_projection(obligation,
1221 skol_trait_predicate.trait_ref.clone(),
1225 debug!("match_projection_obligation_against_definition_bounds: \
1226 matching_bound={:?}",
1228 match matching_bound {
1231 // Repeat the successful match, if any, this time outside of a probe.
1232 let result = self.match_projection(obligation,
1234 skol_trait_predicate.trait_ref.clone(),
1238 self.infcx.pop_skolemized(skol_map, snapshot);
1246 fn match_projection(&mut self,
1247 obligation: &TraitObligation<'tcx>,
1248 trait_bound: ty::PolyTraitRef<'tcx>,
1249 skol_trait_ref: ty::TraitRef<'tcx>,
1250 skol_map: &infer::SkolemizationMap<'tcx>,
1251 snapshot: &infer::CombinedSnapshot)
1254 assert!(!skol_trait_ref.has_escaping_regions());
1255 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
1256 match self.infcx.sub_poly_trait_refs(false,
1258 trait_bound.clone(),
1259 ty::Binder(skol_trait_ref.clone())) {
1260 Ok(InferOk { obligations, .. }) => {
1261 self.inferred_obligations.extend(obligations);
1263 Err(_) => { return false; }
1266 self.infcx.leak_check(false, obligation.cause.span, skol_map, snapshot).is_ok()
1269 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1270 /// supplied to find out whether it is listed among them.
1272 /// Never affects inference environment.
1273 fn assemble_candidates_from_caller_bounds<'o>(&mut self,
1274 stack: &TraitObligationStack<'o, 'tcx>,
1275 candidates: &mut SelectionCandidateSet<'tcx>)
1276 -> Result<(),SelectionError<'tcx>>
1278 debug!("assemble_candidates_from_caller_bounds({:?})",
1282 self.param_env().caller_bounds
1284 .filter_map(|o| o.to_opt_poly_trait_ref());
1286 let matching_bounds =
1288 |bound| self.evaluate_where_clause(stack, bound.clone()).may_apply());
1290 let param_candidates =
1291 matching_bounds.map(|bound| ParamCandidate(bound));
1293 candidates.vec.extend(param_candidates);
1298 fn evaluate_where_clause<'o>(&mut self,
1299 stack: &TraitObligationStack<'o, 'tcx>,
1300 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
1303 self.probe(move |this, _| {
1304 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1305 Ok(obligations) => {
1306 this.evaluate_predicates_recursively(stack.list(), obligations.iter())
1308 Err(()) => EvaluatedToErr
1313 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1314 /// FnMut<..>` where `X` is a closure type.
1316 /// Note: the type parameters on a closure candidate are modeled as *output* type
1317 /// parameters and hence do not affect whether this trait is a match or not. They will be
1318 /// unified during the confirmation step.
1319 fn assemble_closure_candidates(&mut self,
1320 obligation: &TraitObligation<'tcx>,
1321 candidates: &mut SelectionCandidateSet<'tcx>)
1322 -> Result<(),SelectionError<'tcx>>
1324 let kind = match self.tcx().lang_items.fn_trait_kind(obligation.predicate.0.def_id()) {
1326 None => { return Ok(()); }
1329 // ok to skip binder because the substs on closure types never
1330 // touch bound regions, they just capture the in-scope
1331 // type/region parameters
1332 let self_ty = *obligation.self_ty().skip_binder();
1333 let (closure_def_id, substs) = match self_ty.sty {
1334 ty::TyClosure(id, substs) => (id, substs),
1335 ty::TyInfer(ty::TyVar(_)) => {
1336 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1337 candidates.ambiguous = true;
1340 _ => { return Ok(()); }
1343 debug!("assemble_unboxed_candidates: self_ty={:?} kind={:?} obligation={:?}",
1348 match self.infcx.closure_kind(closure_def_id) {
1349 Some(closure_kind) => {
1350 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
1351 if closure_kind.extends(kind) {
1352 candidates.vec.push(ClosureCandidate(closure_def_id, substs, kind));
1356 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1357 candidates.vec.push(ClosureCandidate(closure_def_id, substs, kind));
1364 /// Implement one of the `Fn()` family for a fn pointer.
1365 fn assemble_fn_pointer_candidates(&mut self,
1366 obligation: &TraitObligation<'tcx>,
1367 candidates: &mut SelectionCandidateSet<'tcx>)
1368 -> Result<(),SelectionError<'tcx>>
1370 // We provide impl of all fn traits for fn pointers.
1371 if self.tcx().lang_items.fn_trait_kind(obligation.predicate.def_id()).is_none() {
1375 // ok to skip binder because what we are inspecting doesn't involve bound regions
1376 let self_ty = *obligation.self_ty().skip_binder();
1378 ty::TyInfer(ty::TyVar(_)) => {
1379 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1380 candidates.ambiguous = true; // could wind up being a fn() type
1383 // provide an impl, but only for suitable `fn` pointers
1384 ty::TyFnDef(_, _, &ty::BareFnTy {
1385 unsafety: hir::Unsafety::Normal,
1387 sig: ty::Binder(ty::FnSig {
1393 ty::TyFnPtr(&ty::BareFnTy {
1394 unsafety: hir::Unsafety::Normal,
1396 sig: ty::Binder(ty::FnSig {
1402 candidates.vec.push(FnPointerCandidate);
1411 /// Search for impls that might apply to `obligation`.
1412 fn assemble_candidates_from_impls(&mut self,
1413 obligation: &TraitObligation<'tcx>,
1414 candidates: &mut SelectionCandidateSet<'tcx>)
1415 -> Result<(), SelectionError<'tcx>>
1417 debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
1419 let def = self.tcx().lookup_trait_def(obligation.predicate.def_id());
1421 def.for_each_relevant_impl(
1423 obligation.predicate.0.trait_ref.self_ty(),
1425 self.probe(|this, snapshot| { /* [1] */
1426 match this.match_impl(impl_def_id, obligation, snapshot) {
1428 candidates.vec.push(ImplCandidate(impl_def_id));
1430 // NB: we can safely drop the skol map
1431 // since we are in a probe [1]
1432 mem::drop(skol_map);
1443 fn assemble_candidates_from_default_impls(&mut self,
1444 obligation: &TraitObligation<'tcx>,
1445 candidates: &mut SelectionCandidateSet<'tcx>)
1446 -> Result<(), SelectionError<'tcx>>
1448 // OK to skip binder here because the tests we do below do not involve bound regions
1449 let self_ty = *obligation.self_ty().skip_binder();
1450 debug!("assemble_candidates_from_default_impls(self_ty={:?})", self_ty);
1452 let def_id = obligation.predicate.def_id();
1454 if self.tcx().trait_has_default_impl(def_id) {
1456 ty::TyTrait(..) => {
1457 // For object types, we don't know what the closed
1458 // over types are. For most traits, this means we
1459 // conservatively say nothing; a candidate may be
1460 // added by `assemble_candidates_from_object_ty`.
1461 // However, for the kind of magic reflect trait,
1462 // we consider it to be implemented even for
1463 // object types, because it just lets you reflect
1464 // onto the object type, not into the object's
1466 if self.tcx().has_attr(def_id, "rustc_reflect_like") {
1467 candidates.vec.push(DefaultImplObjectCandidate(def_id));
1471 ty::TyProjection(..) |
1473 // In these cases, we don't know what the actual
1474 // type is. Therefore, we cannot break it down
1475 // into its constituent types. So we don't
1476 // consider the `..` impl but instead just add no
1477 // candidates: this means that typeck will only
1478 // succeed if there is another reason to believe
1479 // that this obligation holds. That could be a
1480 // where-clause or, in the case of an object type,
1481 // it could be that the object type lists the
1482 // trait (e.g. `Foo+Send : Send`). See
1483 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1484 // for an example of a test case that exercises
1487 ty::TyInfer(ty::TyVar(_)) => {
1488 // the defaulted impl might apply, we don't know
1489 candidates.ambiguous = true;
1492 candidates.vec.push(DefaultImplCandidate(def_id.clone()))
1500 /// Search for impls that might apply to `obligation`.
1501 fn assemble_candidates_from_object_ty(&mut self,
1502 obligation: &TraitObligation<'tcx>,
1503 candidates: &mut SelectionCandidateSet<'tcx>)
1505 debug!("assemble_candidates_from_object_ty(self_ty={:?})",
1506 obligation.self_ty().skip_binder());
1508 // Object-safety candidates are only applicable to object-safe
1509 // traits. Including this check is useful because it helps
1510 // inference in cases of traits like `BorrowFrom`, which are
1511 // not object-safe, and which rely on being able to infer the
1512 // self-type from one of the other inputs. Without this check,
1513 // these cases wind up being considered ambiguous due to a
1514 // (spurious) ambiguity introduced here.
1515 let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
1516 if !self.tcx().is_object_safe(predicate_trait_ref.def_id()) {
1520 self.probe(|this, _snapshot| {
1521 // the code below doesn't care about regions, and the
1522 // self-ty here doesn't escape this probe, so just erase
1524 let self_ty = this.tcx().erase_late_bound_regions(&obligation.self_ty());
1525 let poly_trait_ref = match self_ty.sty {
1526 ty::TyTrait(ref data) => {
1527 match this.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) {
1528 Some(bound @ ty::BoundSend) | Some(bound @ ty::BoundSync) => {
1529 if data.builtin_bounds.contains(&bound) {
1530 debug!("assemble_candidates_from_object_ty: matched builtin bound, \
1531 pushing candidate");
1532 candidates.vec.push(BuiltinObjectCandidate);
1539 data.principal.with_self_ty(this.tcx(), self_ty)
1541 ty::TyInfer(ty::TyVar(_)) => {
1542 debug!("assemble_candidates_from_object_ty: ambiguous");
1543 candidates.ambiguous = true; // could wind up being an object type
1551 debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1554 // Count only those upcast versions that match the trait-ref
1555 // we are looking for. Specifically, do not only check for the
1556 // correct trait, but also the correct type parameters.
1557 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1558 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1559 let upcast_trait_refs =
1560 util::supertraits(this.tcx(), poly_trait_ref)
1561 .filter(|upcast_trait_ref| {
1562 this.probe(|this, _| {
1563 let upcast_trait_ref = upcast_trait_ref.clone();
1564 this.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok()
1569 if upcast_trait_refs > 1 {
1570 // can be upcast in many ways; need more type information
1571 candidates.ambiguous = true;
1572 } else if upcast_trait_refs == 1 {
1573 candidates.vec.push(ObjectCandidate);
1578 /// Search for unsizing that might apply to `obligation`.
1579 fn assemble_candidates_for_unsizing(&mut self,
1580 obligation: &TraitObligation<'tcx>,
1581 candidates: &mut SelectionCandidateSet<'tcx>) {
1582 // We currently never consider higher-ranked obligations e.g.
1583 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1584 // because they are a priori invalid, and we could potentially add support
1585 // for them later, it's just that there isn't really a strong need for it.
1586 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1587 // impl, and those are generally applied to concrete types.
1589 // That said, one might try to write a fn with a where clause like
1590 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1591 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1592 // Still, you'd be more likely to write that where clause as
1594 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1595 // obligation above. Should be possible to extend this in the future.
1596 let source = match self.tcx().no_late_bound_regions(&obligation.self_ty()) {
1599 // Don't add any candidates if there are bound regions.
1603 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
1605 debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1608 let may_apply = match (&source.sty, &target.sty) {
1609 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1610 (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => {
1611 // Upcasts permit two things:
1613 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1614 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1616 // Note that neither of these changes requires any
1617 // change at runtime. Eventually this will be
1620 // We always upcast when we can because of reason
1621 // #2 (region bounds).
1622 data_a.principal.def_id() == data_a.principal.def_id() &&
1623 data_a.builtin_bounds.is_superset(&data_b.builtin_bounds)
1627 (_, &ty::TyTrait(_)) => true,
1629 // Ambiguous handling is below T -> Trait, because inference
1630 // variables can still implement Unsize<Trait> and nested
1631 // obligations will have the final say (likely deferred).
1632 (&ty::TyInfer(ty::TyVar(_)), _) |
1633 (_, &ty::TyInfer(ty::TyVar(_))) => {
1634 debug!("assemble_candidates_for_unsizing: ambiguous");
1635 candidates.ambiguous = true;
1640 (&ty::TyArray(_, _), &ty::TySlice(_)) => true,
1642 // Struct<T> -> Struct<U>.
1643 (&ty::TyStruct(def_id_a, _), &ty::TyStruct(def_id_b, _)) => {
1644 def_id_a == def_id_b
1651 candidates.vec.push(BuiltinUnsizeCandidate);
1655 ///////////////////////////////////////////////////////////////////////////
1658 // Winnowing is the process of attempting to resolve ambiguity by
1659 // probing further. During the winnowing process, we unify all
1660 // type variables (ignoring skolemization) and then we also
1661 // attempt to evaluate recursive bounds to see if they are
1664 /// Returns true if `candidate_i` should be dropped in favor of
1665 /// `candidate_j`. Generally speaking we will drop duplicate
1666 /// candidates and prefer where-clause candidates.
1667 /// Returns true if `victim` should be dropped in favor of
1668 /// `other`. Generally speaking we will drop duplicate
1669 /// candidates and prefer where-clause candidates.
1671 /// See the comment for "SelectionCandidate" for more details.
1672 fn candidate_should_be_dropped_in_favor_of<'o>(
1674 victim: &EvaluatedCandidate<'tcx>,
1675 other: &EvaluatedCandidate<'tcx>)
1678 if victim.candidate == other.candidate {
1682 match other.candidate {
1684 ParamCandidate(_) | ProjectionCandidate => match victim.candidate {
1685 DefaultImplCandidate(..) => {
1687 "default implementations shouldn't be recorded \
1688 when there are other valid candidates");
1691 ClosureCandidate(..) |
1692 FnPointerCandidate |
1693 BuiltinObjectCandidate |
1694 BuiltinUnsizeCandidate |
1695 DefaultImplObjectCandidate(..) |
1696 BuiltinCandidate { .. } => {
1697 // We have a where-clause so don't go around looking
1702 ProjectionCandidate => {
1703 // Arbitrarily give param candidates priority
1704 // over projection and object candidates.
1707 ParamCandidate(..) => false,
1709 ImplCandidate(other_def) => {
1710 // See if we can toss out `victim` based on specialization.
1711 // This requires us to know *for sure* that the `other` impl applies
1712 // i.e. EvaluatedToOk:
1713 if other.evaluation == EvaluatedToOk {
1714 if let ImplCandidate(victim_def) = victim.candidate {
1715 let tcx = self.tcx().global_tcx();
1716 return traits::specializes(tcx, other_def, victim_def);
1726 ///////////////////////////////////////////////////////////////////////////
1729 // These cover the traits that are built-in to the language
1730 // itself. This includes `Copy` and `Sized` for sure. For the
1731 // moment, it also includes `Send` / `Sync` and a few others, but
1732 // those will hopefully change to library-defined traits in the
1735 // HACK: if this returns an error, selection exits without considering
1737 fn assemble_builtin_bound_candidates<'o>(&mut self,
1738 conditions: BuiltinImplConditions<'tcx>,
1739 candidates: &mut SelectionCandidateSet<'tcx>)
1740 -> Result<(),SelectionError<'tcx>>
1743 BuiltinImplConditions::Where(nested) => {
1744 debug!("builtin_bound: nested={:?}", nested);
1745 candidates.vec.push(BuiltinCandidate {
1746 has_nested: nested.skip_binder().len() > 0
1750 BuiltinImplConditions::None => { Ok(()) }
1751 BuiltinImplConditions::Ambiguous => {
1752 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
1753 Ok(candidates.ambiguous = true)
1755 BuiltinImplConditions::Never => { Err(Unimplemented) }
1759 fn sized_conditions(&mut self, obligation: &TraitObligation<'tcx>)
1760 -> BuiltinImplConditions<'tcx>
1762 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
1764 // NOTE: binder moved to (*)
1765 let self_ty = self.infcx.shallow_resolve(
1766 obligation.predicate.skip_binder().self_ty());
1769 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
1770 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
1771 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyRawPtr(..) |
1772 ty::TyChar | ty::TyBox(_) | ty::TyRef(..) |
1773 ty::TyArray(..) | ty::TyClosure(..) | ty::TyNever |
1775 // safe for everything
1776 Where(ty::Binder(Vec::new()))
1779 ty::TyStr | ty::TySlice(_) | ty::TyTrait(..) => Never,
1781 ty::TyTuple(tys) => {
1782 // FIXME(#33242) we only need to constrain the last field
1783 Where(ty::Binder(tys.to_vec()))
1786 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
1787 let sized_crit = def.sized_constraint(self.tcx());
1788 // (*) binder moved here
1789 Where(ty::Binder(match sized_crit.sty {
1790 ty::TyTuple(tys) => tys.to_vec().subst(self.tcx(), substs),
1791 ty::TyBool => vec![],
1792 _ => vec![sized_crit.subst(self.tcx(), substs)]
1796 ty::TyProjection(_) | ty::TyParam(_) | ty::TyAnon(..) => None,
1797 ty::TyInfer(ty::TyVar(_)) => Ambiguous,
1799 ty::TyInfer(ty::FreshTy(_))
1800 | ty::TyInfer(ty::FreshIntTy(_))
1801 | ty::TyInfer(ty::FreshFloatTy(_)) => {
1802 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
1808 fn copy_conditions(&mut self, obligation: &TraitObligation<'tcx>)
1809 -> BuiltinImplConditions<'tcx>
1811 // NOTE: binder moved to (*)
1812 let self_ty = self.infcx.shallow_resolve(
1813 obligation.predicate.skip_binder().self_ty());
1815 use self::BuiltinImplConditions::{Ambiguous, None, Never, Where};
1818 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
1819 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
1820 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyChar |
1821 ty::TyRawPtr(..) | ty::TyError | ty::TyNever |
1822 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
1823 Where(ty::Binder(Vec::new()))
1826 ty::TyBox(_) | ty::TyTrait(..) | ty::TyStr | ty::TySlice(..) |
1828 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
1832 ty::TyArray(element_ty, _) => {
1833 // (*) binder moved here
1834 Where(ty::Binder(vec![element_ty]))
1837 ty::TyTuple(tys) => {
1838 // (*) binder moved here
1839 Where(ty::Binder(tys.to_vec()))
1842 ty::TyStruct(..) | ty::TyEnum(..) |
1843 ty::TyProjection(..) | ty::TyParam(..) | ty::TyAnon(..) => {
1844 // Fallback to whatever user-defined impls exist in this case.
1848 ty::TyInfer(ty::TyVar(_)) => {
1849 // Unbound type variable. Might or might not have
1850 // applicable impls and so forth, depending on what
1851 // those type variables wind up being bound to.
1855 ty::TyInfer(ty::FreshTy(_))
1856 | ty::TyInfer(ty::FreshIntTy(_))
1857 | ty::TyInfer(ty::FreshFloatTy(_)) => {
1858 bug!("asked to assemble builtin bounds of unexpected type: {:?}",
1864 /// For default impls, we need to break apart a type into its
1865 /// "constituent types" -- meaning, the types that it contains.
1867 /// Here are some (simple) examples:
1870 /// (i32, u32) -> [i32, u32]
1871 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1872 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1873 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1875 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
1885 ty::TyInfer(ty::IntVar(_)) |
1886 ty::TyInfer(ty::FloatVar(_)) |
1894 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().collect()
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_definition_bounds(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 = obligation.derived_cause(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.input_types();
2181 let assoc_types = data.projection_bounds.iter()
2182 .map(|pb| pb.skip_binder().ty);
2183 let all_types: Vec<_> = input_types.chain(assoc_types)
2186 // reintroduce the two binding levels we skipped, then flatten into one
2187 let all_types = ty::Binder(ty::Binder(all_types));
2188 let all_types = self.tcx().flatten_late_bound_regions(&all_types);
2190 self.vtable_default_impl(obligation, trait_def_id, all_types)
2193 bug!("asked to confirm default object implementation for non-object type: {:?}",
2199 /// See `confirm_default_impl_candidate`
2200 fn vtable_default_impl(&mut self,
2201 obligation: &TraitObligation<'tcx>,
2202 trait_def_id: DefId,
2203 nested: ty::Binder<Vec<Ty<'tcx>>>)
2204 -> VtableDefaultImplData<PredicateObligation<'tcx>>
2206 debug!("vtable_default_impl: nested={:?}", nested);
2208 let cause = obligation.derived_cause(BuiltinDerivedObligation);
2209 let mut obligations = self.collect_predicates_for_types(
2211 obligation.recursion_depth+1,
2215 let trait_obligations = self.in_snapshot(|this, snapshot| {
2216 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2217 let (trait_ref, skol_map) =
2218 this.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot);
2219 let cause = obligation.derived_cause(ImplDerivedObligation);
2220 this.impl_or_trait_obligations(cause,
2221 obligation.recursion_depth + 1,
2228 obligations.extend(trait_obligations);
2230 debug!("vtable_default_impl: obligations={:?}", obligations);
2232 VtableDefaultImplData {
2233 trait_def_id: trait_def_id,
2238 fn confirm_impl_candidate(&mut self,
2239 obligation: &TraitObligation<'tcx>,
2241 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2243 debug!("confirm_impl_candidate({:?},{:?})",
2247 // First, create the substitutions by matching the impl again,
2248 // this time not in a probe.
2249 self.in_snapshot(|this, snapshot| {
2250 let (substs, skol_map) =
2251 this.rematch_impl(impl_def_id, obligation,
2253 debug!("confirm_impl_candidate substs={:?}", substs);
2254 let cause = obligation.derived_cause(ImplDerivedObligation);
2255 this.vtable_impl(impl_def_id, substs, cause,
2256 obligation.recursion_depth + 1,
2261 fn vtable_impl(&mut self,
2263 mut substs: Normalized<'tcx, &'tcx Substs<'tcx>>,
2264 cause: ObligationCause<'tcx>,
2265 recursion_depth: usize,
2266 skol_map: infer::SkolemizationMap<'tcx>,
2267 snapshot: &infer::CombinedSnapshot)
2268 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2270 debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2276 let mut impl_obligations =
2277 self.impl_or_trait_obligations(cause,
2284 debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2288 // Because of RFC447, the impl-trait-ref and obligations
2289 // are sufficient to determine the impl substs, without
2290 // relying on projections in the impl-trait-ref.
2292 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2293 impl_obligations.append(&mut substs.obligations);
2295 VtableImplData { impl_def_id: impl_def_id,
2296 substs: substs.value,
2297 nested: impl_obligations }
2300 fn confirm_object_candidate(&mut self,
2301 obligation: &TraitObligation<'tcx>)
2302 -> VtableObjectData<'tcx, PredicateObligation<'tcx>>
2304 debug!("confirm_object_candidate({:?})",
2307 // FIXME skipping binder here seems wrong -- we should
2308 // probably flatten the binder from the obligation and the
2309 // binder from the object. Have to try to make a broken test
2310 // case that results. -nmatsakis
2311 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2312 let poly_trait_ref = match self_ty.sty {
2313 ty::TyTrait(ref data) => {
2314 data.principal.with_self_ty(self.tcx(), self_ty)
2317 span_bug!(obligation.cause.span,
2318 "object candidate with non-object");
2322 let mut upcast_trait_ref = None;
2326 let tcx = self.tcx();
2328 // We want to find the first supertrait in the list of
2329 // supertraits that we can unify with, and do that
2330 // unification. We know that there is exactly one in the list
2331 // where we can unify because otherwise select would have
2332 // reported an ambiguity. (When we do find a match, also
2333 // record it for later.)
2335 util::supertraits(tcx, poly_trait_ref)
2339 |this, _| this.match_poly_trait_ref(obligation, t))
2341 Ok(_) => { upcast_trait_ref = Some(t); false }
2346 // Additionally, for each of the nonmatching predicates that
2347 // we pass over, we sum up the set of number of vtable
2348 // entries, so that we can compute the offset for the selected
2351 nonmatching.map(|t| tcx.count_own_vtable_entries(t))
2357 upcast_trait_ref: upcast_trait_ref.unwrap(),
2358 vtable_base: vtable_base,
2363 fn confirm_fn_pointer_candidate(&mut self, obligation: &TraitObligation<'tcx>)
2364 -> Result<VtableFnPointerData<'tcx, PredicateObligation<'tcx>>, SelectionError<'tcx>>
2366 debug!("confirm_fn_pointer_candidate({:?})",
2369 // ok to skip binder; it is reintroduced below
2370 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2371 let sig = self_ty.fn_sig();
2373 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2376 util::TupleArgumentsFlag::Yes)
2377 .map_bound(|(trait_ref, _)| trait_ref);
2379 self.confirm_poly_trait_refs(obligation.cause.clone(),
2380 obligation.predicate.to_poly_trait_ref(),
2382 Ok(VtableFnPointerData { fn_ty: self_ty, nested: vec![] })
2385 fn confirm_closure_candidate(&mut self,
2386 obligation: &TraitObligation<'tcx>,
2387 closure_def_id: DefId,
2388 substs: ty::ClosureSubsts<'tcx>,
2389 kind: ty::ClosureKind)
2390 -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>,
2391 SelectionError<'tcx>>
2393 debug!("confirm_closure_candidate({:?},{:?},{:?})",
2401 } = self.closure_trait_ref(obligation, closure_def_id, substs);
2403 debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2408 self.confirm_poly_trait_refs(obligation.cause.clone(),
2409 obligation.predicate.to_poly_trait_ref(),
2412 obligations.push(Obligation::new(
2413 obligation.cause.clone(),
2414 ty::Predicate::ClosureKind(closure_def_id, kind)));
2416 Ok(VtableClosureData {
2417 closure_def_id: closure_def_id,
2418 substs: substs.clone(),
2423 /// In the case of closure types and fn pointers,
2424 /// we currently treat the input type parameters on the trait as
2425 /// outputs. This means that when we have a match we have only
2426 /// considered the self type, so we have to go back and make sure
2427 /// to relate the argument types too. This is kind of wrong, but
2428 /// since we control the full set of impls, also not that wrong,
2429 /// and it DOES yield better error messages (since we don't report
2430 /// errors as if there is no applicable impl, but rather report
2431 /// errors are about mismatched argument types.
2433 /// Here is an example. Imagine we have a closure expression
2434 /// and we desugared it so that the type of the expression is
2435 /// `Closure`, and `Closure` expects an int as argument. Then it
2436 /// is "as if" the compiler generated this impl:
2438 /// impl Fn(int) for Closure { ... }
2440 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2441 /// we have matched the self-type `Closure`. At this point we'll
2442 /// compare the `int` to `usize` and generate an error.
2444 /// Note that this checking occurs *after* the impl has selected,
2445 /// because these output type parameters should not affect the
2446 /// selection of the impl. Therefore, if there is a mismatch, we
2447 /// report an error to the user.
2448 fn confirm_poly_trait_refs(&mut self,
2449 obligation_cause: ObligationCause,
2450 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
2451 expected_trait_ref: ty::PolyTraitRef<'tcx>)
2452 -> Result<(), SelectionError<'tcx>>
2454 let origin = TypeOrigin::RelateOutputImplTypes(obligation_cause.span);
2456 let obligation_trait_ref = obligation_trait_ref.clone();
2457 self.infcx.sub_poly_trait_refs(false,
2459 expected_trait_ref.clone(),
2460 obligation_trait_ref.clone())
2461 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
2462 .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
2465 fn confirm_builtin_unsize_candidate(&mut self,
2466 obligation: &TraitObligation<'tcx>,)
2467 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>,
2468 SelectionError<'tcx>> {
2469 let tcx = self.tcx();
2471 // assemble_candidates_for_unsizing should ensure there are no late bound
2472 // regions here. See the comment there for more details.
2473 let source = self.infcx.shallow_resolve(
2474 tcx.no_late_bound_regions(&obligation.self_ty()).unwrap());
2475 let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
2476 let target = self.infcx.shallow_resolve(target);
2478 debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2481 let mut nested = vec![];
2482 match (&source.sty, &target.sty) {
2483 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2484 (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => {
2485 // See assemble_candidates_for_unsizing for more info.
2486 let new_trait = tcx.mk_trait(ty::TraitObject {
2487 principal: data_a.principal,
2488 region_bound: data_b.region_bound,
2489 builtin_bounds: data_b.builtin_bounds,
2490 projection_bounds: data_a.projection_bounds.clone(),
2492 let origin = TypeOrigin::Misc(obligation.cause.span);
2493 let InferOk { obligations, .. } =
2494 self.infcx.sub_types(false, origin, new_trait, target)
2495 .map_err(|_| Unimplemented)?;
2496 self.inferred_obligations.extend(obligations);
2498 // Register one obligation for 'a: 'b.
2499 let cause = ObligationCause::new(obligation.cause.span,
2500 obligation.cause.body_id,
2501 ObjectCastObligation(target));
2502 let outlives = ty::OutlivesPredicate(data_a.region_bound,
2503 data_b.region_bound);
2504 nested.push(Obligation::with_depth(cause,
2505 obligation.recursion_depth + 1,
2506 ty::Binder(outlives).to_predicate()));
2510 (_, &ty::TyTrait(ref data)) => {
2511 let mut object_dids = Some(data.principal.def_id()).into_iter();
2513 // data.builtin_bounds.iter().flat_map(|bound| {
2514 // tcx.lang_items.from_builtin_kind(bound).ok()
2516 // .chain(Some(data.principal.def_id()));
2517 if let Some(did) = object_dids.find(|did| {
2518 !tcx.is_object_safe(*did)
2520 return Err(TraitNotObjectSafe(did))
2523 let cause = ObligationCause::new(obligation.cause.span,
2524 obligation.cause.body_id,
2525 ObjectCastObligation(target));
2526 let mut push = |predicate| {
2527 nested.push(Obligation::with_depth(cause.clone(),
2528 obligation.recursion_depth + 1,
2532 // Create the obligation for casting from T to Trait.
2533 push(data.principal.with_self_ty(tcx, source).to_predicate());
2535 // We can only make objects from sized types.
2536 let mut builtin_bounds = data.builtin_bounds;
2537 builtin_bounds.insert(ty::BoundSized);
2539 // Create additional obligations for all the various builtin
2540 // bounds attached to the object cast. (In other words, if the
2541 // object type is Foo+Send, this would create an obligation
2542 // for the Send check.)
2543 for bound in &builtin_bounds {
2544 if let Ok(tr) = tcx.trait_ref_for_builtin_bound(bound, source) {
2545 push(tr.to_predicate());
2547 return Err(Unimplemented);
2551 // Create obligations for the projection predicates.
2552 for bound in &data.projection_bounds {
2553 push(bound.with_self_ty(tcx, source).to_predicate());
2556 // If the type is `Foo+'a`, ensures that the type
2557 // being cast to `Foo+'a` outlives `'a`:
2558 let outlives = ty::OutlivesPredicate(source, data.region_bound);
2559 push(ty::Binder(outlives).to_predicate());
2563 (&ty::TyArray(a, _), &ty::TySlice(b)) => {
2564 let origin = TypeOrigin::Misc(obligation.cause.span);
2565 let InferOk { obligations, .. } =
2566 self.infcx.sub_types(false, origin, a, b)
2567 .map_err(|_| Unimplemented)?;
2568 self.inferred_obligations.extend(obligations);
2571 // Struct<T> -> Struct<U>.
2572 (&ty::TyStruct(def, substs_a), &ty::TyStruct(_, substs_b)) => {
2575 .map(|f| f.unsubst_ty())
2576 .collect::<Vec<_>>();
2578 // The last field of the structure has to exist and contain type parameters.
2579 let field = if let Some(&field) = fields.last() {
2582 return Err(Unimplemented);
2584 let mut ty_params = BitVector::new(substs_a.types().count());
2585 let mut found = false;
2586 for ty in field.walk() {
2587 if let ty::TyParam(p) = ty.sty {
2588 ty_params.insert(p.idx as usize);
2593 return Err(Unimplemented);
2596 // Replace type parameters used in unsizing with
2597 // TyError and ensure they do not affect any other fields.
2598 // This could be checked after type collection for any struct
2599 // with a potentially unsized trailing field.
2600 let params = substs_a.params().iter().enumerate().map(|(i, &k)| {
2601 if ty_params.contains(i) {
2602 Kind::from(tcx.types.err)
2607 let substs = Substs::new(tcx, params);
2608 for &ty in fields.split_last().unwrap().1 {
2609 if ty.subst(tcx, substs).references_error() {
2610 return Err(Unimplemented);
2614 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2615 let inner_source = field.subst(tcx, substs_a);
2616 let inner_target = field.subst(tcx, substs_b);
2618 // Check that the source structure with the target's
2619 // type parameters is a subtype of the target.
2620 let params = substs_a.params().iter().enumerate().map(|(i, &k)| {
2621 if ty_params.contains(i) {
2622 Kind::from(substs_b.type_at(i))
2627 let new_struct = tcx.mk_struct(def, Substs::new(tcx, params));
2628 let origin = TypeOrigin::Misc(obligation.cause.span);
2629 let InferOk { obligations, .. } =
2630 self.infcx.sub_types(false, origin, new_struct, target)
2631 .map_err(|_| Unimplemented)?;
2632 self.inferred_obligations.extend(obligations);
2634 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2635 nested.push(tcx.predicate_for_trait_def(
2636 obligation.cause.clone(),
2637 obligation.predicate.def_id(),
2638 obligation.recursion_depth + 1,
2646 Ok(VtableBuiltinData { nested: nested })
2649 ///////////////////////////////////////////////////////////////////////////
2652 // Matching is a common path used for both evaluation and
2653 // confirmation. It basically unifies types that appear in impls
2654 // and traits. This does affect the surrounding environment;
2655 // therefore, when used during evaluation, match routines must be
2656 // run inside of a `probe()` so that their side-effects are
2659 fn rematch_impl(&mut self,
2661 obligation: &TraitObligation<'tcx>,
2662 snapshot: &infer::CombinedSnapshot)
2663 -> (Normalized<'tcx, &'tcx Substs<'tcx>>,
2664 infer::SkolemizationMap<'tcx>)
2666 match self.match_impl(impl_def_id, obligation, snapshot) {
2667 Ok((substs, skol_map)) => (substs, skol_map),
2669 bug!("Impl {:?} was matchable against {:?} but now is not",
2676 fn match_impl(&mut self,
2678 obligation: &TraitObligation<'tcx>,
2679 snapshot: &infer::CombinedSnapshot)
2680 -> Result<(Normalized<'tcx, &'tcx Substs<'tcx>>,
2681 infer::SkolemizationMap<'tcx>), ()>
2683 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
2685 // Before we create the substitutions and everything, first
2686 // consider a "quick reject". This avoids creating more types
2687 // and so forth that we need to.
2688 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
2692 let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions(
2693 &obligation.predicate,
2695 let skol_obligation_trait_ref = skol_obligation.trait_ref;
2697 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span,
2700 let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
2703 let impl_trait_ref =
2704 project::normalize_with_depth(self,
2705 obligation.cause.clone(),
2706 obligation.recursion_depth + 1,
2709 debug!("match_impl(impl_def_id={:?}, obligation={:?}, \
2710 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
2714 skol_obligation_trait_ref);
2716 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
2717 let InferOk { obligations, .. } =
2718 self.infcx.eq_trait_refs(false,
2720 impl_trait_ref.value.clone(),
2721 skol_obligation_trait_ref)
2723 debug!("match_impl: failed eq_trait_refs due to `{}`", e);
2726 self.inferred_obligations.extend(obligations);
2728 if let Err(e) = self.infcx.leak_check(false,
2729 obligation.cause.span,
2732 debug!("match_impl: failed leak check due to `{}`", e);
2736 debug!("match_impl: success impl_substs={:?}", impl_substs);
2739 obligations: impl_trait_ref.obligations
2743 fn fast_reject_trait_refs(&mut self,
2744 obligation: &TraitObligation,
2745 impl_trait_ref: &ty::TraitRef)
2748 // We can avoid creating type variables and doing the full
2749 // substitution if we find that any of the input types, when
2750 // simplified, do not match.
2752 obligation.predicate.skip_binder().input_types()
2753 .zip(impl_trait_ref.input_types())
2754 .any(|(obligation_ty, impl_ty)| {
2755 let simplified_obligation_ty =
2756 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
2757 let simplified_impl_ty =
2758 fast_reject::simplify_type(self.tcx(), impl_ty, false);
2760 simplified_obligation_ty.is_some() &&
2761 simplified_impl_ty.is_some() &&
2762 simplified_obligation_ty != simplified_impl_ty
2766 /// Normalize `where_clause_trait_ref` and try to match it against
2767 /// `obligation`. If successful, return any predicates that
2768 /// result from the normalization. Normalization is necessary
2769 /// because where-clauses are stored in the parameter environment
2771 fn match_where_clause_trait_ref(&mut self,
2772 obligation: &TraitObligation<'tcx>,
2773 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
2774 -> Result<Vec<PredicateObligation<'tcx>>,()>
2776 self.match_poly_trait_ref(obligation, where_clause_trait_ref)?;
2780 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2781 /// obligation is satisfied.
2782 fn match_poly_trait_ref(&mut self,
2783 obligation: &TraitObligation<'tcx>,
2784 poly_trait_ref: ty::PolyTraitRef<'tcx>)
2787 debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
2791 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
2792 self.infcx.sub_poly_trait_refs(false,
2795 obligation.predicate.to_poly_trait_ref())
2796 .map(|InferOk { obligations, .. }| self.inferred_obligations.extend(obligations))
2800 ///////////////////////////////////////////////////////////////////////////
2803 fn match_fresh_trait_refs(&self,
2804 previous: &ty::PolyTraitRef<'tcx>,
2805 current: &ty::PolyTraitRef<'tcx>)
2808 let mut matcher = ty::_match::Match::new(self.tcx());
2809 matcher.relate(previous, current).is_ok()
2812 fn push_stack<'o,'s:'o>(&mut self,
2813 previous_stack: TraitObligationStackList<'s, 'tcx>,
2814 obligation: &'o TraitObligation<'tcx>)
2815 -> TraitObligationStack<'o, 'tcx>
2817 let fresh_trait_ref =
2818 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
2820 TraitObligationStack {
2821 obligation: obligation,
2822 fresh_trait_ref: fresh_trait_ref,
2823 previous: previous_stack,
2827 fn closure_trait_ref_unnormalized(&mut self,
2828 obligation: &TraitObligation<'tcx>,
2829 closure_def_id: DefId,
2830 substs: ty::ClosureSubsts<'tcx>)
2831 -> ty::PolyTraitRef<'tcx>
2833 let closure_type = self.infcx.closure_type(closure_def_id, substs);
2834 let ty::Binder((trait_ref, _)) =
2835 self.tcx().closure_trait_ref_and_return_type(obligation.predicate.def_id(),
2836 obligation.predicate.0.self_ty(), // (1)
2838 util::TupleArgumentsFlag::No);
2839 // (1) Feels icky to skip the binder here, but OTOH we know
2840 // that the self-type is an unboxed closure type and hence is
2841 // in fact unparameterized (or at least does not reference any
2842 // regions bound in the obligation). Still probably some
2843 // refactoring could make this nicer.
2845 ty::Binder(trait_ref)
2848 fn closure_trait_ref(&mut self,
2849 obligation: &TraitObligation<'tcx>,
2850 closure_def_id: DefId,
2851 substs: ty::ClosureSubsts<'tcx>)
2852 -> Normalized<'tcx, ty::PolyTraitRef<'tcx>>
2854 let trait_ref = self.closure_trait_ref_unnormalized(
2855 obligation, closure_def_id, substs);
2857 // A closure signature can contain associated types which
2858 // must be normalized.
2859 normalize_with_depth(self,
2860 obligation.cause.clone(),
2861 obligation.recursion_depth+1,
2865 /// Returns the obligations that are implied by instantiating an
2866 /// impl or trait. The obligations are substituted and fully
2867 /// normalized. This is used when confirming an impl or default
2869 fn impl_or_trait_obligations(&mut self,
2870 cause: ObligationCause<'tcx>,
2871 recursion_depth: usize,
2872 def_id: DefId, // of impl or trait
2873 substs: &Substs<'tcx>, // for impl or trait
2874 skol_map: infer::SkolemizationMap<'tcx>,
2875 snapshot: &infer::CombinedSnapshot)
2876 -> Vec<PredicateObligation<'tcx>>
2878 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
2879 let tcx = self.tcx();
2881 // To allow for one-pass evaluation of the nested obligation,
2882 // each predicate must be preceded by the obligations required
2884 // for example, if we have:
2885 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
2886 // the impl will have the following predicates:
2887 // <V as Iterator>::Item = U,
2888 // U: Iterator, U: Sized,
2889 // V: Iterator, V: Sized,
2890 // <U as Iterator>::Item: Copy
2891 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2892 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2893 // `$1: Copy`, so we must ensure the obligations are emitted in
2895 let predicates = tcx.lookup_predicates(def_id);
2896 assert_eq!(predicates.parent, None);
2897 let predicates = predicates.predicates.iter().flat_map(|predicate| {
2898 let predicate = 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)
2911 impl<'tcx> TraitObligation<'tcx> {
2912 #[allow(unused_comparisons)]
2913 pub fn derived_cause(&self,
2914 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
2915 -> ObligationCause<'tcx>
2918 * Creates a cause for obligations that are derived from
2919 * `obligation` by a recursive search (e.g., for a builtin
2920 * bound, or eventually a `impl Foo for ..`). If `obligation`
2921 * is itself a derived obligation, this is just a clone, but
2922 * otherwise we create a "derived obligation" cause so as to
2923 * keep track of the original root obligation for error
2927 let obligation = self;
2929 // NOTE(flaper87): As of now, it keeps track of the whole error
2930 // chain. Ideally, we should have a way to configure this either
2931 // by using -Z verbose or just a CLI argument.
2932 if obligation.recursion_depth >= 0 {
2933 let derived_cause = DerivedObligationCause {
2934 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
2935 parent_code: Rc::new(obligation.cause.code.clone())
2937 let derived_code = variant(derived_cause);
2938 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
2940 obligation.cause.clone()
2945 impl<'tcx> SelectionCache<'tcx> {
2946 pub fn new() -> SelectionCache<'tcx> {
2948 hashmap: RefCell::new(FnvHashMap())
2953 impl<'tcx> EvaluationCache<'tcx> {
2954 pub fn new() -> EvaluationCache<'tcx> {
2956 hashmap: RefCell::new(FnvHashMap())
2961 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {
2962 fn list(&'o self) -> TraitObligationStackList<'o,'tcx> {
2963 TraitObligationStackList::with(self)
2966 fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> {
2971 #[derive(Copy, Clone)]
2972 struct TraitObligationStackList<'o,'tcx:'o> {
2973 head: Option<&'o TraitObligationStack<'o,'tcx>>
2976 impl<'o,'tcx> TraitObligationStackList<'o,'tcx> {
2977 fn empty() -> TraitObligationStackList<'o,'tcx> {
2978 TraitObligationStackList { head: None }
2981 fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> {
2982 TraitObligationStackList { head: Some(r) }
2986 impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{
2987 type Item = &'o TraitObligationStack<'o,'tcx>;
2989 fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> {
3000 impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> {
3001 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
3002 write!(f, "TraitObligationStack({:?})", self.obligation)
3006 impl EvaluationResult {
3007 fn may_apply(&self) -> bool {
3011 EvaluatedToUnknown => true,
3013 EvaluatedToErr => false
3018 impl MethodMatchResult {
3019 pub fn may_apply(&self) -> bool {
3021 MethodMatched(_) => true,
3022 MethodAmbiguous(_) => true,
3023 MethodDidNotMatch => false,