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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.
4 //
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.
10
11 //! See `README.md` for high-level documentation
12
13 pub use self::MethodMatchResult::*;
14 pub use self::MethodMatchedData::*;
15 use self::SelectionCandidate::*;
16 use self::BuiltinBoundConditions::*;
17 use self::EvaluationResult::*;
18
19 use super::coherence;
20 use super::DerivedObligationCause;
21 use super::project;
22 use super::project::{normalize_with_depth, Normalized};
23 use super::{PredicateObligation, TraitObligation, ObligationCause};
24 use super::report_overflow_error;
25 use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation};
26 use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch};
27 use super::{ObjectCastObligation, Obligation};
28 use super::TraitNotObjectSafe;
29 use super::Selection;
30 use super::SelectionResult;
31 use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure,
32 VtableFnPointer, VtableObject, VtableDefaultImpl};
33 use super::{VtableImplData, VtableObjectData, VtableBuiltinData,
34 VtableClosureData, VtableDefaultImplData};
35 use super::object_safety;
36 use super::util;
37
38 use middle::def_id::DefId;
39 use middle::infer;
40 use middle::infer::{InferCtxt, TypeFreshener, TypeOrigin};
41 use middle::subst::{Subst, Substs, TypeSpace};
42 use middle::ty::{self, ToPredicate, ToPolyTraitRef, Ty, TypeFoldable};
43 use middle::ty::fast_reject;
44 use middle::ty::relate::TypeRelation;
45
46 use std::cell::RefCell;
47 use std::fmt;
48 use std::rc::Rc;
49 use syntax::abi::Abi;
50 use rustc_front::hir;
51 use util::common::ErrorReported;
52 use util::nodemap::FnvHashMap;
53
54 pub struct SelectionContext<'cx, 'tcx:'cx> {
55 infcx: &'cx InferCtxt<'cx, 'tcx>,
56
57 /// Freshener used specifically for skolemizing entries on the
58 /// obligation stack. This ensures that all entries on the stack
59 /// at one time will have the same set of skolemized entries,
60 /// which is important for checking for trait bounds that
61 /// recursively require themselves.
62 freshener: TypeFreshener<'cx, 'tcx>,
63
64 /// If true, indicates that the evaluation should be conservative
65 /// and consider the possibility of types outside this crate.
66 /// This comes up primarily when resolving ambiguity. Imagine
67 /// there is some trait reference `$0 : Bar` where `$0` is an
68 /// inference variable. If `intercrate` is true, then we can never
69 /// say for sure that this reference is not implemented, even if
70 /// there are *no impls at all for `Bar`*, because `$0` could be
71 /// bound to some type that in a downstream crate that implements
72 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
73 /// though, we set this to false, because we are only interested
74 /// in types that the user could actually have written --- in
75 /// other words, we consider `$0 : Bar` to be unimplemented if
76 /// there is no type that the user could *actually name* that
77 /// would satisfy it. This avoids crippling inference, basically.
78
79 intercrate: bool,
80 }
81
82 // A stack that walks back up the stack frame.
83 struct TraitObligationStack<'prev, 'tcx: 'prev> {
84 obligation: &'prev TraitObligation<'tcx>,
85
86 /// Trait ref from `obligation` but skolemized with the
87 /// selection-context's freshener. Used to check for recursion.
88 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
89
90 previous: TraitObligationStackList<'prev, 'tcx>,
91 }
92
93 #[derive(Clone)]
94 pub struct SelectionCache<'tcx> {
95 hashmap: RefCell<FnvHashMap<ty::TraitRef<'tcx>,
96 SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
97 }
98
99 pub enum MethodMatchResult {
100 MethodMatched(MethodMatchedData),
101 MethodAmbiguous(/* list of impls that could apply */ Vec<DefId>),
102 MethodDidNotMatch,
103 }
104
105 #[derive(Copy, Clone, Debug)]
106 pub enum MethodMatchedData {
107 // In the case of a precise match, we don't really need to store
108 // how the match was found. So don't.
109 PreciseMethodMatch,
110
111 // In the case of a coercion, we need to know the precise impl so
112 // that we can determine the type to which things were coerced.
113 CoerciveMethodMatch(/* impl we matched */ DefId)
114 }
115
116 /// The selection process begins by considering all impls, where
117 /// clauses, and so forth that might resolve an obligation. Sometimes
118 /// we'll be able to say definitively that (e.g.) an impl does not
119 /// apply to the obligation: perhaps it is defined for `usize` but the
120 /// obligation is for `int`. In that case, we drop the impl out of the
121 /// list. But the other cases are considered *candidates*.
122 ///
123 /// For selection to succeed, there must be exactly one matching
124 /// candidate. If the obligation is fully known, this is guaranteed
125 /// by coherence. However, if the obligation contains type parameters
126 /// or variables, there may be multiple such impls.
127 ///
128 /// It is not a real problem if multiple matching impls exist because
129 /// of type variables - it just means the obligation isn't sufficiently
130 /// elaborated. In that case we report an ambiguity, and the caller can
131 /// try again after more type information has been gathered or report a
132 /// "type annotations required" error.
133 ///
134 /// However, with type parameters, this can be a real problem - type
135 /// parameters don't unify with regular types, but they *can* unify
136 /// with variables from blanket impls, and (unless we know its bounds
137 /// will always be satisfied) picking the blanket impl will be wrong
138 /// for at least *some* substitutions. To make this concrete, if we have
139 ///
140 /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; }
141 /// impl<T: fmt::Debug> AsDebug for T {
142 /// type Out = T;
143 /// fn debug(self) -> fmt::Debug { self }
144 /// }
145 /// fn foo<T: AsDebug>(t: T) { println!("{:?}", <T as AsDebug>::debug(t)); }
146 ///
147 /// we can't just use the impl to resolve the <T as AsDebug> obligation
148 /// - a type from another crate (that doesn't implement fmt::Debug) could
149 /// implement AsDebug.
150 ///
151 /// Because where-clauses match the type exactly, multiple clauses can
152 /// only match if there are unresolved variables, and we can mostly just
153 /// report this ambiguity in that case. This is still a problem - we can't
154 /// *do anything* with ambiguities that involve only regions. This is issue
155 /// #21974.
156 ///
157 /// If a single where-clause matches and there are no inference
158 /// variables left, then it definitely matches and we can just select
159 /// it.
160 ///
161 /// In fact, we even select the where-clause when the obligation contains
162 /// inference variables. The can lead to inference making "leaps of logic",
163 /// for example in this situation:
164 ///
165 /// pub trait Foo<T> { fn foo(&self) -> T; }
166 /// impl<T> Foo<()> for T { fn foo(&self) { } }
167 /// impl Foo<bool> for bool { fn foo(&self) -> bool { *self } }
168 ///
169 /// pub fn foo<T>(t: T) where T: Foo<bool> {
170 /// println!("{:?}", <T as Foo<_>>::foo(&t));
171 /// }
172 /// fn main() { foo(false); }
173 ///
174 /// Here the obligation <T as Foo<$0>> can be matched by both the blanket
175 /// impl and the where-clause. We select the where-clause and unify $0=bool,
176 /// so the program prints "false". However, if the where-clause is omitted,
177 /// the blanket impl is selected, we unify $0=(), and the program prints
178 /// "()".
179 ///
180 /// Exactly the same issues apply to projection and object candidates, except
181 /// that we can have both a projection candidate and a where-clause candidate
182 /// for the same obligation. In that case either would do (except that
183 /// different "leaps of logic" would occur if inference variables are
184 /// present), and we just pick the where-clause. This is, for example,
185 /// required for associated types to work in default impls, as the bounds
186 /// are visible both as projection bounds and as where-clauses from the
187 /// parameter environment.
188 #[derive(PartialEq,Eq,Debug,Clone)]
189 enum SelectionCandidate<'tcx> {
190 BuiltinCandidate(ty::BuiltinBound),
191 ParamCandidate(ty::PolyTraitRef<'tcx>),
192 ImplCandidate(DefId),
193 DefaultImplCandidate(DefId),
194 DefaultImplObjectCandidate(DefId),
195
196 /// This is a trait matching with a projected type as `Self`, and
197 /// we found an applicable bound in the trait definition.
198 ProjectionCandidate,
199
200 /// Implementation of a `Fn`-family trait by one of the
201 /// anonymous types generated for a `||` expression.
202 ClosureCandidate(/* closure */ DefId, &'tcx ty::ClosureSubsts<'tcx>),
203
204 /// Implementation of a `Fn`-family trait by one of the anonymous
205 /// types generated for a fn pointer type (e.g., `fn(int)->int`)
206 FnPointerCandidate,
207
208 ObjectCandidate,
209
210 BuiltinObjectCandidate,
211
212 BuiltinUnsizeCandidate,
213 }
214
215 struct SelectionCandidateSet<'tcx> {
216 // a list of candidates that definitely apply to the current
217 // obligation (meaning: types unify).
218 vec: Vec<SelectionCandidate<'tcx>>,
219
220 // if this is true, then there were candidates that might or might
221 // not have applied, but we couldn't tell. This occurs when some
222 // of the input types are type variables, in which case there are
223 // various "builtin" rules that might or might not trigger.
224 ambiguous: bool,
225 }
226
227 enum BuiltinBoundConditions<'tcx> {
228 If(ty::Binder<Vec<Ty<'tcx>>>),
229 ParameterBuiltin,
230 AmbiguousBuiltin
231 }
232
233 #[derive(Copy, Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
234 /// The result of trait evaluation. The order is important
235 /// here as the evaluation of a list is the maximum of the
236 /// evaluations.
237 enum EvaluationResult {
238 /// Evaluation successful
239 EvaluatedToOk,
240 /// Evaluation failed because of recursion - treated as ambiguous
241 EvaluatedToUnknown,
242 /// Evaluation is known to be ambiguous
243 EvaluatedToAmbig,
244 /// Evaluation failed
245 EvaluatedToErr,
246 }
247
248 #[derive(Clone)]
249 pub struct EvaluationCache<'tcx> {
250 hashmap: RefCell<FnvHashMap<ty::PolyTraitRef<'tcx>, EvaluationResult>>
251 }
252
253 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
254 pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>)
255 -> SelectionContext<'cx, 'tcx> {
256 SelectionContext {
257 infcx: infcx,
258 freshener: infcx.freshener(),
259 intercrate: false,
260 }
261 }
262
263 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>)
264 -> SelectionContext<'cx, 'tcx> {
265 SelectionContext {
266 infcx: infcx,
267 freshener: infcx.freshener(),
268 intercrate: true,
269 }
270 }
271
272 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {
273 self.infcx
274 }
275
276 pub fn tcx(&self) -> &'cx ty::ctxt<'tcx> {
277 self.infcx.tcx
278 }
279
280 pub fn param_env(&self) -> &'cx ty::ParameterEnvironment<'cx, 'tcx> {
281 self.infcx.param_env()
282 }
283
284 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'tcx> {
285 self.infcx
286 }
287
288 ///////////////////////////////////////////////////////////////////////////
289 // Selection
290 //
291 // The selection phase tries to identify *how* an obligation will
292 // be resolved. For example, it will identify which impl or
293 // parameter bound is to be used. The process can be inconclusive
294 // if the self type in the obligation is not fully inferred. Selection
295 // can result in an error in one of two ways:
296 //
297 // 1. If no applicable impl or parameter bound can be found.
298 // 2. If the output type parameters in the obligation do not match
299 // those specified by the impl/bound. For example, if the obligation
300 // is `Vec<Foo>:Iterable<Bar>`, but the impl specifies
301 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
302
303 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
304 /// type environment by performing unification.
305 pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
306 -> SelectionResult<'tcx, Selection<'tcx>> {
307 debug!("select({:?})", obligation);
308 assert!(!obligation.predicate.has_escaping_regions());
309
310 let dep_node = obligation.predicate.dep_node();
311 let _task = self.tcx().dep_graph.in_task(dep_node);
312
313 let stack = self.push_stack(TraitObligationStackList::empty(), obligation);
314 match try!(self.candidate_from_obligation(&stack)) {
315 None => {
316 self.consider_unification_despite_ambiguity(obligation);
317 Ok(None)
318 }
319 Some(candidate) => Ok(Some(try!(self.confirm_candidate(obligation, candidate)))),
320 }
321 }
322
323 /// In the particular case of unboxed closure obligations, we can
324 /// sometimes do some amount of unification for the
325 /// argument/return types even though we can't yet fully match obligation.
326 /// The particular case we are interesting in is an obligation of the form:
327 ///
328 /// C : FnFoo<A>
329 ///
330 /// where `C` is an unboxed closure type and `FnFoo` is one of the
331 /// `Fn` traits. Because we know that users cannot write impls for closure types
332 /// themselves, the only way that `C : FnFoo` can fail to match is under two
333 /// conditions:
334 ///
335 /// 1. The closure kind for `C` is not yet known, because inference isn't complete.
336 /// 2. The closure kind for `C` *is* known, but doesn't match what is needed.
337 /// For example, `C` may be a `FnOnce` closure, but a `Fn` closure is needed.
338 ///
339 /// In either case, we always know what argument types are
340 /// expected by `C`, no matter what kind of `Fn` trait it
341 /// eventually matches. So we can go ahead and unify the argument
342 /// types, even though the end result is ambiguous.
343 ///
344 /// Note that this is safe *even if* the trait would never be
345 /// matched (case 2 above). After all, in that case, an error will
346 /// result, so it kind of doesn't matter what we do --- unifying
347 /// the argument types can only be helpful to the user, because
348 /// once they patch up the kind of closure that is expected, the
349 /// argment types won't really change.
350 fn consider_unification_despite_ambiguity(&mut self, obligation: &TraitObligation<'tcx>) {
351 // Is this a `C : FnFoo(...)` trait reference for some trait binding `FnFoo`?
352 match self.tcx().lang_items.fn_trait_kind(obligation.predicate.0.def_id()) {
353 Some(_) => { }
354 None => { return; }
355 }
356
357 // Is the self-type a closure type? We ignore bindings here
358 // because if it is a closure type, it must be a closure type from
359 // within this current fn, and hence none of the higher-ranked
360 // lifetimes can appear inside the self-type.
361 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
362 let (closure_def_id, substs) = match self_ty.sty {
363 ty::TyClosure(id, ref substs) => (id, substs),
364 _ => { return; }
365 };
366 assert!(!substs.has_escaping_regions());
367
368 // It is OK to call the unnormalized variant here - this is only
369 // reached for TyClosure: Fn inputs where the closure kind is
370 // still unknown, which should only occur in typeck where the
371 // closure type is already normalized.
372 let closure_trait_ref = self.closure_trait_ref_unnormalized(obligation,
373 closure_def_id,
374 substs);
375
376 match self.confirm_poly_trait_refs(obligation.cause.clone(),
377 obligation.predicate.to_poly_trait_ref(),
378 closure_trait_ref) {
379 Ok(()) => { }
380 Err(_) => { /* Silently ignore errors. */ }
381 }
382 }
383
384 ///////////////////////////////////////////////////////////////////////////
385 // EVALUATION
386 //
387 // Tests whether an obligation can be selected or whether an impl
388 // can be applied to particular types. It skips the "confirmation"
389 // step and hence completely ignores output type parameters.
390 //
391 // The result is "true" if the obligation *may* hold and "false" if
392 // we can be sure it does not.
393
394
395 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
396 pub fn evaluate_obligation(&mut self,
397 obligation: &PredicateObligation<'tcx>)
398 -> bool
399 {
400 debug!("evaluate_obligation({:?})",
401 obligation);
402
403 self.infcx.probe(|_| {
404 self.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
405 .may_apply()
406 })
407 }
408
409 /// Evaluates whether the obligation `obligation` can be satisfied,
410 /// and returns `false` if not certain. However, this is not entirely
411 /// accurate if inference variables are involved.
412 pub fn evaluate_obligation_conservatively(&mut self,
413 obligation: &PredicateObligation<'tcx>)
414 -> bool
415 {
416 debug!("evaluate_obligation_conservatively({:?})",
417 obligation);
418
419 self.infcx.probe(|_| {
420 self.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
421 == EvaluatedToOk
422 })
423 }
424
425 /// Evaluates the predicates in `predicates` recursively. Note that
426 /// this applies projections in the predicates, and therefore
427 /// is run within an inference probe.
428 fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
429 stack: TraitObligationStackList<'o, 'tcx>,
430 predicates: I)
431 -> EvaluationResult
432 where I : Iterator<Item=&'a PredicateObligation<'tcx>>, 'tcx:'a
433 {
434 let mut result = EvaluatedToOk;
435 for obligation in predicates {
436 let eval = self.evaluate_predicate_recursively(stack, obligation);
437 debug!("evaluate_predicate_recursively({:?}) = {:?}",
438 obligation, eval);
439 match eval {
440 EvaluatedToErr => { return EvaluatedToErr; }
441 EvaluatedToAmbig => { result = EvaluatedToAmbig; }
442 EvaluatedToUnknown => {
443 if result < EvaluatedToUnknown {
444 result = EvaluatedToUnknown;
445 }
446 }
447 EvaluatedToOk => { }
448 }
449 }
450 result
451 }
452
453 fn evaluate_predicate_recursively<'o>(&mut self,
454 previous_stack: TraitObligationStackList<'o, 'tcx>,
455 obligation: &PredicateObligation<'tcx>)
456 -> EvaluationResult
457 {
458 debug!("evaluate_predicate_recursively({:?})",
459 obligation);
460
461 // Check the cache from the tcx of predicates that we know
462 // have been proven elsewhere. This cache only contains
463 // predicates that are global in scope and hence unaffected by
464 // the current environment.
465 if self.tcx().fulfilled_predicates.borrow().check_duplicate(&obligation.predicate) {
466 return EvaluatedToOk;
467 }
468
469 match obligation.predicate {
470 ty::Predicate::Trait(ref t) => {
471 assert!(!t.has_escaping_regions());
472 let obligation = obligation.with(t.clone());
473 self.evaluate_obligation_recursively(previous_stack, &obligation)
474 }
475
476 ty::Predicate::Equate(ref p) => {
477 // does this code ever run?
478 match self.infcx.equality_predicate(obligation.cause.span, p) {
479 Ok(()) => EvaluatedToOk,
480 Err(_) => EvaluatedToErr
481 }
482 }
483
484 ty::Predicate::WellFormed(ty) => {
485 match ty::wf::obligations(self.infcx, obligation.cause.body_id,
486 ty, obligation.cause.span) {
487 Some(obligations) =>
488 self.evaluate_predicates_recursively(previous_stack, obligations.iter()),
489 None =>
490 EvaluatedToAmbig,
491 }
492 }
493
494 ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => {
495 // we do not consider region relationships when
496 // evaluating trait matches
497 EvaluatedToOk
498 }
499
500 ty::Predicate::ObjectSafe(trait_def_id) => {
501 if object_safety::is_object_safe(self.tcx(), trait_def_id) {
502 EvaluatedToOk
503 } else {
504 EvaluatedToErr
505 }
506 }
507
508 ty::Predicate::Projection(ref data) => {
509 let project_obligation = obligation.with(data.clone());
510 match project::poly_project_and_unify_type(self, &project_obligation) {
511 Ok(Some(subobligations)) => {
512 self.evaluate_predicates_recursively(previous_stack,
513 subobligations.iter())
514 }
515 Ok(None) => {
516 EvaluatedToAmbig
517 }
518 Err(_) => {
519 EvaluatedToErr
520 }
521 }
522 }
523 }
524 }
525
526 fn evaluate_obligation_recursively<'o>(&mut self,
527 previous_stack: TraitObligationStackList<'o, 'tcx>,
528 obligation: &TraitObligation<'tcx>)
529 -> EvaluationResult
530 {
531 debug!("evaluate_obligation_recursively({:?})",
532 obligation);
533
534 let stack = self.push_stack(previous_stack, obligation);
535 let fresh_trait_ref = stack.fresh_trait_ref;
536 if let Some(result) = self.check_evaluation_cache(fresh_trait_ref) {
537 debug!("CACHE HIT: EVAL({:?})={:?}",
538 fresh_trait_ref,
539 result);
540 return result;
541 }
542
543 let result = self.evaluate_stack(&stack);
544
545 debug!("CACHE MISS: EVAL({:?})={:?}",
546 fresh_trait_ref,
547 result);
548 self.insert_evaluation_cache(fresh_trait_ref, result);
549
550 result
551 }
552
553 fn evaluate_stack<'o>(&mut self,
554 stack: &TraitObligationStack<'o, 'tcx>)
555 -> EvaluationResult
556 {
557 // In intercrate mode, whenever any of the types are unbound,
558 // there can always be an impl. Even if there are no impls in
559 // this crate, perhaps the type would be unified with
560 // something from another crate that does provide an impl.
561 //
562 // In intracrate mode, we must still be conservative. The reason is
563 // that we want to avoid cycles. Imagine an impl like:
564 //
565 // impl<T:Eq> Eq for Vec<T>
566 //
567 // and a trait reference like `$0 : Eq` where `$0` is an
568 // unbound variable. When we evaluate this trait-reference, we
569 // will unify `$0` with `Vec<$1>` (for some fresh variable
570 // `$1`), on the condition that `$1 : Eq`. We will then wind
571 // up with many candidates (since that are other `Eq` impls
572 // that apply) and try to winnow things down. This results in
573 // a recursive evaluation that `$1 : Eq` -- as you can
574 // imagine, this is just where we started. To avoid that, we
575 // check for unbound variables and return an ambiguous (hence possible)
576 // match if we've seen this trait before.
577 //
578 // This suffices to allow chains like `FnMut` implemented in
579 // terms of `Fn` etc, but we could probably make this more
580 // precise still.
581 let input_types = stack.fresh_trait_ref.0.input_types();
582 let unbound_input_types = input_types.iter().any(|ty| ty.is_fresh());
583 if unbound_input_types && self.intercrate {
584 debug!("evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
585 stack.fresh_trait_ref);
586 return EvaluatedToAmbig;
587 }
588 if unbound_input_types &&
589 stack.iter().skip(1).any(
590 |prev| self.match_fresh_trait_refs(&stack.fresh_trait_ref,
591 &prev.fresh_trait_ref))
592 {
593 debug!("evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
594 stack.fresh_trait_ref);
595 return EvaluatedToUnknown;
596 }
597
598 // If there is any previous entry on the stack that precisely
599 // matches this obligation, then we can assume that the
600 // obligation is satisfied for now (still all other conditions
601 // must be met of course). One obvious case this comes up is
602 // marker traits like `Send`. Think of a linked list:
603 //
604 // struct List<T> { data: T, next: Option<Box<List<T>>> {
605 //
606 // `Box<List<T>>` will be `Send` if `T` is `Send` and
607 // `Option<Box<List<T>>>` is `Send`, and in turn
608 // `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
609 // `Send`.
610 //
611 // Note that we do this comparison using the `fresh_trait_ref`
612 // fields. Because these have all been skolemized using
613 // `self.freshener`, we can be sure that (a) this will not
614 // affect the inferencer state and (b) that if we see two
615 // skolemized types with the same index, they refer to the
616 // same unbound type variable.
617 if
618 stack.iter()
619 .skip(1) // skip top-most frame
620 .any(|prev| stack.fresh_trait_ref == prev.fresh_trait_ref)
621 {
622 debug!("evaluate_stack({:?}) --> recursive",
623 stack.fresh_trait_ref);
624 return EvaluatedToOk;
625 }
626
627 match self.candidate_from_obligation(stack) {
628 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
629 Ok(None) => EvaluatedToAmbig,
630 Err(..) => EvaluatedToErr
631 }
632 }
633
634 /// Further evaluate `candidate` to decide whether all type parameters match and whether nested
635 /// obligations are met. Returns true if `candidate` remains viable after this further
636 /// scrutiny.
637 fn evaluate_candidate<'o>(&mut self,
638 stack: &TraitObligationStack<'o, 'tcx>,
639 candidate: &SelectionCandidate<'tcx>)
640 -> EvaluationResult
641 {
642 debug!("evaluate_candidate: depth={} candidate={:?}",
643 stack.obligation.recursion_depth, candidate);
644 let result = self.infcx.probe(|_| {
645 let candidate = (*candidate).clone();
646 match self.confirm_candidate(stack.obligation, candidate) {
647 Ok(selection) => {
648 self.evaluate_predicates_recursively(
649 stack.list(),
650 selection.nested_obligations().iter())
651 }
652 Err(..) => EvaluatedToErr
653 }
654 });
655 debug!("evaluate_candidate: depth={} result={:?}",
656 stack.obligation.recursion_depth, result);
657 result
658 }
659
660 fn pick_evaluation_cache(&self) -> &EvaluationCache<'tcx> {
661 // see comment in `pick_candidate_cache`
662 if self.intercrate ||
663 !self.param_env().caller_bounds.is_empty()
664 {
665 &self.param_env().evaluation_cache
666 } else
667 {
668 &self.tcx().evaluation_cache
669 }
670 }
671
672 fn check_evaluation_cache(&self, trait_ref: ty::PolyTraitRef<'tcx>)
673 -> Option<EvaluationResult>
674 {
675 let cache = self.pick_evaluation_cache();
676 cache.hashmap.borrow().get(&trait_ref).cloned()
677 }
678
679 fn insert_evaluation_cache(&mut self,
680 trait_ref: ty::PolyTraitRef<'tcx>,
681 result: EvaluationResult)
682 {
683 // Avoid caching results that depend on more than just the trait-ref:
684 // The stack can create EvaluatedToUnknown, and closure signatures
685 // being yet uninferred can create "spurious" EvaluatedToAmbig
686 // and EvaluatedToOk.
687 if result == EvaluatedToUnknown ||
688 ((result == EvaluatedToAmbig || result == EvaluatedToOk)
689 && trait_ref.has_closure_types())
690 {
691 return;
692 }
693
694 let cache = self.pick_evaluation_cache();
695 cache.hashmap.borrow_mut().insert(trait_ref, result);
696 }
697
698 ///////////////////////////////////////////////////////////////////////////
699 // CANDIDATE ASSEMBLY
700 //
701 // The selection process begins by examining all in-scope impls,
702 // caller obligations, and so forth and assembling a list of
703 // candidates. See `README.md` and the `Candidate` type for more
704 // details.
705
706 fn candidate_from_obligation<'o>(&mut self,
707 stack: &TraitObligationStack<'o, 'tcx>)
708 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
709 {
710 // Watch out for overflow. This intentionally bypasses (and does
711 // not update) the cache.
712 let recursion_limit = self.infcx.tcx.sess.recursion_limit.get();
713 if stack.obligation.recursion_depth >= recursion_limit {
714 report_overflow_error(self.infcx(), &stack.obligation, true);
715 }
716
717 // Check the cache. Note that we skolemize the trait-ref
718 // separately rather than using `stack.fresh_trait_ref` -- this
719 // is because we want the unbound variables to be replaced
720 // with fresh skolemized types starting from index 0.
721 let cache_fresh_trait_pred =
722 self.infcx.freshen(stack.obligation.predicate.clone());
723 debug!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
724 cache_fresh_trait_pred,
725 stack);
726 assert!(!stack.obligation.predicate.has_escaping_regions());
727
728 match self.check_candidate_cache(&cache_fresh_trait_pred) {
729 Some(c) => {
730 debug!("CACHE HIT: SELECT({:?})={:?}",
731 cache_fresh_trait_pred,
732 c);
733 return c;
734 }
735 None => { }
736 }
737
738 // If no match, compute result and insert into cache.
739 let candidate = self.candidate_from_obligation_no_cache(stack);
740
741 if self.should_update_candidate_cache(&cache_fresh_trait_pred, &candidate) {
742 debug!("CACHE MISS: SELECT({:?})={:?}",
743 cache_fresh_trait_pred, candidate);
744 self.insert_candidate_cache(cache_fresh_trait_pred, candidate.clone());
745 }
746
747 candidate
748 }
749
750 fn candidate_from_obligation_no_cache<'o>(&mut self,
751 stack: &TraitObligationStack<'o, 'tcx>)
752 -> SelectionResult<'tcx, SelectionCandidate<'tcx>>
753 {
754 if stack.obligation.predicate.references_error() {
755 // If we encounter a `TyError`, we generally prefer the
756 // most "optimistic" result in response -- that is, the
757 // one least likely to report downstream errors. But
758 // because this routine is shared by coherence and by
759 // trait selection, there isn't an obvious "right" choice
760 // here in that respect, so we opt to just return
761 // ambiguity and let the upstream clients sort it out.
762 return Ok(None);
763 }
764
765 if !self.is_knowable(stack) {
766 debug!("intercrate not knowable");
767 return Ok(None);
768 }
769
770 let candidate_set = try!(self.assemble_candidates(stack));
771
772 if candidate_set.ambiguous {
773 debug!("candidate set contains ambig");
774 return Ok(None);
775 }
776
777 let mut candidates = candidate_set.vec;
778
779 debug!("assembled {} candidates for {:?}: {:?}",
780 candidates.len(),
781 stack,
782 candidates);
783
784 // At this point, we know that each of the entries in the
785 // candidate set is *individually* applicable. Now we have to
786 // figure out if they contain mutual incompatibilities. This
787 // frequently arises if we have an unconstrained input type --
788 // for example, we are looking for $0:Eq where $0 is some
789 // unconstrained type variable. In that case, we'll get a
790 // candidate which assumes $0 == int, one that assumes $0 ==
791 // usize, etc. This spells an ambiguity.
792
793 // If there is more than one candidate, first winnow them down
794 // by considering extra conditions (nested obligations and so
795 // forth). We don't winnow if there is exactly one
796 // candidate. This is a relatively minor distinction but it
797 // can lead to better inference and error-reporting. An
798 // example would be if there was an impl:
799 //
800 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
801 //
802 // and we were to see some code `foo.push_clone()` where `boo`
803 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
804 // we were to winnow, we'd wind up with zero candidates.
805 // Instead, we select the right impl now but report `Bar does
806 // not implement Clone`.
807 if candidates.len() > 1 {
808 candidates.retain(|c| self.evaluate_candidate(stack, c).may_apply())
809 }
810
811 // If there are STILL multiple candidate, we can further reduce
812 // the list by dropping duplicates.
813 if candidates.len() > 1 {
814 let mut i = 0;
815 while i < candidates.len() {
816 let is_dup =
817 (0..candidates.len())
818 .filter(|&j| i != j)
819 .any(|j| self.candidate_should_be_dropped_in_favor_of(&candidates[i],
820 &candidates[j]));
821 if is_dup {
822 debug!("Dropping candidate #{}/{}: {:?}",
823 i, candidates.len(), candidates[i]);
824 candidates.swap_remove(i);
825 } else {
826 debug!("Retaining candidate #{}/{}: {:?}",
827 i, candidates.len(), candidates[i]);
828 i += 1;
829 }
830 }
831 }
832
833 // If there are *STILL* multiple candidates, give up and
834 // report ambiguity.
835 if candidates.len() > 1 {
836 debug!("multiple matches, ambig");
837 return Ok(None);
838 }
839
840
841 // If there are *NO* candidates, that there are no impls --
842 // that we know of, anyway. Note that in the case where there
843 // are unbound type variables within the obligation, it might
844 // be the case that you could still satisfy the obligation
845 // from another crate by instantiating the type variables with
846 // a type from another crate that does have an impl. This case
847 // is checked for in `evaluate_stack` (and hence users
848 // who might care about this case, like coherence, should use
849 // that function).
850 if candidates.is_empty() {
851 return Err(Unimplemented);
852 }
853
854 // Just one candidate left.
855 let candidate = candidates.pop().unwrap();
856
857 match candidate {
858 ImplCandidate(def_id) => {
859 match self.tcx().trait_impl_polarity(def_id) {
860 Some(hir::ImplPolarity::Negative) => return Err(Unimplemented),
861 _ => {}
862 }
863 }
864 _ => {}
865 }
866
867 Ok(Some(candidate))
868 }
869
870 fn is_knowable<'o>(&mut self,
871 stack: &TraitObligationStack<'o, 'tcx>)
872 -> bool
873 {
874 debug!("is_knowable(intercrate={})", self.intercrate);
875
876 if !self.intercrate {
877 return true;
878 }
879
880 let obligation = &stack.obligation;
881 let predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
882
883 // ok to skip binder because of the nature of the
884 // trait-ref-is-knowable check, which does not care about
885 // bound regions
886 let trait_ref = &predicate.skip_binder().trait_ref;
887
888 coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
889 }
890
891 fn pick_candidate_cache(&self) -> &SelectionCache<'tcx> {
892 // If there are any where-clauses in scope, then we always use
893 // a cache local to this particular scope. Otherwise, we
894 // switch to a global cache. We used to try and draw
895 // finer-grained distinctions, but that led to a serious of
896 // annoying and weird bugs like #22019 and #18290. This simple
897 // rule seems to be pretty clearly safe and also still retains
898 // a very high hit rate (~95% when compiling rustc).
899 if !self.param_env().caller_bounds.is_empty() {
900 return &self.param_env().selection_cache;
901 }
902
903 // Avoid using the master cache during coherence and just rely
904 // on the local cache. This effectively disables caching
905 // during coherence. It is really just a simplification to
906 // avoid us having to fear that coherence results "pollute"
907 // the master cache. Since coherence executes pretty quickly,
908 // it's not worth going to more trouble to increase the
909 // hit-rate I don't think.
910 if self.intercrate {
911 return &self.param_env().selection_cache;
912 }
913
914 // Otherwise, we can use the global cache.
915 &self.tcx().selection_cache
916 }
917
918 fn check_candidate_cache(&mut self,
919 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
920 -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
921 {
922 let cache = self.pick_candidate_cache();
923 let hashmap = cache.hashmap.borrow();
924 hashmap.get(&cache_fresh_trait_pred.0.trait_ref).cloned()
925 }
926
927 fn insert_candidate_cache(&mut self,
928 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
929 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
930 {
931 let cache = self.pick_candidate_cache();
932 let mut hashmap = cache.hashmap.borrow_mut();
933 hashmap.insert(cache_fresh_trait_pred.0.trait_ref.clone(), candidate);
934 }
935
936 fn should_update_candidate_cache(&mut self,
937 cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>,
938 candidate: &SelectionResult<'tcx, SelectionCandidate<'tcx>>)
939 -> bool
940 {
941 // In general, it's a good idea to cache results, even
942 // ambiguous ones, to save us some trouble later. But we have
943 // to be careful not to cache results that could be
944 // invalidated later by advances in inference. Normally, this
945 // is not an issue, because any inference variables whose
946 // types are not yet bound are "freshened" in the cache key,
947 // which means that if we later get the same request once that
948 // type variable IS bound, we'll have a different cache key.
949 // For example, if we have `Vec<_#0t> : Foo`, and `_#0t` is
950 // not yet known, we may cache the result as `None`. But if
951 // later `_#0t` is bound to `Bar`, then when we freshen we'll
952 // have `Vec<Bar> : Foo` as the cache key.
953 //
954 // HOWEVER, it CAN happen that we get an ambiguity result in
955 // one particular case around closures where the cache key
956 // would not change. That is when the precise types of the
957 // upvars that a closure references have not yet been figured
958 // out (i.e., because it is not yet known if they are captured
959 // by ref, and if by ref, what kind of ref). In these cases,
960 // when matching a builtin bound, we will yield back an
961 // ambiguous result. But the *cache key* is just the closure type,
962 // it doesn't capture the state of the upvar computation.
963 //
964 // To avoid this trap, just don't cache ambiguous results if
965 // the self-type contains no inference byproducts (that really
966 // shouldn't happen in other circumstances anyway, given
967 // coherence).
968
969 match *candidate {
970 Ok(Some(_)) | Err(_) => true,
971 Ok(None) => {
972 cache_fresh_trait_pred.0.trait_ref.substs.types.has_infer_types()
973 }
974 }
975 }
976
977 fn assemble_candidates<'o>(&mut self,
978 stack: &TraitObligationStack<'o, 'tcx>)
979 -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
980 {
981 let TraitObligationStack { obligation, .. } = *stack;
982 let ref obligation = Obligation {
983 cause: obligation.cause.clone(),
984 recursion_depth: obligation.recursion_depth,
985 predicate: self.infcx().resolve_type_vars_if_possible(&obligation.predicate)
986 };
987
988 if obligation.predicate.skip_binder().self_ty().is_ty_var() {
989 // FIXME(#20297): Self is a type variable (e.g. `_: AsRef<str>`).
990 //
991 // This is somewhat problematic, as the current scheme can't really
992 // handle it turning to be a projection. This does end up as truly
993 // ambiguous in most cases anyway.
994 //
995 // Until this is fixed, take the fast path out - this also improves
996 // performance by preventing assemble_candidates_from_impls from
997 // matching every impl for this trait.
998 return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
999 }
1000
1001 let mut candidates = SelectionCandidateSet {
1002 vec: Vec::new(),
1003 ambiguous: false
1004 };
1005
1006 // Other bounds. Consider both in-scope bounds from fn decl
1007 // and applicable impls. There is a certain set of precedence rules here.
1008
1009 match self.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) {
1010 Some(ty::BoundCopy) => {
1011 debug!("obligation self ty is {:?}",
1012 obligation.predicate.0.self_ty());
1013
1014 // User-defined copy impls are permitted, but only for
1015 // structs and enums.
1016 try!(self.assemble_candidates_from_impls(obligation, &mut candidates));
1017
1018 // For other types, we'll use the builtin rules.
1019 try!(self.assemble_builtin_bound_candidates(ty::BoundCopy,
1020 obligation,
1021 &mut candidates));
1022 }
1023 Some(bound @ ty::BoundSized) => {
1024 // Sized is never implementable by end-users, it is
1025 // always automatically computed.
1026 try!(self.assemble_builtin_bound_candidates(bound,
1027 obligation,
1028 &mut candidates));
1029 }
1030
1031 None if self.tcx().lang_items.unsize_trait() ==
1032 Some(obligation.predicate.def_id()) => {
1033 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
1034 }
1035
1036 Some(ty::BoundSend) |
1037 Some(ty::BoundSync) |
1038 None => {
1039 try!(self.assemble_closure_candidates(obligation, &mut candidates));
1040 try!(self.assemble_fn_pointer_candidates(obligation, &mut candidates));
1041 try!(self.assemble_candidates_from_impls(obligation, &mut candidates));
1042 self.assemble_candidates_from_object_ty(obligation, &mut candidates);
1043 }
1044 }
1045
1046 self.assemble_candidates_from_projected_tys(obligation, &mut candidates);
1047 try!(self.assemble_candidates_from_caller_bounds(stack, &mut candidates));
1048 // Default implementations have lower priority, so we only
1049 // consider triggering a default if there is no other impl that can apply.
1050 if candidates.vec.is_empty() {
1051 try!(self.assemble_candidates_from_default_impls(obligation, &mut candidates));
1052 }
1053 debug!("candidate list size: {}", candidates.vec.len());
1054 Ok(candidates)
1055 }
1056
1057 fn assemble_candidates_from_projected_tys(&mut self,
1058 obligation: &TraitObligation<'tcx>,
1059 candidates: &mut SelectionCandidateSet<'tcx>)
1060 {
1061 debug!("assemble_candidates_for_projected_tys({:?})", obligation);
1062
1063 // FIXME(#20297) -- just examining the self-type is very simplistic
1064
1065 // before we go into the whole skolemization thing, just
1066 // quickly check if the self-type is a projection at all.
1067 let trait_def_id = match obligation.predicate.0.trait_ref.self_ty().sty {
1068 ty::TyProjection(ref data) => data.trait_ref.def_id,
1069 ty::TyInfer(ty::TyVar(_)) => {
1070 self.tcx().sess.span_bug(obligation.cause.span,
1071 "Self=_ should have been handled by assemble_candidates");
1072 }
1073 _ => { return; }
1074 };
1075
1076 debug!("assemble_candidates_for_projected_tys: trait_def_id={:?}",
1077 trait_def_id);
1078
1079 let result = self.infcx.probe(|snapshot| {
1080 self.match_projection_obligation_against_bounds_from_trait(obligation,
1081 snapshot)
1082 });
1083
1084 if result {
1085 candidates.vec.push(ProjectionCandidate);
1086 }
1087 }
1088
1089 fn match_projection_obligation_against_bounds_from_trait(
1090 &mut self,
1091 obligation: &TraitObligation<'tcx>,
1092 snapshot: &infer::CombinedSnapshot)
1093 -> bool
1094 {
1095 let poly_trait_predicate =
1096 self.infcx().resolve_type_vars_if_possible(&obligation.predicate);
1097 let (skol_trait_predicate, skol_map) =
1098 self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot);
1099 debug!("match_projection_obligation_against_bounds_from_trait: \
1100 skol_trait_predicate={:?} skol_map={:?}",
1101 skol_trait_predicate,
1102 skol_map);
1103
1104 let projection_trait_ref = match skol_trait_predicate.trait_ref.self_ty().sty {
1105 ty::TyProjection(ref data) => &data.trait_ref,
1106 _ => {
1107 self.tcx().sess.span_bug(
1108 obligation.cause.span,
1109 &format!("match_projection_obligation_against_bounds_from_trait() called \
1110 but self-ty not a projection: {:?}",
1111 skol_trait_predicate.trait_ref.self_ty()));
1112 }
1113 };
1114 debug!("match_projection_obligation_against_bounds_from_trait: \
1115 projection_trait_ref={:?}",
1116 projection_trait_ref);
1117
1118 let trait_predicates = self.tcx().lookup_predicates(projection_trait_ref.def_id);
1119 let bounds = trait_predicates.instantiate(self.tcx(), projection_trait_ref.substs);
1120 debug!("match_projection_obligation_against_bounds_from_trait: \
1121 bounds={:?}",
1122 bounds);
1123
1124 let matching_bound =
1125 util::elaborate_predicates(self.tcx(), bounds.predicates.into_vec())
1126 .filter_to_traits()
1127 .find(
1128 |bound| self.infcx.probe(
1129 |_| self.match_projection(obligation,
1130 bound.clone(),
1131 skol_trait_predicate.trait_ref.clone(),
1132 &skol_map,
1133 snapshot)));
1134
1135 debug!("match_projection_obligation_against_bounds_from_trait: \
1136 matching_bound={:?}",
1137 matching_bound);
1138 match matching_bound {
1139 None => false,
1140 Some(bound) => {
1141 // Repeat the successful match, if any, this time outside of a probe.
1142 let result = self.match_projection(obligation,
1143 bound,
1144 skol_trait_predicate.trait_ref.clone(),
1145 &skol_map,
1146 snapshot);
1147 assert!(result);
1148 true
1149 }
1150 }
1151 }
1152
1153 fn match_projection(&mut self,
1154 obligation: &TraitObligation<'tcx>,
1155 trait_bound: ty::PolyTraitRef<'tcx>,
1156 skol_trait_ref: ty::TraitRef<'tcx>,
1157 skol_map: &infer::SkolemizationMap,
1158 snapshot: &infer::CombinedSnapshot)
1159 -> bool
1160 {
1161 assert!(!skol_trait_ref.has_escaping_regions());
1162 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
1163 match self.infcx.sub_poly_trait_refs(false,
1164 origin,
1165 trait_bound.clone(),
1166 ty::Binder(skol_trait_ref.clone())) {
1167 Ok(()) => { }
1168 Err(_) => { return false; }
1169 }
1170
1171 self.infcx.leak_check(skol_map, snapshot).is_ok()
1172 }
1173
1174 /// Given an obligation like `<SomeTrait for T>`, search the obligations that the caller
1175 /// supplied to find out whether it is listed among them.
1176 ///
1177 /// Never affects inference environment.
1178 fn assemble_candidates_from_caller_bounds<'o>(&mut self,
1179 stack: &TraitObligationStack<'o, 'tcx>,
1180 candidates: &mut SelectionCandidateSet<'tcx>)
1181 -> Result<(),SelectionError<'tcx>>
1182 {
1183 debug!("assemble_candidates_from_caller_bounds({:?})",
1184 stack.obligation);
1185
1186 let all_bounds =
1187 self.param_env().caller_bounds
1188 .iter()
1189 .filter_map(|o| o.to_opt_poly_trait_ref());
1190
1191 let matching_bounds =
1192 all_bounds.filter(
1193 |bound| self.evaluate_where_clause(stack, bound.clone()).may_apply());
1194
1195 let param_candidates =
1196 matching_bounds.map(|bound| ParamCandidate(bound));
1197
1198 candidates.vec.extend(param_candidates);
1199
1200 Ok(())
1201 }
1202
1203 fn evaluate_where_clause<'o>(&mut self,
1204 stack: &TraitObligationStack<'o, 'tcx>,
1205 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
1206 -> EvaluationResult
1207 {
1208 self.infcx().probe(move |_| {
1209 match self.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1210 Ok(obligations) => {
1211 self.evaluate_predicates_recursively(stack.list(), obligations.iter())
1212 }
1213 Err(()) => EvaluatedToErr
1214 }
1215 })
1216 }
1217
1218 /// Check for the artificial impl that the compiler will create for an obligation like `X :
1219 /// FnMut<..>` where `X` is a closure type.
1220 ///
1221 /// Note: the type parameters on a closure candidate are modeled as *output* type
1222 /// parameters and hence do not affect whether this trait is a match or not. They will be
1223 /// unified during the confirmation step.
1224 fn assemble_closure_candidates(&mut self,
1225 obligation: &TraitObligation<'tcx>,
1226 candidates: &mut SelectionCandidateSet<'tcx>)
1227 -> Result<(),SelectionError<'tcx>>
1228 {
1229 let kind = match self.tcx().lang_items.fn_trait_kind(obligation.predicate.0.def_id()) {
1230 Some(k) => k,
1231 None => { return Ok(()); }
1232 };
1233
1234 // ok to skip binder because the substs on closure types never
1235 // touch bound regions, they just capture the in-scope
1236 // type/region parameters
1237 let self_ty = *obligation.self_ty().skip_binder();
1238 let (closure_def_id, substs) = match self_ty.sty {
1239 ty::TyClosure(id, ref substs) => (id, substs),
1240 ty::TyInfer(ty::TyVar(_)) => {
1241 debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
1242 candidates.ambiguous = true;
1243 return Ok(());
1244 }
1245 _ => { return Ok(()); }
1246 };
1247
1248 debug!("assemble_unboxed_candidates: self_ty={:?} kind={:?} obligation={:?}",
1249 self_ty,
1250 kind,
1251 obligation);
1252
1253 match self.infcx.closure_kind(closure_def_id) {
1254 Some(closure_kind) => {
1255 debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
1256 if closure_kind.extends(kind) {
1257 candidates.vec.push(ClosureCandidate(closure_def_id, substs));
1258 }
1259 }
1260 None => {
1261 debug!("assemble_unboxed_candidates: closure_kind not yet known");
1262 candidates.ambiguous = true;
1263 }
1264 }
1265
1266 Ok(())
1267 }
1268
1269 /// Implement one of the `Fn()` family for a fn pointer.
1270 fn assemble_fn_pointer_candidates(&mut self,
1271 obligation: &TraitObligation<'tcx>,
1272 candidates: &mut SelectionCandidateSet<'tcx>)
1273 -> Result<(),SelectionError<'tcx>>
1274 {
1275 // We provide impl of all fn traits for fn pointers.
1276 if self.tcx().lang_items.fn_trait_kind(obligation.predicate.def_id()).is_none() {
1277 return Ok(());
1278 }
1279
1280 // ok to skip binder because what we are inspecting doesn't involve bound regions
1281 let self_ty = *obligation.self_ty().skip_binder();
1282 match self_ty.sty {
1283 ty::TyInfer(ty::TyVar(_)) => {
1284 debug!("assemble_fn_pointer_candidates: ambiguous self-type");
1285 candidates.ambiguous = true; // could wind up being a fn() type
1286 }
1287
1288 // provide an impl, but only for suitable `fn` pointers
1289 ty::TyBareFn(_, &ty::BareFnTy {
1290 unsafety: hir::Unsafety::Normal,
1291 abi: Abi::Rust,
1292 sig: ty::Binder(ty::FnSig {
1293 inputs: _,
1294 output: ty::FnConverging(_),
1295 variadic: false
1296 })
1297 }) => {
1298 candidates.vec.push(FnPointerCandidate);
1299 }
1300
1301 _ => { }
1302 }
1303
1304 Ok(())
1305 }
1306
1307 /// Search for impls that might apply to `obligation`.
1308 fn assemble_candidates_from_impls(&mut self,
1309 obligation: &TraitObligation<'tcx>,
1310 candidates: &mut SelectionCandidateSet<'tcx>)
1311 -> Result<(), SelectionError<'tcx>>
1312 {
1313 debug!("assemble_candidates_from_impls(obligation={:?})", obligation);
1314
1315 let def = self.tcx().lookup_trait_def(obligation.predicate.def_id());
1316
1317 def.for_each_relevant_impl(
1318 self.tcx(),
1319 obligation.predicate.0.trait_ref.self_ty(),
1320 |impl_def_id| {
1321 self.infcx.probe(|snapshot| {
1322 if let Ok(_) = self.match_impl(impl_def_id, obligation, snapshot) {
1323 candidates.vec.push(ImplCandidate(impl_def_id));
1324 }
1325 });
1326 }
1327 );
1328
1329 Ok(())
1330 }
1331
1332 fn assemble_candidates_from_default_impls(&mut self,
1333 obligation: &TraitObligation<'tcx>,
1334 candidates: &mut SelectionCandidateSet<'tcx>)
1335 -> Result<(), SelectionError<'tcx>>
1336 {
1337 // OK to skip binder here because the tests we do below do not involve bound regions
1338 let self_ty = *obligation.self_ty().skip_binder();
1339 debug!("assemble_candidates_from_default_impls(self_ty={:?})", self_ty);
1340
1341 let def_id = obligation.predicate.def_id();
1342
1343 if self.tcx().trait_has_default_impl(def_id) {
1344 match self_ty.sty {
1345 ty::TyTrait(..) => {
1346 // For object types, we don't know what the closed
1347 // over types are. For most traits, this means we
1348 // conservatively say nothing; a candidate may be
1349 // added by `assemble_candidates_from_object_ty`.
1350 // However, for the kind of magic reflect trait,
1351 // we consider it to be implemented even for
1352 // object types, because it just lets you reflect
1353 // onto the object type, not into the object's
1354 // interior.
1355 if self.tcx().has_attr(def_id, "rustc_reflect_like") {
1356 candidates.vec.push(DefaultImplObjectCandidate(def_id));
1357 }
1358 }
1359 ty::TyParam(..) |
1360 ty::TyProjection(..) => {
1361 // In these cases, we don't know what the actual
1362 // type is. Therefore, we cannot break it down
1363 // into its constituent types. So we don't
1364 // consider the `..` impl but instead just add no
1365 // candidates: this means that typeck will only
1366 // succeed if there is another reason to believe
1367 // that this obligation holds. That could be a
1368 // where-clause or, in the case of an object type,
1369 // it could be that the object type lists the
1370 // trait (e.g. `Foo+Send : Send`). See
1371 // `compile-fail/typeck-default-trait-impl-send-param.rs`
1372 // for an example of a test case that exercises
1373 // this path.
1374 }
1375 ty::TyInfer(ty::TyVar(_)) => {
1376 // the defaulted impl might apply, we don't know
1377 candidates.ambiguous = true;
1378 }
1379 _ => {
1380 candidates.vec.push(DefaultImplCandidate(def_id.clone()))
1381 }
1382 }
1383 }
1384
1385 Ok(())
1386 }
1387
1388 /// Search for impls that might apply to `obligation`.
1389 fn assemble_candidates_from_object_ty(&mut self,
1390 obligation: &TraitObligation<'tcx>,
1391 candidates: &mut SelectionCandidateSet<'tcx>)
1392 {
1393 debug!("assemble_candidates_from_object_ty(self_ty={:?})",
1394 obligation.self_ty().skip_binder());
1395
1396 // Object-safety candidates are only applicable to object-safe
1397 // traits. Including this check is useful because it helps
1398 // inference in cases of traits like `BorrowFrom`, which are
1399 // not object-safe, and which rely on being able to infer the
1400 // self-type from one of the other inputs. Without this check,
1401 // these cases wind up being considered ambiguous due to a
1402 // (spurious) ambiguity introduced here.
1403 let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
1404 if !object_safety::is_object_safe(self.tcx(), predicate_trait_ref.def_id()) {
1405 return;
1406 }
1407
1408 self.infcx.commit_if_ok(|snapshot| {
1409 let (self_ty, _) =
1410 self.infcx().skolemize_late_bound_regions(&obligation.self_ty(), snapshot);
1411 let poly_trait_ref = match self_ty.sty {
1412 ty::TyTrait(ref data) => {
1413 match self.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) {
1414 Some(bound @ ty::BoundSend) | Some(bound @ ty::BoundSync) => {
1415 if data.bounds.builtin_bounds.contains(&bound) {
1416 debug!("assemble_candidates_from_object_ty: matched builtin bound, \
1417 pushing candidate");
1418 candidates.vec.push(BuiltinObjectCandidate);
1419 return Ok(());
1420 }
1421 }
1422 _ => {}
1423 }
1424
1425 data.principal_trait_ref_with_self_ty(self.tcx(), self_ty)
1426 }
1427 ty::TyInfer(ty::TyVar(_)) => {
1428 debug!("assemble_candidates_from_object_ty: ambiguous");
1429 candidates.ambiguous = true; // could wind up being an object type
1430 return Ok(());
1431 }
1432 _ => {
1433 return Ok(());
1434 }
1435 };
1436
1437 debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}",
1438 poly_trait_ref);
1439
1440 // Count only those upcast versions that match the trait-ref
1441 // we are looking for. Specifically, do not only check for the
1442 // correct trait, but also the correct type parameters.
1443 // For example, we may be trying to upcast `Foo` to `Bar<i32>`,
1444 // but `Foo` is declared as `trait Foo : Bar<u32>`.
1445 let upcast_trait_refs =
1446 util::supertraits(self.tcx(), poly_trait_ref)
1447 .filter(|upcast_trait_ref| {
1448 self.infcx.probe(|_| {
1449 let upcast_trait_ref = upcast_trait_ref.clone();
1450 self.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok()
1451 })
1452 })
1453 .count();
1454
1455 if upcast_trait_refs > 1 {
1456 // can be upcast in many ways; need more type information
1457 candidates.ambiguous = true;
1458 } else if upcast_trait_refs == 1 {
1459 candidates.vec.push(ObjectCandidate);
1460 }
1461
1462 Ok::<(),()>(())
1463 }).unwrap();
1464 }
1465
1466 /// Search for unsizing that might apply to `obligation`.
1467 fn assemble_candidates_for_unsizing(&mut self,
1468 obligation: &TraitObligation<'tcx>,
1469 candidates: &mut SelectionCandidateSet<'tcx>) {
1470 // We currently never consider higher-ranked obligations e.g.
1471 // `for<'a> &'a T: Unsize<Trait+'a>` to be implemented. This is not
1472 // because they are a priori invalid, and we could potentially add support
1473 // for them later, it's just that there isn't really a strong need for it.
1474 // A `T: Unsize<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
1475 // impl, and those are generally applied to concrete types.
1476 //
1477 // That said, one might try to write a fn with a where clause like
1478 // for<'a> Foo<'a, T>: Unsize<Foo<'a, Trait>>
1479 // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`.
1480 // Still, you'd be more likely to write that where clause as
1481 // T: Trait
1482 // so it seems ok if we (conservatively) fail to accept that `Unsize`
1483 // obligation above. Should be possible to extend this in the future.
1484 let source = match self.tcx().no_late_bound_regions(&obligation.self_ty()) {
1485 Some(t) => t,
1486 None => {
1487 // Don't add any candidates if there are bound regions.
1488 return;
1489 }
1490 };
1491 let target = obligation.predicate.0.input_types()[0];
1492
1493 debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})",
1494 source, target);
1495
1496 let may_apply = match (&source.sty, &target.sty) {
1497 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
1498 (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => {
1499 // Upcasts permit two things:
1500 //
1501 // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo`
1502 // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b`
1503 //
1504 // Note that neither of these changes requires any
1505 // change at runtime. Eventually this will be
1506 // generalized.
1507 //
1508 // We always upcast when we can because of reason
1509 // #2 (region bounds).
1510 data_a.principal.def_id() == data_a.principal.def_id() &&
1511 data_a.bounds.builtin_bounds.is_superset(&data_b.bounds.builtin_bounds)
1512 }
1513
1514 // T -> Trait.
1515 (_, &ty::TyTrait(_)) => true,
1516
1517 // Ambiguous handling is below T -> Trait, because inference
1518 // variables can still implement Unsize<Trait> and nested
1519 // obligations will have the final say (likely deferred).
1520 (&ty::TyInfer(ty::TyVar(_)), _) |
1521 (_, &ty::TyInfer(ty::TyVar(_))) => {
1522 debug!("assemble_candidates_for_unsizing: ambiguous");
1523 candidates.ambiguous = true;
1524 false
1525 }
1526
1527 // [T; n] -> [T].
1528 (&ty::TyArray(_, _), &ty::TySlice(_)) => true,
1529
1530 // Struct<T> -> Struct<U>.
1531 (&ty::TyStruct(def_id_a, _), &ty::TyStruct(def_id_b, _)) => {
1532 def_id_a == def_id_b
1533 }
1534
1535 _ => false
1536 };
1537
1538 if may_apply {
1539 candidates.vec.push(BuiltinUnsizeCandidate);
1540 }
1541 }
1542
1543 ///////////////////////////////////////////////////////////////////////////
1544 // WINNOW
1545 //
1546 // Winnowing is the process of attempting to resolve ambiguity by
1547 // probing further. During the winnowing process, we unify all
1548 // type variables (ignoring skolemization) and then we also
1549 // attempt to evaluate recursive bounds to see if they are
1550 // satisfied.
1551
1552 /// Returns true if `candidate_i` should be dropped in favor of
1553 /// `candidate_j`. Generally speaking we will drop duplicate
1554 /// candidates and prefer where-clause candidates.
1555 /// Returns true if `victim` should be dropped in favor of
1556 /// `other`. Generally speaking we will drop duplicate
1557 /// candidates and prefer where-clause candidates.
1558 ///
1559 /// See the comment for "SelectionCandidate" for more details.
1560 fn candidate_should_be_dropped_in_favor_of<'o>(&mut self,
1561 victim: &SelectionCandidate<'tcx>,
1562 other: &SelectionCandidate<'tcx>)
1563 -> bool
1564 {
1565 if victim == other {
1566 return true;
1567 }
1568
1569 match other {
1570 &ObjectCandidate |
1571 &ParamCandidate(_) | &ProjectionCandidate => match victim {
1572 &DefaultImplCandidate(..) => {
1573 self.tcx().sess.bug(
1574 "default implementations shouldn't be recorded \
1575 when there are other valid candidates");
1576 }
1577 &ImplCandidate(..) |
1578 &ClosureCandidate(..) |
1579 &FnPointerCandidate |
1580 &BuiltinObjectCandidate |
1581 &BuiltinUnsizeCandidate |
1582 &DefaultImplObjectCandidate(..) |
1583 &BuiltinCandidate(..) => {
1584 // We have a where-clause so don't go around looking
1585 // for impls.
1586 true
1587 }
1588 &ObjectCandidate |
1589 &ProjectionCandidate => {
1590 // Arbitrarily give param candidates priority
1591 // over projection and object candidates.
1592 true
1593 },
1594 &ParamCandidate(..) => false,
1595 },
1596 _ => false
1597 }
1598 }
1599
1600 ///////////////////////////////////////////////////////////////////////////
1601 // BUILTIN BOUNDS
1602 //
1603 // These cover the traits that are built-in to the language
1604 // itself. This includes `Copy` and `Sized` for sure. For the
1605 // moment, it also includes `Send` / `Sync` and a few others, but
1606 // those will hopefully change to library-defined traits in the
1607 // future.
1608
1609 fn assemble_builtin_bound_candidates<'o>(&mut self,
1610 bound: ty::BuiltinBound,
1611 obligation: &TraitObligation<'tcx>,
1612 candidates: &mut SelectionCandidateSet<'tcx>)
1613 -> Result<(),SelectionError<'tcx>>
1614 {
1615 match self.builtin_bound(bound, obligation) {
1616 Ok(If(..)) => {
1617 debug!("builtin_bound: bound={:?}",
1618 bound);
1619 candidates.vec.push(BuiltinCandidate(bound));
1620 Ok(())
1621 }
1622 Ok(ParameterBuiltin) => { Ok(()) }
1623 Ok(AmbiguousBuiltin) => {
1624 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
1625 Ok(candidates.ambiguous = true)
1626 }
1627 Err(e) => { Err(e) }
1628 }
1629 }
1630
1631 fn builtin_bound(&mut self,
1632 bound: ty::BuiltinBound,
1633 obligation: &TraitObligation<'tcx>)
1634 -> Result<BuiltinBoundConditions<'tcx>,SelectionError<'tcx>>
1635 {
1636 // Note: these tests operate on types that may contain bound
1637 // regions. To be proper, we ought to skolemize here, but we
1638 // forego the skolemization and defer it until the
1639 // confirmation step.
1640
1641 let self_ty = self.infcx.shallow_resolve(obligation.predicate.0.self_ty());
1642 return match self_ty.sty {
1643 ty::TyInfer(ty::IntVar(_)) |
1644 ty::TyInfer(ty::FloatVar(_)) |
1645 ty::TyUint(_) |
1646 ty::TyInt(_) |
1647 ty::TyBool |
1648 ty::TyFloat(_) |
1649 ty::TyBareFn(..) |
1650 ty::TyChar => {
1651 // safe for everything
1652 ok_if(Vec::new())
1653 }
1654
1655 ty::TyBox(_) => { // Box<T>
1656 match bound {
1657 ty::BoundCopy => Err(Unimplemented),
1658
1659 ty::BoundSized => ok_if(Vec::new()),
1660
1661 ty::BoundSync | ty::BoundSend => {
1662 self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()");
1663 }
1664 }
1665 }
1666
1667 ty::TyRawPtr(..) => { // *const T, *mut T
1668 match bound {
1669 ty::BoundCopy | ty::BoundSized => ok_if(Vec::new()),
1670
1671 ty::BoundSync | ty::BoundSend => {
1672 self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()");
1673 }
1674 }
1675 }
1676
1677 ty::TyTrait(ref data) => {
1678 match bound {
1679 ty::BoundSized => Err(Unimplemented),
1680 ty::BoundCopy => {
1681 if data.bounds.builtin_bounds.contains(&bound) {
1682 ok_if(Vec::new())
1683 } else {
1684 // Recursively check all supertraits to find out if any further
1685 // bounds are required and thus we must fulfill.
1686 let principal =
1687 data.principal_trait_ref_with_self_ty(self.tcx(),
1688 self.tcx().types.err);
1689 let copy_def_id = obligation.predicate.def_id();
1690 for tr in util::supertraits(self.tcx(), principal) {
1691 if tr.def_id() == copy_def_id {
1692 return ok_if(Vec::new())
1693 }
1694 }
1695
1696 Err(Unimplemented)
1697 }
1698 }
1699 ty::BoundSync | ty::BoundSend => {
1700 self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()");
1701 }
1702 }
1703 }
1704
1705 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl }) => {
1706 // &mut T or &T
1707 match bound {
1708 ty::BoundCopy => {
1709 match mutbl {
1710 // &mut T is affine and hence never `Copy`
1711 hir::MutMutable => Err(Unimplemented),
1712
1713 // &T is always copyable
1714 hir::MutImmutable => ok_if(Vec::new()),
1715 }
1716 }
1717
1718 ty::BoundSized => ok_if(Vec::new()),
1719
1720 ty::BoundSync | ty::BoundSend => {
1721 self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()");
1722 }
1723 }
1724 }
1725
1726 ty::TyArray(element_ty, _) => {
1727 // [T; n]
1728 match bound {
1729 ty::BoundCopy => ok_if(vec![element_ty]),
1730 ty::BoundSized => ok_if(Vec::new()),
1731 ty::BoundSync | ty::BoundSend => {
1732 self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()");
1733 }
1734 }
1735 }
1736
1737 ty::TyStr | ty::TySlice(_) => {
1738 match bound {
1739 ty::BoundSync | ty::BoundSend => {
1740 self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()");
1741 }
1742
1743 ty::BoundCopy | ty::BoundSized => Err(Unimplemented),
1744 }
1745 }
1746
1747 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1748 ty::TyTuple(ref tys) => ok_if(tys.clone()),
1749
1750 ty::TyClosure(_, ref substs) => {
1751 // FIXME -- This case is tricky. In the case of by-ref
1752 // closures particularly, we need the results of
1753 // inference to decide how to reflect the type of each
1754 // upvar (the upvar may have type `T`, but the runtime
1755 // type could be `&mut`, `&`, or just `T`). For now,
1756 // though, we'll do this unsoundly and assume that all
1757 // captures are by value. Really what we ought to do
1758 // is reserve judgement and then intertwine this
1759 // analysis with closure inference.
1760
1761 // Unboxed closures shouldn't be
1762 // implicitly copyable
1763 if bound == ty::BoundCopy {
1764 return Ok(ParameterBuiltin);
1765 }
1766
1767 // Upvars are always local variables or references to
1768 // local variables, and local variables cannot be
1769 // unsized, so the closure struct as a whole must be
1770 // Sized.
1771 if bound == ty::BoundSized {
1772 return ok_if(Vec::new());
1773 }
1774
1775 ok_if(substs.upvar_tys.clone())
1776 }
1777
1778 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
1779 let types: Vec<Ty> = def.all_fields().map(|f| {
1780 f.ty(self.tcx(), substs)
1781 }).collect();
1782 nominal(bound, types)
1783 }
1784
1785 ty::TyProjection(_) | ty::TyParam(_) => {
1786 // Note: A type parameter is only considered to meet a
1787 // particular bound if there is a where clause telling
1788 // us that it does, and that case is handled by
1789 // `assemble_candidates_from_caller_bounds()`.
1790 Ok(ParameterBuiltin)
1791 }
1792
1793 ty::TyInfer(ty::TyVar(_)) => {
1794 // Unbound type variable. Might or might not have
1795 // applicable impls and so forth, depending on what
1796 // those type variables wind up being bound to.
1797 debug!("assemble_builtin_bound_candidates: ambiguous builtin");
1798 Ok(AmbiguousBuiltin)
1799 }
1800
1801 ty::TyError => ok_if(Vec::new()),
1802
1803 ty::TyInfer(ty::FreshTy(_))
1804 | ty::TyInfer(ty::FreshIntTy(_))
1805 | ty::TyInfer(ty::FreshFloatTy(_)) => {
1806 self.tcx().sess.bug(
1807 &format!(
1808 "asked to assemble builtin bounds of unexpected type: {:?}",
1809 self_ty));
1810 }
1811 };
1812
1813 fn ok_if<'tcx>(v: Vec<Ty<'tcx>>)
1814 -> Result<BuiltinBoundConditions<'tcx>, SelectionError<'tcx>> {
1815 Ok(If(ty::Binder(v)))
1816 }
1817
1818 fn nominal<'cx, 'tcx>(bound: ty::BuiltinBound,
1819 types: Vec<Ty<'tcx>>)
1820 -> Result<BuiltinBoundConditions<'tcx>, SelectionError<'tcx>>
1821 {
1822 // First check for markers and other nonsense.
1823 match bound {
1824 // Fallback to whatever user-defined impls exist in this case.
1825 ty::BoundCopy => Ok(ParameterBuiltin),
1826
1827 // Sized if all the component types are sized.
1828 ty::BoundSized => ok_if(types),
1829
1830 // Shouldn't be coming through here.
1831 ty::BoundSend | ty::BoundSync => unreachable!(),
1832 }
1833 }
1834 }
1835
1836 /// For default impls, we need to break apart a type into its
1837 /// "constituent types" -- meaning, the types that it contains.
1838 ///
1839 /// Here are some (simple) examples:
1840 ///
1841 /// ```
1842 /// (i32, u32) -> [i32, u32]
1843 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1844 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1845 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1846 /// ```
1847 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
1848 match t.sty {
1849 ty::TyUint(_) |
1850 ty::TyInt(_) |
1851 ty::TyBool |
1852 ty::TyFloat(_) |
1853 ty::TyBareFn(..) |
1854 ty::TyStr |
1855 ty::TyError |
1856 ty::TyInfer(ty::IntVar(_)) |
1857 ty::TyInfer(ty::FloatVar(_)) |
1858 ty::TyChar => {
1859 Vec::new()
1860 }
1861
1862 ty::TyTrait(..) |
1863 ty::TyParam(..) |
1864 ty::TyProjection(..) |
1865 ty::TyInfer(ty::TyVar(_)) |
1866 ty::TyInfer(ty::FreshTy(_)) |
1867 ty::TyInfer(ty::FreshIntTy(_)) |
1868 ty::TyInfer(ty::FreshFloatTy(_)) => {
1869 self.tcx().sess.bug(
1870 &format!(
1871 "asked to assemble constituent types of unexpected type: {:?}",
1872 t));
1873 }
1874
1875 ty::TyBox(referent_ty) => { // Box<T>
1876 vec![referent_ty]
1877 }
1878
1879 ty::TyRawPtr(ty::TypeAndMut { ty: element_ty, ..}) |
1880 ty::TyRef(_, ty::TypeAndMut { ty: element_ty, ..}) => {
1881 vec![element_ty]
1882 },
1883
1884 ty::TyArray(element_ty, _) | ty::TySlice(element_ty) => {
1885 vec![element_ty]
1886 }
1887
1888 ty::TyTuple(ref tys) => {
1889 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1890 tys.clone()
1891 }
1892
1893 ty::TyClosure(_, ref substs) => {
1894 // FIXME(#27086). We are invariant w/r/t our
1895 // substs.func_substs, but we don't see them as
1896 // constituent types; this seems RIGHT but also like
1897 // something that a normal type couldn't simulate. Is
1898 // this just a gap with the way that PhantomData and
1899 // OIBIT interact? That is, there is no way to say
1900 // "make me invariant with respect to this TYPE, but
1901 // do not act as though I can reach it"
1902 substs.upvar_tys.clone()
1903 }
1904
1905 // for `PhantomData<T>`, we pass `T`
1906 ty::TyStruct(def, substs) if def.is_phantom_data() => {
1907 substs.types.get_slice(TypeSpace).to_vec()
1908 }
1909
1910 ty::TyStruct(def, substs) | ty::TyEnum(def, substs) => {
1911 def.all_fields()
1912 .map(|f| f.ty(self.tcx(), substs))
1913 .collect()
1914 }
1915 }
1916 }
1917
1918 fn collect_predicates_for_types(&mut self,
1919 obligation: &TraitObligation<'tcx>,
1920 trait_def_id: DefId,
1921 types: ty::Binder<Vec<Ty<'tcx>>>)
1922 -> Vec<PredicateObligation<'tcx>>
1923 {
1924 let derived_cause = match self.tcx().lang_items.to_builtin_kind(trait_def_id) {
1925 Some(_) => {
1926 self.derived_cause(obligation, BuiltinDerivedObligation)
1927 },
1928 None => {
1929 self.derived_cause(obligation, ImplDerivedObligation)
1930 }
1931 };
1932
1933 // Because the types were potentially derived from
1934 // higher-ranked obligations they may reference late-bound
1935 // regions. For example, `for<'a> Foo<&'a int> : Copy` would
1936 // yield a type like `for<'a> &'a int`. In general, we
1937 // maintain the invariant that we never manipulate bound
1938 // regions, so we have to process these bound regions somehow.
1939 //
1940 // The strategy is to:
1941 //
1942 // 1. Instantiate those regions to skolemized regions (e.g.,
1943 // `for<'a> &'a int` becomes `&0 int`.
1944 // 2. Produce something like `&'0 int : Copy`
1945 // 3. Re-bind the regions back to `for<'a> &'a int : Copy`
1946
1947 // Move the binder into the individual types
1948 let bound_types: Vec<ty::Binder<Ty<'tcx>>> =
1949 types.skip_binder()
1950 .iter()
1951 .map(|&nested_ty| ty::Binder(nested_ty))
1952 .collect();
1953
1954 // For each type, produce a vector of resulting obligations
1955 let obligations: Result<Vec<Vec<_>>, _> = bound_types.iter().map(|nested_ty| {
1956 self.infcx.commit_if_ok(|snapshot| {
1957 let (skol_ty, skol_map) =
1958 self.infcx().skolemize_late_bound_regions(nested_ty, snapshot);
1959 let Normalized { value: normalized_ty, mut obligations } =
1960 project::normalize_with_depth(self,
1961 obligation.cause.clone(),
1962 obligation.recursion_depth + 1,
1963 &skol_ty);
1964 let skol_obligation =
1965 util::predicate_for_trait_def(self.tcx(),
1966 derived_cause.clone(),
1967 trait_def_id,
1968 obligation.recursion_depth + 1,
1969 normalized_ty,
1970 vec![]);
1971 obligations.push(skol_obligation);
1972 Ok(self.infcx().plug_leaks(skol_map, snapshot, &obligations))
1973 })
1974 }).collect();
1975
1976 // Flatten those vectors (couldn't do it above due `collect`)
1977 match obligations {
1978 Ok(obligations) => obligations.into_iter().flat_map(|o| o).collect(),
1979 Err(ErrorReported) => Vec::new(),
1980 }
1981 }
1982
1983 ///////////////////////////////////////////////////////////////////////////
1984 // CONFIRMATION
1985 //
1986 // Confirmation unifies the output type parameters of the trait
1987 // with the values found in the obligation, possibly yielding a
1988 // type error. See `README.md` for more details.
1989
1990 fn confirm_candidate(&mut self,
1991 obligation: &TraitObligation<'tcx>,
1992 candidate: SelectionCandidate<'tcx>)
1993 -> Result<Selection<'tcx>,SelectionError<'tcx>>
1994 {
1995 debug!("confirm_candidate({:?}, {:?})",
1996 obligation,
1997 candidate);
1998
1999 match candidate {
2000 BuiltinCandidate(builtin_bound) => {
2001 Ok(VtableBuiltin(
2002 try!(self.confirm_builtin_candidate(obligation, builtin_bound))))
2003 }
2004
2005 ParamCandidate(param) => {
2006 let obligations = self.confirm_param_candidate(obligation, param);
2007 Ok(VtableParam(obligations))
2008 }
2009
2010 DefaultImplCandidate(trait_def_id) => {
2011 let data = self.confirm_default_impl_candidate(obligation, trait_def_id);
2012 Ok(VtableDefaultImpl(data))
2013 }
2014
2015 DefaultImplObjectCandidate(trait_def_id) => {
2016 let data = self.confirm_default_impl_object_candidate(obligation, trait_def_id);
2017 Ok(VtableDefaultImpl(data))
2018 }
2019
2020 ImplCandidate(impl_def_id) => {
2021 let vtable_impl =
2022 try!(self.confirm_impl_candidate(obligation, impl_def_id));
2023 Ok(VtableImpl(vtable_impl))
2024 }
2025
2026 ClosureCandidate(closure_def_id, substs) => {
2027 let vtable_closure =
2028 try!(self.confirm_closure_candidate(obligation, closure_def_id, substs));
2029 Ok(VtableClosure(vtable_closure))
2030 }
2031
2032 BuiltinObjectCandidate => {
2033 // This indicates something like `(Trait+Send) :
2034 // Send`. In this case, we know that this holds
2035 // because that's what the object type is telling us,
2036 // and there's really no additional obligations to
2037 // prove and no types in particular to unify etc.
2038 Ok(VtableParam(Vec::new()))
2039 }
2040
2041 ObjectCandidate => {
2042 let data = self.confirm_object_candidate(obligation);
2043 Ok(VtableObject(data))
2044 }
2045
2046 FnPointerCandidate => {
2047 let fn_type =
2048 try!(self.confirm_fn_pointer_candidate(obligation));
2049 Ok(VtableFnPointer(fn_type))
2050 }
2051
2052 ProjectionCandidate => {
2053 self.confirm_projection_candidate(obligation);
2054 Ok(VtableParam(Vec::new()))
2055 }
2056
2057 BuiltinUnsizeCandidate => {
2058 let data = try!(self.confirm_builtin_unsize_candidate(obligation));
2059 Ok(VtableBuiltin(data))
2060 }
2061 }
2062 }
2063
2064 fn confirm_projection_candidate(&mut self,
2065 obligation: &TraitObligation<'tcx>)
2066 {
2067 let _: Result<(),()> =
2068 self.infcx.commit_if_ok(|snapshot| {
2069 let result =
2070 self.match_projection_obligation_against_bounds_from_trait(obligation,
2071 snapshot);
2072 assert!(result);
2073 Ok(())
2074 });
2075 }
2076
2077 fn confirm_param_candidate(&mut self,
2078 obligation: &TraitObligation<'tcx>,
2079 param: ty::PolyTraitRef<'tcx>)
2080 -> Vec<PredicateObligation<'tcx>>
2081 {
2082 debug!("confirm_param_candidate({:?},{:?})",
2083 obligation,
2084 param);
2085
2086 // During evaluation, we already checked that this
2087 // where-clause trait-ref could be unified with the obligation
2088 // trait-ref. Repeat that unification now without any
2089 // transactional boundary; it should not fail.
2090 match self.match_where_clause_trait_ref(obligation, param.clone()) {
2091 Ok(obligations) => obligations,
2092 Err(()) => {
2093 self.tcx().sess.bug(
2094 &format!("Where clause `{:?}` was applicable to `{:?}` but now is not",
2095 param,
2096 obligation));
2097 }
2098 }
2099 }
2100
2101 fn confirm_builtin_candidate(&mut self,
2102 obligation: &TraitObligation<'tcx>,
2103 bound: ty::BuiltinBound)
2104 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>,
2105 SelectionError<'tcx>>
2106 {
2107 debug!("confirm_builtin_candidate({:?})",
2108 obligation);
2109
2110 match try!(self.builtin_bound(bound, obligation)) {
2111 If(nested) => Ok(self.vtable_builtin_data(obligation, bound, nested)),
2112 AmbiguousBuiltin | ParameterBuiltin => {
2113 self.tcx().sess.span_bug(
2114 obligation.cause.span,
2115 &format!("builtin bound for {:?} was ambig",
2116 obligation));
2117 }
2118 }
2119 }
2120
2121 fn vtable_builtin_data(&mut self,
2122 obligation: &TraitObligation<'tcx>,
2123 bound: ty::BuiltinBound,
2124 nested: ty::Binder<Vec<Ty<'tcx>>>)
2125 -> VtableBuiltinData<PredicateObligation<'tcx>>
2126 {
2127 debug!("vtable_builtin_data(obligation={:?}, bound={:?}, nested={:?})",
2128 obligation, bound, nested);
2129
2130 let trait_def = match self.tcx().lang_items.from_builtin_kind(bound) {
2131 Ok(def_id) => def_id,
2132 Err(_) => {
2133 self.tcx().sess.bug("builtin trait definition not found");
2134 }
2135 };
2136
2137 let obligations = self.collect_predicates_for_types(obligation, trait_def, nested);
2138
2139 debug!("vtable_builtin_data: obligations={:?}",
2140 obligations);
2141
2142 VtableBuiltinData { nested: obligations }
2143 }
2144
2145 /// This handles the case where a `impl Foo for ..` impl is being used.
2146 /// The idea is that the impl applies to `X : Foo` if the following conditions are met:
2147 ///
2148 /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds
2149 /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds.
2150 fn confirm_default_impl_candidate(&mut self,
2151 obligation: &TraitObligation<'tcx>,
2152 trait_def_id: DefId)
2153 -> VtableDefaultImplData<PredicateObligation<'tcx>>
2154 {
2155 debug!("confirm_default_impl_candidate({:?}, {:?})",
2156 obligation,
2157 trait_def_id);
2158
2159 // binder is moved below
2160 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2161 let types = self.constituent_types_for_ty(self_ty);
2162 self.vtable_default_impl(obligation, trait_def_id, ty::Binder(types))
2163 }
2164
2165 fn confirm_default_impl_object_candidate(&mut self,
2166 obligation: &TraitObligation<'tcx>,
2167 trait_def_id: DefId)
2168 -> VtableDefaultImplData<PredicateObligation<'tcx>>
2169 {
2170 debug!("confirm_default_impl_object_candidate({:?}, {:?})",
2171 obligation,
2172 trait_def_id);
2173
2174 assert!(self.tcx().has_attr(trait_def_id, "rustc_reflect_like"));
2175
2176 // OK to skip binder, it is reintroduced below
2177 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2178 match self_ty.sty {
2179 ty::TyTrait(ref data) => {
2180 // OK to skip the binder, it is reintroduced below
2181 let input_types = data.principal.skip_binder().substs.types.get_slice(TypeSpace);
2182 let assoc_types = data.bounds.projection_bounds
2183 .iter()
2184 .map(|pb| pb.skip_binder().ty);
2185 let all_types: Vec<_> = input_types.iter().cloned()
2186 .chain(assoc_types)
2187 .collect();
2188
2189 // reintroduce the two binding levels we skipped, then flatten into one
2190 let all_types = ty::Binder(ty::Binder(all_types));
2191 let all_types = self.tcx().flatten_late_bound_regions(&all_types);
2192
2193 self.vtable_default_impl(obligation, trait_def_id, all_types)
2194 }
2195 _ => {
2196 self.tcx().sess.bug(
2197 &format!(
2198 "asked to confirm default object implementation for non-object type: {:?}",
2199 self_ty));
2200 }
2201 }
2202 }
2203
2204 /// See `confirm_default_impl_candidate`
2205 fn vtable_default_impl(&mut self,
2206 obligation: &TraitObligation<'tcx>,
2207 trait_def_id: DefId,
2208 nested: ty::Binder<Vec<Ty<'tcx>>>)
2209 -> VtableDefaultImplData<PredicateObligation<'tcx>>
2210 {
2211 debug!("vtable_default_impl_data: nested={:?}", nested);
2212
2213 let mut obligations = self.collect_predicates_for_types(obligation,
2214 trait_def_id,
2215 nested);
2216
2217 let trait_obligations: Result<Vec<_>,()> = self.infcx.commit_if_ok(|snapshot| {
2218 let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
2219 let (trait_ref, skol_map) =
2220 self.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot);
2221 Ok(self.impl_or_trait_obligations(obligation.cause.clone(),
2222 obligation.recursion_depth + 1,
2223 trait_def_id,
2224 &trait_ref.substs,
2225 skol_map,
2226 snapshot))
2227 });
2228
2229 // no Errors in that code above
2230 obligations.append(&mut trait_obligations.unwrap());
2231
2232 debug!("vtable_default_impl_data: obligations={:?}", obligations);
2233
2234 VtableDefaultImplData {
2235 trait_def_id: trait_def_id,
2236 nested: obligations
2237 }
2238 }
2239
2240 fn confirm_impl_candidate(&mut self,
2241 obligation: &TraitObligation<'tcx>,
2242 impl_def_id: DefId)
2243 -> Result<VtableImplData<'tcx, PredicateObligation<'tcx>>,
2244 SelectionError<'tcx>>
2245 {
2246 debug!("confirm_impl_candidate({:?},{:?})",
2247 obligation,
2248 impl_def_id);
2249
2250 // First, create the substitutions by matching the impl again,
2251 // this time not in a probe.
2252 self.infcx.commit_if_ok(|snapshot| {
2253 let (substs, skol_map) =
2254 self.rematch_impl(impl_def_id, obligation,
2255 snapshot);
2256 debug!("confirm_impl_candidate substs={:?}", substs);
2257 Ok(self.vtable_impl(impl_def_id, substs, obligation.cause.clone(),
2258 obligation.recursion_depth + 1, skol_map, snapshot))
2259 })
2260 }
2261
2262 fn vtable_impl(&mut self,
2263 impl_def_id: DefId,
2264 mut substs: Normalized<'tcx, Substs<'tcx>>,
2265 cause: ObligationCause<'tcx>,
2266 recursion_depth: usize,
2267 skol_map: infer::SkolemizationMap,
2268 snapshot: &infer::CombinedSnapshot)
2269 -> VtableImplData<'tcx, PredicateObligation<'tcx>>
2270 {
2271 debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})",
2272 impl_def_id,
2273 substs,
2274 recursion_depth,
2275 skol_map);
2276
2277 let mut impl_obligations =
2278 self.impl_or_trait_obligations(cause,
2279 recursion_depth,
2280 impl_def_id,
2281 &substs.value,
2282 skol_map,
2283 snapshot);
2284
2285 debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}",
2286 impl_def_id,
2287 impl_obligations);
2288
2289 // Because of RFC447, the impl-trait-ref and obligations
2290 // are sufficient to determine the impl substs, without
2291 // relying on projections in the impl-trait-ref.
2292 //
2293 // e.g. `impl<U: Tr, V: Iterator<Item=U>> Foo<<U as Tr>::T> for V`
2294 impl_obligations.append(&mut substs.obligations);
2295
2296 VtableImplData { impl_def_id: impl_def_id,
2297 substs: substs.value,
2298 nested: impl_obligations }
2299 }
2300
2301 fn confirm_object_candidate(&mut self,
2302 obligation: &TraitObligation<'tcx>)
2303 -> VtableObjectData<'tcx>
2304 {
2305 debug!("confirm_object_candidate({:?})",
2306 obligation);
2307
2308 // FIXME skipping binder here seems wrong -- we should
2309 // probably flatten the binder from the obligation and the
2310 // binder from the object. Have to try to make a broken test
2311 // case that results. -nmatsakis
2312 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2313 let poly_trait_ref = match self_ty.sty {
2314 ty::TyTrait(ref data) => {
2315 data.principal_trait_ref_with_self_ty(self.tcx(), self_ty)
2316 }
2317 _ => {
2318 self.tcx().sess.span_bug(obligation.cause.span,
2319 "object candidate with non-object");
2320 }
2321 };
2322
2323 let mut upcast_trait_ref = None;
2324 let vtable_base;
2325
2326 {
2327 // We want to find the first supertrait in the list of
2328 // supertraits that we can unify with, and do that
2329 // unification. We know that there is exactly one in the list
2330 // where we can unify because otherwise select would have
2331 // reported an ambiguity. (When we do find a match, also
2332 // record it for later.)
2333 let nonmatching =
2334 util::supertraits(self.tcx(), poly_trait_ref)
2335 .take_while(|&t| {
2336 match
2337 self.infcx.commit_if_ok(
2338 |_| self.match_poly_trait_ref(obligation, t))
2339 {
2340 Ok(_) => { upcast_trait_ref = Some(t); false }
2341 Err(_) => { true }
2342 }
2343 });
2344
2345 // Additionally, for each of the nonmatching predicates that
2346 // we pass over, we sum up the set of number of vtable
2347 // entries, so that we can compute the offset for the selected
2348 // trait.
2349 vtable_base =
2350 nonmatching.map(|t| util::count_own_vtable_entries(self.tcx(), t))
2351 .sum();
2352
2353 }
2354
2355 VtableObjectData {
2356 upcast_trait_ref: upcast_trait_ref.unwrap(),
2357 vtable_base: vtable_base,
2358 }
2359 }
2360
2361 fn confirm_fn_pointer_candidate(&mut self,
2362 obligation: &TraitObligation<'tcx>)
2363 -> Result<ty::Ty<'tcx>,SelectionError<'tcx>>
2364 {
2365 debug!("confirm_fn_pointer_candidate({:?})",
2366 obligation);
2367
2368 // ok to skip binder; it is reintroduced below
2369 let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder());
2370 let sig = self_ty.fn_sig();
2371 let trait_ref =
2372 util::closure_trait_ref_and_return_type(self.tcx(),
2373 obligation.predicate.def_id(),
2374 self_ty,
2375 sig,
2376 util::TupleArgumentsFlag::Yes)
2377 .map_bound(|(trait_ref, _)| trait_ref);
2378
2379 try!(self.confirm_poly_trait_refs(obligation.cause.clone(),
2380 obligation.predicate.to_poly_trait_ref(),
2381 trait_ref));
2382 Ok(self_ty)
2383 }
2384
2385 fn confirm_closure_candidate(&mut self,
2386 obligation: &TraitObligation<'tcx>,
2387 closure_def_id: DefId,
2388 substs: &ty::ClosureSubsts<'tcx>)
2389 -> Result<VtableClosureData<'tcx, PredicateObligation<'tcx>>,
2390 SelectionError<'tcx>>
2391 {
2392 debug!("confirm_closure_candidate({:?},{:?},{:?})",
2393 obligation,
2394 closure_def_id,
2395 substs);
2396
2397 let Normalized {
2398 value: trait_ref,
2399 obligations
2400 } = self.closure_trait_ref(obligation, closure_def_id, substs);
2401
2402 debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})",
2403 closure_def_id,
2404 trait_ref,
2405 obligations);
2406
2407 try!(self.confirm_poly_trait_refs(obligation.cause.clone(),
2408 obligation.predicate.to_poly_trait_ref(),
2409 trait_ref));
2410
2411 Ok(VtableClosureData {
2412 closure_def_id: closure_def_id,
2413 substs: substs.clone(),
2414 nested: obligations
2415 })
2416 }
2417
2418 /// In the case of closure types and fn pointers,
2419 /// we currently treat the input type parameters on the trait as
2420 /// outputs. This means that when we have a match we have only
2421 /// considered the self type, so we have to go back and make sure
2422 /// to relate the argument types too. This is kind of wrong, but
2423 /// since we control the full set of impls, also not that wrong,
2424 /// and it DOES yield better error messages (since we don't report
2425 /// errors as if there is no applicable impl, but rather report
2426 /// errors are about mismatched argument types.
2427 ///
2428 /// Here is an example. Imagine we have a closure expression
2429 /// and we desugared it so that the type of the expression is
2430 /// `Closure`, and `Closure` expects an int as argument. Then it
2431 /// is "as if" the compiler generated this impl:
2432 ///
2433 /// impl Fn(int) for Closure { ... }
2434 ///
2435 /// Now imagine our obligation is `Fn(usize) for Closure`. So far
2436 /// we have matched the self-type `Closure`. At this point we'll
2437 /// compare the `int` to `usize` and generate an error.
2438 ///
2439 /// Note that this checking occurs *after* the impl has selected,
2440 /// because these output type parameters should not affect the
2441 /// selection of the impl. Therefore, if there is a mismatch, we
2442 /// report an error to the user.
2443 fn confirm_poly_trait_refs(&mut self,
2444 obligation_cause: ObligationCause,
2445 obligation_trait_ref: ty::PolyTraitRef<'tcx>,
2446 expected_trait_ref: ty::PolyTraitRef<'tcx>)
2447 -> Result<(), SelectionError<'tcx>>
2448 {
2449 let origin = TypeOrigin::RelateOutputImplTypes(obligation_cause.span);
2450
2451 let obligation_trait_ref = obligation_trait_ref.clone();
2452 match self.infcx.sub_poly_trait_refs(false,
2453 origin,
2454 expected_trait_ref.clone(),
2455 obligation_trait_ref.clone()) {
2456 Ok(()) => Ok(()),
2457 Err(e) => Err(OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
2458 }
2459 }
2460
2461 fn confirm_builtin_unsize_candidate(&mut self,
2462 obligation: &TraitObligation<'tcx>,)
2463 -> Result<VtableBuiltinData<PredicateObligation<'tcx>>,
2464 SelectionError<'tcx>> {
2465 let tcx = self.tcx();
2466
2467 // assemble_candidates_for_unsizing should ensure there are no late bound
2468 // regions here. See the comment there for more details.
2469 let source = self.infcx.shallow_resolve(
2470 tcx.no_late_bound_regions(&obligation.self_ty()).unwrap());
2471 let target = self.infcx.shallow_resolve(obligation.predicate.0.input_types()[0]);
2472
2473 debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})",
2474 source, target);
2475
2476 let mut nested = vec![];
2477 match (&source.sty, &target.sty) {
2478 // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
2479 (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => {
2480 // See assemble_candidates_for_unsizing for more info.
2481 let bounds = ty::ExistentialBounds {
2482 region_bound: data_b.bounds.region_bound,
2483 builtin_bounds: data_b.bounds.builtin_bounds,
2484 projection_bounds: data_a.bounds.projection_bounds.clone(),
2485 };
2486
2487 let new_trait = tcx.mk_trait(data_a.principal.clone(), bounds);
2488 let origin = TypeOrigin::Misc(obligation.cause.span);
2489 if self.infcx.sub_types(false, origin, new_trait, target).is_err() {
2490 return Err(Unimplemented);
2491 }
2492
2493 // Register one obligation for 'a: 'b.
2494 let cause = ObligationCause::new(obligation.cause.span,
2495 obligation.cause.body_id,
2496 ObjectCastObligation(target));
2497 let outlives = ty::OutlivesPredicate(data_a.bounds.region_bound,
2498 data_b.bounds.region_bound);
2499 nested.push(Obligation::with_depth(cause,
2500 obligation.recursion_depth + 1,
2501 ty::Binder(outlives).to_predicate()));
2502 }
2503
2504 // T -> Trait.
2505 (_, &ty::TyTrait(ref data)) => {
2506 let object_did = data.principal_def_id();
2507 if !object_safety::is_object_safe(tcx, object_did) {
2508 return Err(TraitNotObjectSafe(object_did));
2509 }
2510
2511 let cause = ObligationCause::new(obligation.cause.span,
2512 obligation.cause.body_id,
2513 ObjectCastObligation(target));
2514 let mut push = |predicate| {
2515 nested.push(Obligation::with_depth(cause.clone(),
2516 obligation.recursion_depth + 1,
2517 predicate));
2518 };
2519
2520 // Create the obligation for casting from T to Trait.
2521 push(data.principal_trait_ref_with_self_ty(tcx, source).to_predicate());
2522
2523 // We can only make objects from sized types.
2524 let mut builtin_bounds = data.bounds.builtin_bounds;
2525 builtin_bounds.insert(ty::BoundSized);
2526
2527 // Create additional obligations for all the various builtin
2528 // bounds attached to the object cast. (In other words, if the
2529 // object type is Foo+Send, this would create an obligation
2530 // for the Send check.)
2531 for bound in &builtin_bounds {
2532 if let Ok(tr) = util::trait_ref_for_builtin_bound(tcx, bound, source) {
2533 push(tr.to_predicate());
2534 } else {
2535 return Err(Unimplemented);
2536 }
2537 }
2538
2539 // Create obligations for the projection predicates.
2540 for bound in data.projection_bounds_with_self_ty(tcx, source) {
2541 push(bound.to_predicate());
2542 }
2543
2544 // If the type is `Foo+'a`, ensures that the type
2545 // being cast to `Foo+'a` outlives `'a`:
2546 let outlives = ty::OutlivesPredicate(source,
2547 data.bounds.region_bound);
2548 push(ty::Binder(outlives).to_predicate());
2549 }
2550
2551 // [T; n] -> [T].
2552 (&ty::TyArray(a, _), &ty::TySlice(b)) => {
2553 let origin = TypeOrigin::Misc(obligation.cause.span);
2554 if self.infcx.sub_types(false, origin, a, b).is_err() {
2555 return Err(Unimplemented);
2556 }
2557 }
2558
2559 // Struct<T> -> Struct<U>.
2560 (&ty::TyStruct(def, substs_a), &ty::TyStruct(_, substs_b)) => {
2561 let fields = def
2562 .all_fields()
2563 .map(|f| f.unsubst_ty())
2564 .collect::<Vec<_>>();
2565
2566 // The last field of the structure has to exist and contain type parameters.
2567 let field = if let Some(&field) = fields.last() {
2568 field
2569 } else {
2570 return Err(Unimplemented);
2571 };
2572 let mut ty_params = vec![];
2573 for ty in field.walk() {
2574 if let ty::TyParam(p) = ty.sty {
2575 assert!(p.space == TypeSpace);
2576 let idx = p.idx as usize;
2577 if !ty_params.contains(&idx) {
2578 ty_params.push(idx);
2579 }
2580 }
2581 }
2582 if ty_params.is_empty() {
2583 return Err(Unimplemented);
2584 }
2585
2586 // Replace type parameters used in unsizing with
2587 // TyError and ensure they do not affect any other fields.
2588 // This could be checked after type collection for any struct
2589 // with a potentially unsized trailing field.
2590 let mut new_substs = substs_a.clone();
2591 for &i in &ty_params {
2592 new_substs.types.get_mut_slice(TypeSpace)[i] = tcx.types.err;
2593 }
2594 for &ty in fields.split_last().unwrap().1 {
2595 if ty.subst(tcx, &new_substs).references_error() {
2596 return Err(Unimplemented);
2597 }
2598 }
2599
2600 // Extract Field<T> and Field<U> from Struct<T> and Struct<U>.
2601 let inner_source = field.subst(tcx, substs_a);
2602 let inner_target = field.subst(tcx, substs_b);
2603
2604 // Check that the source structure with the target's
2605 // type parameters is a subtype of the target.
2606 for &i in &ty_params {
2607 let param_b = *substs_b.types.get(TypeSpace, i);
2608 new_substs.types.get_mut_slice(TypeSpace)[i] = param_b;
2609 }
2610 let new_struct = tcx.mk_struct(def, tcx.mk_substs(new_substs));
2611 let origin = TypeOrigin::Misc(obligation.cause.span);
2612 if self.infcx.sub_types(false, origin, new_struct, target).is_err() {
2613 return Err(Unimplemented);
2614 }
2615
2616 // Construct the nested Field<T>: Unsize<Field<U>> predicate.
2617 nested.push(util::predicate_for_trait_def(tcx,
2618 obligation.cause.clone(),
2619 obligation.predicate.def_id(),
2620 obligation.recursion_depth + 1,
2621 inner_source,
2622 vec![inner_target]));
2623 }
2624
2625 _ => unreachable!()
2626 };
2627
2628 Ok(VtableBuiltinData { nested: nested })
2629 }
2630
2631 ///////////////////////////////////////////////////////////////////////////
2632 // Matching
2633 //
2634 // Matching is a common path used for both evaluation and
2635 // confirmation. It basically unifies types that appear in impls
2636 // and traits. This does affect the surrounding environment;
2637 // therefore, when used during evaluation, match routines must be
2638 // run inside of a `probe()` so that their side-effects are
2639 // contained.
2640
2641 fn rematch_impl(&mut self,
2642 impl_def_id: DefId,
2643 obligation: &TraitObligation<'tcx>,
2644 snapshot: &infer::CombinedSnapshot)
2645 -> (Normalized<'tcx, Substs<'tcx>>, infer::SkolemizationMap)
2646 {
2647 match self.match_impl(impl_def_id, obligation, snapshot) {
2648 Ok((substs, skol_map)) => (substs, skol_map),
2649 Err(()) => {
2650 self.tcx().sess.bug(
2651 &format!("Impl {:?} was matchable against {:?} but now is not",
2652 impl_def_id,
2653 obligation));
2654 }
2655 }
2656 }
2657
2658 fn match_impl(&mut self,
2659 impl_def_id: DefId,
2660 obligation: &TraitObligation<'tcx>,
2661 snapshot: &infer::CombinedSnapshot)
2662 -> Result<(Normalized<'tcx, Substs<'tcx>>,
2663 infer::SkolemizationMap), ()>
2664 {
2665 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
2666
2667 // Before we create the substitutions and everything, first
2668 // consider a "quick reject". This avoids creating more types
2669 // and so forth that we need to.
2670 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
2671 return Err(());
2672 }
2673
2674 let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions(
2675 &obligation.predicate,
2676 snapshot);
2677 let skol_obligation_trait_ref = skol_obligation.trait_ref;
2678
2679 let impl_substs = util::fresh_type_vars_for_impl(self.infcx,
2680 obligation.cause.span,
2681 impl_def_id);
2682
2683 let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
2684 &impl_substs);
2685
2686 let impl_trait_ref =
2687 project::normalize_with_depth(self,
2688 obligation.cause.clone(),
2689 obligation.recursion_depth + 1,
2690 &impl_trait_ref);
2691
2692 debug!("match_impl(impl_def_id={:?}, obligation={:?}, \
2693 impl_trait_ref={:?}, skol_obligation_trait_ref={:?})",
2694 impl_def_id,
2695 obligation,
2696 impl_trait_ref,
2697 skol_obligation_trait_ref);
2698
2699 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
2700 if let Err(e) = self.infcx.eq_trait_refs(false,
2701 origin,
2702 impl_trait_ref.value.clone(),
2703 skol_obligation_trait_ref) {
2704 debug!("match_impl: failed eq_trait_refs due to `{}`", e);
2705 return Err(());
2706 }
2707
2708 if let Err(e) = self.infcx.leak_check(&skol_map, snapshot) {
2709 debug!("match_impl: failed leak check due to `{}`", e);
2710 return Err(());
2711 }
2712
2713 debug!("match_impl: success impl_substs={:?}", impl_substs);
2714 Ok((Normalized {
2715 value: impl_substs,
2716 obligations: impl_trait_ref.obligations
2717 }, skol_map))
2718 }
2719
2720 fn fast_reject_trait_refs(&mut self,
2721 obligation: &TraitObligation,
2722 impl_trait_ref: &ty::TraitRef)
2723 -> bool
2724 {
2725 // We can avoid creating type variables and doing the full
2726 // substitution if we find that any of the input types, when
2727 // simplified, do not match.
2728
2729 obligation.predicate.0.input_types().iter()
2730 .zip(impl_trait_ref.input_types())
2731 .any(|(&obligation_ty, &impl_ty)| {
2732 let simplified_obligation_ty =
2733 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
2734 let simplified_impl_ty =
2735 fast_reject::simplify_type(self.tcx(), impl_ty, false);
2736
2737 simplified_obligation_ty.is_some() &&
2738 simplified_impl_ty.is_some() &&
2739 simplified_obligation_ty != simplified_impl_ty
2740 })
2741 }
2742
2743 /// Normalize `where_clause_trait_ref` and try to match it against
2744 /// `obligation`. If successful, return any predicates that
2745 /// result from the normalization. Normalization is necessary
2746 /// because where-clauses are stored in the parameter environment
2747 /// unnormalized.
2748 fn match_where_clause_trait_ref(&mut self,
2749 obligation: &TraitObligation<'tcx>,
2750 where_clause_trait_ref: ty::PolyTraitRef<'tcx>)
2751 -> Result<Vec<PredicateObligation<'tcx>>,()>
2752 {
2753 try!(self.match_poly_trait_ref(obligation, where_clause_trait_ref));
2754 Ok(Vec::new())
2755 }
2756
2757 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2758 /// obligation is satisfied.
2759 fn match_poly_trait_ref(&self,
2760 obligation: &TraitObligation<'tcx>,
2761 poly_trait_ref: ty::PolyTraitRef<'tcx>)
2762 -> Result<(),()>
2763 {
2764 debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
2765 obligation,
2766 poly_trait_ref);
2767
2768 let origin = TypeOrigin::RelateOutputImplTypes(obligation.cause.span);
2769 match self.infcx.sub_poly_trait_refs(false,
2770 origin,
2771 poly_trait_ref,
2772 obligation.predicate.to_poly_trait_ref()) {
2773 Ok(()) => Ok(()),
2774 Err(_) => Err(()),
2775 }
2776 }
2777
2778 ///////////////////////////////////////////////////////////////////////////
2779 // Miscellany
2780
2781 fn match_fresh_trait_refs(&self,
2782 previous: &ty::PolyTraitRef<'tcx>,
2783 current: &ty::PolyTraitRef<'tcx>)
2784 -> bool
2785 {
2786 let mut matcher = ty::_match::Match::new(self.tcx());
2787 matcher.relate(previous, current).is_ok()
2788 }
2789
2790 fn push_stack<'o,'s:'o>(&mut self,
2791 previous_stack: TraitObligationStackList<'s, 'tcx>,
2792 obligation: &'o TraitObligation<'tcx>)
2793 -> TraitObligationStack<'o, 'tcx>
2794 {
2795 let fresh_trait_ref =
2796 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
2797
2798 TraitObligationStack {
2799 obligation: obligation,
2800 fresh_trait_ref: fresh_trait_ref,
2801 previous: previous_stack,
2802 }
2803 }
2804
2805 fn closure_trait_ref_unnormalized(&mut self,
2806 obligation: &TraitObligation<'tcx>,
2807 closure_def_id: DefId,
2808 substs: &ty::ClosureSubsts<'tcx>)
2809 -> ty::PolyTraitRef<'tcx>
2810 {
2811 let closure_type = self.infcx.closure_type(closure_def_id, substs);
2812 let ty::Binder((trait_ref, _)) =
2813 util::closure_trait_ref_and_return_type(self.tcx(),
2814 obligation.predicate.def_id(),
2815 obligation.predicate.0.self_ty(), // (1)
2816 &closure_type.sig,
2817 util::TupleArgumentsFlag::No);
2818 // (1) Feels icky to skip the binder here, but OTOH we know
2819 // that the self-type is an unboxed closure type and hence is
2820 // in fact unparameterized (or at least does not reference any
2821 // regions bound in the obligation). Still probably some
2822 // refactoring could make this nicer.
2823
2824 ty::Binder(trait_ref)
2825 }
2826
2827 fn closure_trait_ref(&mut self,
2828 obligation: &TraitObligation<'tcx>,
2829 closure_def_id: DefId,
2830 substs: &ty::ClosureSubsts<'tcx>)
2831 -> Normalized<'tcx, ty::PolyTraitRef<'tcx>>
2832 {
2833 let trait_ref = self.closure_trait_ref_unnormalized(
2834 obligation, closure_def_id, substs);
2835
2836 // A closure signature can contain associated types which
2837 // must be normalized.
2838 normalize_with_depth(self,
2839 obligation.cause.clone(),
2840 obligation.recursion_depth+1,
2841 &trait_ref)
2842 }
2843
2844 /// Returns the obligations that are implied by instantiating an
2845 /// impl or trait. The obligations are substituted and fully
2846 /// normalized. This is used when confirming an impl or default
2847 /// impl.
2848 fn impl_or_trait_obligations(&mut self,
2849 cause: ObligationCause<'tcx>,
2850 recursion_depth: usize,
2851 def_id: DefId, // of impl or trait
2852 substs: &Substs<'tcx>, // for impl or trait
2853 skol_map: infer::SkolemizationMap,
2854 snapshot: &infer::CombinedSnapshot)
2855 -> Vec<PredicateObligation<'tcx>>
2856 {
2857 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
2858 let tcx = self.tcx();
2859
2860 // To allow for one-pass evaluation of the nested obligation,
2861 // each predicate must be preceded by the obligations required
2862 // to normalize it.
2863 // for example, if we have:
2864 // impl<U: Iterator, V: Iterator<Item=U>> Foo for V where U::Item: Copy
2865 // the impl will have the following predicates:
2866 // <V as Iterator>::Item = U,
2867 // U: Iterator, U: Sized,
2868 // V: Iterator, V: Sized,
2869 // <U as Iterator>::Item: Copy
2870 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2871 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2872 // `$1: Copy`, so we must ensure the obligations are emitted in
2873 // that order.
2874 let predicates = tcx
2875 .lookup_predicates(def_id)
2876 .predicates.iter()
2877 .flat_map(|predicate| {
2878 let predicate =
2879 normalize_with_depth(self, cause.clone(), recursion_depth,
2880 &predicate.subst(tcx, substs));
2881 predicate.obligations.into_iter().chain(
2882 Some(Obligation {
2883 cause: cause.clone(),
2884 recursion_depth: recursion_depth,
2885 predicate: predicate.value
2886 }))
2887 }).collect();
2888 self.infcx().plug_leaks(skol_map, snapshot, &predicates)
2889 }
2890
2891 #[allow(unused_comparisons)]
2892 fn derived_cause(&self,
2893 obligation: &TraitObligation<'tcx>,
2894 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>)
2895 -> ObligationCause<'tcx>
2896 {
2897 /*!
2898 * Creates a cause for obligations that are derived from
2899 * `obligation` by a recursive search (e.g., for a builtin
2900 * bound, or eventually a `impl Foo for ..`). If `obligation`
2901 * is itself a derived obligation, this is just a clone, but
2902 * otherwise we create a "derived obligation" cause so as to
2903 * keep track of the original root obligation for error
2904 * reporting.
2905 */
2906
2907 // NOTE(flaper87): As of now, it keeps track of the whole error
2908 // chain. Ideally, we should have a way to configure this either
2909 // by using -Z verbose or just a CLI argument.
2910 if obligation.recursion_depth >= 0 {
2911 let derived_cause = DerivedObligationCause {
2912 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
2913 parent_code: Rc::new(obligation.cause.code.clone())
2914 };
2915 let derived_code = variant(derived_cause);
2916 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
2917 } else {
2918 obligation.cause.clone()
2919 }
2920 }
2921 }
2922
2923 impl<'tcx> SelectionCache<'tcx> {
2924 pub fn new() -> SelectionCache<'tcx> {
2925 SelectionCache {
2926 hashmap: RefCell::new(FnvHashMap())
2927 }
2928 }
2929 }
2930
2931 impl<'tcx> EvaluationCache<'tcx> {
2932 pub fn new() -> EvaluationCache<'tcx> {
2933 EvaluationCache {
2934 hashmap: RefCell::new(FnvHashMap())
2935 }
2936 }
2937 }
2938
2939 impl<'o,'tcx> TraitObligationStack<'o,'tcx> {
2940 fn list(&'o self) -> TraitObligationStackList<'o,'tcx> {
2941 TraitObligationStackList::with(self)
2942 }
2943
2944 fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> {
2945 self.list()
2946 }
2947 }
2948
2949 #[derive(Copy, Clone)]
2950 struct TraitObligationStackList<'o,'tcx:'o> {
2951 head: Option<&'o TraitObligationStack<'o,'tcx>>
2952 }
2953
2954 impl<'o,'tcx> TraitObligationStackList<'o,'tcx> {
2955 fn empty() -> TraitObligationStackList<'o,'tcx> {
2956 TraitObligationStackList { head: None }
2957 }
2958
2959 fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> {
2960 TraitObligationStackList { head: Some(r) }
2961 }
2962 }
2963
2964 impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{
2965 type Item = &'o TraitObligationStack<'o,'tcx>;
2966
2967 fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> {
2968 match self.head {
2969 Some(o) => {
2970 *self = o.previous;
2971 Some(o)
2972 }
2973 None => None
2974 }
2975 }
2976 }
2977
2978 impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> {
2979 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
2980 write!(f, "TraitObligationStack({:?})", self.obligation)
2981 }
2982 }
2983
2984 impl EvaluationResult {
2985 fn may_apply(&self) -> bool {
2986 match *self {
2987 EvaluatedToOk |
2988 EvaluatedToAmbig |
2989 EvaluatedToUnknown => true,
2990
2991 EvaluatedToErr => false
2992 }
2993 }
2994 }
2995
2996 impl MethodMatchResult {
2997 pub fn may_apply(&self) -> bool {
2998 match *self {
2999 MethodMatched(_) => true,
3000 MethodAmbiguous(_) => true,
3001 MethodDidNotMatch => false,
3002 }
3003 }
3004 }