// Copyright 2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! See `README.md` for high-level documentation #![allow(dead_code)] // FIXME -- just temporarily pub use self::MethodMatchResult::*; pub use self::MethodMatchedData::*; use self::SelectionCandidate::*; use self::BuiltinBoundConditions::*; use self::EvaluationResult::*; use super::coherence; use super::DerivedObligationCause; use super::project; use super::project::{normalize_with_depth, Normalized}; use super::{PredicateObligation, TraitObligation, ObligationCause}; use super::report_overflow_error; use super::{ObligationCauseCode, BuiltinDerivedObligation, ImplDerivedObligation}; use super::{SelectionError, Unimplemented, OutputTypeParameterMismatch}; use super::{ObjectCastObligation, Obligation}; use super::TraitNotObjectSafe; use super::Selection; use super::SelectionResult; use super::{VtableBuiltin, VtableImpl, VtableParam, VtableClosure, VtableFnPointer, VtableObject, VtableDefaultImpl}; use super::{VtableImplData, VtableObjectData, VtableBuiltinData, VtableClosureData, VtableDefaultImplData}; use super::object_safety; use super::util; use middle::fast_reject; use middle::subst::{Subst, Substs, TypeSpace}; use middle::ty::{self, ToPredicate, RegionEscape, ToPolyTraitRef, Ty, HasTypeFlags}; use middle::infer; use middle::infer::{InferCtxt, TypeFreshener}; use middle::ty_fold::TypeFoldable; use middle::ty_match; use middle::ty_relate::TypeRelation; use std::cell::RefCell; use std::fmt; use std::rc::Rc; use syntax::{abi, ast}; use util::common::ErrorReported; use util::nodemap::FnvHashMap; pub struct SelectionContext<'cx, 'tcx:'cx> { infcx: &'cx InferCtxt<'cx, 'tcx>, /// Freshener used specifically for skolemizing entries on the /// obligation stack. This ensures that all entries on the stack /// at one time will have the same set of skolemized entries, /// which is important for checking for trait bounds that /// recursively require themselves. freshener: TypeFreshener<'cx, 'tcx>, /// If true, indicates that the evaluation should be conservative /// and consider the possibility of types outside this crate. /// This comes up primarily when resolving ambiguity. Imagine /// there is some trait reference `$0 : Bar` where `$0` is an /// inference variable. If `intercrate` is true, then we can never /// say for sure that this reference is not implemented, even if /// there are *no impls at all for `Bar`*, because `$0` could be /// bound to some type that in a downstream crate that implements /// `Bar`. This is the suitable mode for coherence. Elsewhere, /// though, we set this to false, because we are only interested /// in types that the user could actually have written --- in /// other words, we consider `$0 : Bar` to be unimplemented if /// there is no type that the user could *actually name* that /// would satisfy it. This avoids crippling inference, basically. intercrate: bool, } // A stack that walks back up the stack frame. struct TraitObligationStack<'prev, 'tcx: 'prev> { obligation: &'prev TraitObligation<'tcx>, /// Trait ref from `obligation` but skolemized with the /// selection-context's freshener. Used to check for recursion. fresh_trait_ref: ty::PolyTraitRef<'tcx>, previous: TraitObligationStackList<'prev, 'tcx>, } #[derive(Clone)] pub struct SelectionCache<'tcx> { hashmap: RefCell, SelectionResult<'tcx, SelectionCandidate<'tcx>>>>, } pub enum MethodMatchResult { MethodMatched(MethodMatchedData), MethodAmbiguous(/* list of impls that could apply */ Vec), MethodDidNotMatch, } #[derive(Copy, Clone, Debug)] pub enum MethodMatchedData { // In the case of a precise match, we don't really need to store // how the match was found. So don't. PreciseMethodMatch, // In the case of a coercion, we need to know the precise impl so // that we can determine the type to which things were coerced. CoerciveMethodMatch(/* impl we matched */ ast::DefId) } /// The selection process begins by considering all impls, where /// clauses, and so forth that might resolve an obligation. Sometimes /// we'll be able to say definitively that (e.g.) an impl does not /// apply to the obligation: perhaps it is defined for `usize` but the /// obligation is for `int`. In that case, we drop the impl out of the /// list. But the other cases are considered *candidates*. /// /// For selection to succeed, there must be exactly one matching /// candidate. If the obligation is fully known, this is guaranteed /// by coherence. However, if the obligation contains type parameters /// or variables, there may be multiple such impls. /// /// It is not a real problem if multiple matching impls exist because /// of type variables - it just means the obligation isn't sufficiently /// elaborated. In that case we report an ambiguity, and the caller can /// try again after more type information has been gathered or report a /// "type annotations required" error. /// /// However, with type parameters, this can be a real problem - type /// parameters don't unify with regular types, but they *can* unify /// with variables from blanket impls, and (unless we know its bounds /// will always be satisfied) picking the blanket impl will be wrong /// for at least *some* substitutions. To make this concrete, if we have /// /// trait AsDebug { type Out : fmt::Debug; fn debug(self) -> Self::Out; } /// impl AsDebug for T { /// type Out = T; /// fn debug(self) -> fmt::Debug { self } /// } /// fn foo(t: T) { println!("{:?}", ::debug(t)); } /// /// we can't just use the impl to resolve the obligation /// - a type from another crate (that doesn't implement fmt::Debug) could /// implement AsDebug. /// /// Because where-clauses match the type exactly, multiple clauses can /// only match if there are unresolved variables, and we can mostly just /// report this ambiguity in that case. This is still a problem - we can't /// *do anything* with ambiguities that involve only regions. This is issue /// #21974. /// /// If a single where-clause matches and there are no inference /// variables left, then it definitely matches and we can just select /// it. /// /// In fact, we even select the where-clause when the obligation contains /// inference variables. The can lead to inference making "leaps of logic", /// for example in this situation: /// /// pub trait Foo { fn foo(&self) -> T; } /// impl Foo<()> for T { fn foo(&self) { } } /// impl Foo for bool { fn foo(&self) -> bool { *self } } /// /// pub fn foo(t: T) where T: Foo { /// println!("{:?}", >::foo(&t)); /// } /// fn main() { foo(false); } /// /// Here the obligation > can be matched by both the blanket /// impl and the where-clause. We select the where-clause and unify $0=bool, /// so the program prints "false". However, if the where-clause is omitted, /// the blanket impl is selected, we unify $0=(), and the program prints /// "()". /// /// Exactly the same issues apply to projection and object candidates, except /// that we can have both a projection candidate and a where-clause candidate /// for the same obligation. In that case either would do (except that /// different "leaps of logic" would occur if inference variables are /// present), and we just pick the projection. This is, for example, /// required for associated types to work in default impls, as the bounds /// are visible both as projection bounds and as where-clauses from the /// parameter environment. #[derive(PartialEq,Eq,Debug,Clone)] enum SelectionCandidate<'tcx> { PhantomFnCandidate, BuiltinCandidate(ty::BuiltinBound), ParamCandidate(ty::PolyTraitRef<'tcx>), ImplCandidate(ast::DefId), DefaultImplCandidate(ast::DefId), DefaultImplObjectCandidate(ast::DefId), /// This is a trait matching with a projected type as `Self`, and /// we found an applicable bound in the trait definition. ProjectionCandidate, /// Implementation of a `Fn`-family trait by one of the /// anonymous types generated for a `||` expression. ClosureCandidate(/* closure */ ast::DefId, Substs<'tcx>), /// Implementation of a `Fn`-family trait by one of the anonymous /// types generated for a fn pointer type (e.g., `fn(int)->int`) FnPointerCandidate, ObjectCandidate, BuiltinObjectCandidate, BuiltinUnsizeCandidate, ErrorCandidate, } struct SelectionCandidateSet<'tcx> { // a list of candidates that definitely apply to the current // obligation (meaning: types unify). vec: Vec>, // if this is true, then there were candidates that might or might // not have applied, but we couldn't tell. This occurs when some // of the input types are type variables, in which case there are // various "builtin" rules that might or might not trigger. ambiguous: bool, } enum BuiltinBoundConditions<'tcx> { If(ty::Binder>>), ParameterBuiltin, AmbiguousBuiltin } #[derive(Debug)] enum EvaluationResult<'tcx> { EvaluatedToOk, EvaluatedToAmbig, EvaluatedToErr(SelectionError<'tcx>), } impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> { pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> { SelectionContext { infcx: infcx, freshener: infcx.freshener(), intercrate: false, } } pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> { SelectionContext { infcx: infcx, freshener: infcx.freshener(), intercrate: true, } } pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> { self.infcx } pub fn tcx(&self) -> &'cx ty::ctxt<'tcx> { self.infcx.tcx } pub fn param_env(&self) -> &'cx ty::ParameterEnvironment<'cx, 'tcx> { self.infcx.param_env() } pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'tcx> { self.infcx } /////////////////////////////////////////////////////////////////////////// // Selection // // The selection phase tries to identify *how* an obligation will // be resolved. For example, it will identify which impl or // parameter bound is to be used. The process can be inconclusive // if the self type in the obligation is not fully inferred. Selection // can result in an error in one of two ways: // // 1. If no applicable impl or parameter bound can be found. // 2. If the output type parameters in the obligation do not match // those specified by the impl/bound. For example, if the obligation // is `Vec:Iterable`, but the impl specifies // `impl Iterable for Vec`, than an error would result. /// Attempts to satisfy the obligation. If successful, this will affect the surrounding /// type environment by performing unification. pub fn select(&mut self, obligation: &TraitObligation<'tcx>) -> SelectionResult<'tcx, Selection<'tcx>> { debug!("select({:?})", obligation); assert!(!obligation.predicate.has_escaping_regions()); let stack = self.push_stack(TraitObligationStackList::empty(), obligation); match try!(self.candidate_from_obligation(&stack)) { None => { self.consider_unification_despite_ambiguity(obligation); Ok(None) } Some(candidate) => Ok(Some(try!(self.confirm_candidate(obligation, candidate)))), } } /// In the particular case of unboxed closure obligations, we can /// sometimes do some amount of unification for the /// argument/return types even though we can't yet fully match obligation. /// The particular case we are interesting in is an obligation of the form: /// /// C : FnFoo /// /// where `C` is an unboxed closure type and `FnFoo` is one of the /// `Fn` traits. Because we know that users cannot write impls for closure types /// themselves, the only way that `C : FnFoo` can fail to match is under two /// conditions: /// /// 1. The closure kind for `C` is not yet known, because inference isn't complete. /// 2. The closure kind for `C` *is* known, but doesn't match what is needed. /// For example, `C` may be a `FnOnce` closure, but a `Fn` closure is needed. /// /// In either case, we always know what argument types are /// expected by `C`, no matter what kind of `Fn` trait it /// eventually matches. So we can go ahead and unify the argument /// types, even though the end result is ambiguous. /// /// Note that this is safe *even if* the trait would never be /// matched (case 2 above). After all, in that case, an error will /// result, so it kind of doesn't matter what we do --- unifying /// the argument types can only be helpful to the user, because /// once they patch up the kind of closure that is expected, the /// argment types won't really change. fn consider_unification_despite_ambiguity(&mut self, obligation: &TraitObligation<'tcx>) { // Is this a `C : FnFoo(...)` trait reference for some trait binding `FnFoo`? match self.tcx().lang_items.fn_trait_kind(obligation.predicate.0.def_id()) { Some(_) => { } None => { return; } } // Is the self-type a closure type? We ignore bindings here // because if it is a closure type, it must be a closure type from // within this current fn, and hence none of the higher-ranked // lifetimes can appear inside the self-type. let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder()); let (closure_def_id, substs) = match self_ty.sty { ty::TyClosure(id, ref substs) => (id, substs.clone()), _ => { return; } }; assert!(!substs.has_escaping_regions()); // It is OK to call the unnormalized variant here - this is only // reached for TyClosure: Fn inputs where the closure kind is // still unknown, which should only occur in typeck where the // closure type is already normalized. let closure_trait_ref = self.closure_trait_ref_unnormalized(obligation, closure_def_id, substs); match self.confirm_poly_trait_refs(obligation.cause.clone(), obligation.predicate.to_poly_trait_ref(), closure_trait_ref) { Ok(()) => { } Err(_) => { /* Silently ignore errors. */ } } } /////////////////////////////////////////////////////////////////////////// // EVALUATION // // Tests whether an obligation can be selected or whether an impl // can be applied to particular types. It skips the "confirmation" // step and hence completely ignores output type parameters. // // The result is "true" if the obligation *may* hold and "false" if // we can be sure it does not. /// Evaluates whether the obligation `obligation` can be satisfied (by any means). pub fn evaluate_obligation(&mut self, obligation: &PredicateObligation<'tcx>) -> bool { debug!("evaluate_obligation({:?})", obligation); self.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation) .may_apply() } fn evaluate_builtin_bound_recursively<'o>(&mut self, bound: ty::BuiltinBound, previous_stack: &TraitObligationStack<'o, 'tcx>, ty: Ty<'tcx>) -> EvaluationResult<'tcx> { let obligation = util::predicate_for_builtin_bound( self.tcx(), previous_stack.obligation.cause.clone(), bound, previous_stack.obligation.recursion_depth + 1, ty); match obligation { Ok(obligation) => { self.evaluate_predicate_recursively(previous_stack.list(), &obligation) } Err(ErrorReported) => { EvaluatedToOk } } } fn evaluate_predicates_recursively<'a,'o,I>(&mut self, stack: TraitObligationStackList<'o, 'tcx>, predicates: I) -> EvaluationResult<'tcx> where I : Iterator>, 'tcx:'a { let mut result = EvaluatedToOk; for obligation in predicates { match self.evaluate_predicate_recursively(stack, obligation) { EvaluatedToErr(e) => { return EvaluatedToErr(e); } EvaluatedToAmbig => { result = EvaluatedToAmbig; } EvaluatedToOk => { } } } result } fn evaluate_predicate_recursively<'o>(&mut self, previous_stack: TraitObligationStackList<'o, 'tcx>, obligation: &PredicateObligation<'tcx>) -> EvaluationResult<'tcx> { debug!("evaluate_predicate_recursively({:?})", obligation); // Check the cache from the tcx of predicates that we know // have been proven elsewhere. This cache only contains // predicates that are global in scope and hence unaffected by // the current environment. if self.tcx().fulfilled_predicates.borrow().is_duplicate(&obligation.predicate) { return EvaluatedToOk; } match obligation.predicate { ty::Predicate::Trait(ref t) => { assert!(!t.has_escaping_regions()); let obligation = obligation.with(t.clone()); self.evaluate_obligation_recursively(previous_stack, &obligation) } ty::Predicate::Equate(ref p) => { let result = self.infcx.probe(|_| { self.infcx.equality_predicate(obligation.cause.span, p) }); match result { Ok(()) => EvaluatedToOk, Err(_) => EvaluatedToErr(Unimplemented), } } ty::Predicate::TypeOutlives(..) | ty::Predicate::RegionOutlives(..) => { // we do not consider region relationships when // evaluating trait matches EvaluatedToOk } ty::Predicate::Projection(ref data) => { self.infcx.probe(|_| { let project_obligation = obligation.with(data.clone()); match project::poly_project_and_unify_type(self, &project_obligation) { Ok(Some(subobligations)) => { self.evaluate_predicates_recursively(previous_stack, subobligations.iter()) } Ok(None) => { EvaluatedToAmbig } Err(_) => { EvaluatedToErr(Unimplemented) } } }) } } } fn evaluate_obligation_recursively<'o>(&mut self, previous_stack: TraitObligationStackList<'o, 'tcx>, obligation: &TraitObligation<'tcx>) -> EvaluationResult<'tcx> { debug!("evaluate_obligation_recursively({:?})", obligation); let stack = self.push_stack(previous_stack, obligation); let result = self.evaluate_stack(&stack); debug!("result: {:?}", result); result } fn evaluate_stack<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> EvaluationResult<'tcx> { // In intercrate mode, whenever any of the types are unbound, // there can always be an impl. Even if there are no impls in // this crate, perhaps the type would be unified with // something from another crate that does provide an impl. // // In intracrate mode, we must still be conservative. The reason is // that we want to avoid cycles. Imagine an impl like: // // impl Eq for Vec // // and a trait reference like `$0 : Eq` where `$0` is an // unbound variable. When we evaluate this trait-reference, we // will unify `$0` with `Vec<$1>` (for some fresh variable // `$1`), on the condition that `$1 : Eq`. We will then wind // up with many candidates (since that are other `Eq` impls // that apply) and try to winnow things down. This results in // a recursive evaluation that `$1 : Eq` -- as you can // imagine, this is just where we started. To avoid that, we // check for unbound variables and return an ambiguous (hence possible) // match if we've seen this trait before. // // This suffices to allow chains like `FnMut` implemented in // terms of `Fn` etc, but we could probably make this more // precise still. let input_types = stack.fresh_trait_ref.0.input_types(); let unbound_input_types = input_types.iter().any(|ty| ty.is_fresh()); if unbound_input_types && (self.intercrate || stack.iter().skip(1).any( |prev| self.match_fresh_trait_refs(&stack.fresh_trait_ref, &prev.fresh_trait_ref))) { debug!("evaluate_stack({:?}) --> unbound argument, recursion --> ambiguous", stack.fresh_trait_ref); return EvaluatedToAmbig; } // If there is any previous entry on the stack that precisely // matches this obligation, then we can assume that the // obligation is satisfied for now (still all other conditions // must be met of course). One obvious case this comes up is // marker traits like `Send`. Think of a linked list: // // struct List { data: T, next: Option>> { // // `Box>` will be `Send` if `T` is `Send` and // `Option>>` is `Send`, and in turn // `Option>>` is `Send` if `Box>` is // `Send`. // // Note that we do this comparison using the `fresh_trait_ref` // fields. Because these have all been skolemized using // `self.freshener`, we can be sure that (a) this will not // affect the inferencer state and (b) that if we see two // skolemized types with the same index, they refer to the // same unbound type variable. if stack.iter() .skip(1) // skip top-most frame .any(|prev| stack.fresh_trait_ref == prev.fresh_trait_ref) { debug!("evaluate_stack({:?}) --> recursive", stack.fresh_trait_ref); return EvaluatedToOk; } match self.candidate_from_obligation(stack) { Ok(Some(c)) => self.winnow_candidate(stack, &c), Ok(None) => EvaluatedToAmbig, Err(e) => EvaluatedToErr(e), } } /// Evaluates whether the impl with id `impl_def_id` could be applied to the self type /// `obligation_self_ty`. This can be used either for trait or inherent impls. pub fn evaluate_impl(&mut self, impl_def_id: ast::DefId, obligation: &TraitObligation<'tcx>) -> bool { debug!("evaluate_impl(impl_def_id={:?}, obligation={:?})", impl_def_id, obligation); self.infcx.probe(|snapshot| { match self.match_impl(impl_def_id, obligation, snapshot) { Ok((substs, skol_map)) => { let vtable_impl = self.vtable_impl(impl_def_id, substs, obligation.cause.clone(), obligation.recursion_depth + 1, skol_map, snapshot); self.winnow_selection(TraitObligationStackList::empty(), VtableImpl(vtable_impl)).may_apply() } Err(()) => { false } } }) } /////////////////////////////////////////////////////////////////////////// // CANDIDATE ASSEMBLY // // The selection process begins by examining all in-scope impls, // caller obligations, and so forth and assembling a list of // candidates. See `README.md` and the `Candidate` type for more // details. fn candidate_from_obligation<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { // Watch out for overflow. This intentionally bypasses (and does // not update) the cache. let recursion_limit = self.infcx.tcx.sess.recursion_limit.get(); if stack.obligation.recursion_depth >= recursion_limit { report_overflow_error(self.infcx(), &stack.obligation); } // Check the cache. Note that we skolemize the trait-ref // separately rather than using `stack.fresh_trait_ref` -- this // is because we want the unbound variables to be replaced // with fresh skolemized types starting from index 0. let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate.clone()); debug!("candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})", cache_fresh_trait_pred, stack); assert!(!stack.obligation.predicate.has_escaping_regions()); match self.check_candidate_cache(&cache_fresh_trait_pred) { Some(c) => { debug!("CACHE HIT: cache_fresh_trait_pred={:?}, candidate={:?}", cache_fresh_trait_pred, c); return c; } None => { } } // If no match, compute result and insert into cache. let candidate = self.candidate_from_obligation_no_cache(stack); if self.should_update_candidate_cache(&cache_fresh_trait_pred, &candidate) { debug!("CACHE MISS: cache_fresh_trait_pred={:?}, candidate={:?}", cache_fresh_trait_pred, candidate); self.insert_candidate_cache(cache_fresh_trait_pred, candidate.clone()); } candidate } fn candidate_from_obligation_no_cache<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> { if stack.obligation.predicate.0.self_ty().references_error() { return Ok(Some(ErrorCandidate)); } if !self.is_knowable(stack) { debug!("intercrate not knowable"); return Ok(None); } let candidate_set = try!(self.assemble_candidates(stack)); if candidate_set.ambiguous { debug!("candidate set contains ambig"); return Ok(None); } let mut candidates = candidate_set.vec; debug!("assembled {} candidates for {:?}: {:?}", candidates.len(), stack, candidates); // At this point, we know that each of the entries in the // candidate set is *individually* applicable. Now we have to // figure out if they contain mutual incompatibilities. This // frequently arises if we have an unconstrained input type -- // for example, we are looking for $0:Eq where $0 is some // unconstrained type variable. In that case, we'll get a // candidate which assumes $0 == int, one that assumes $0 == // usize, etc. This spells an ambiguity. // If there is more than one candidate, first winnow them down // by considering extra conditions (nested obligations and so // forth). We don't winnow if there is exactly one // candidate. This is a relatively minor distinction but it // can lead to better inference and error-reporting. An // example would be if there was an impl: // // impl Vec { fn push_clone(...) { ... } } // // and we were to see some code `foo.push_clone()` where `boo` // is a `Vec` and `Bar` does not implement `Clone`. If // we were to winnow, we'd wind up with zero candidates. // Instead, we select the right impl now but report `Bar does // not implement Clone`. if candidates.len() > 1 { candidates.retain(|c| self.winnow_candidate(stack, c).may_apply()) } // If there are STILL multiple candidate, we can further reduce // the list by dropping duplicates. if candidates.len() > 1 { let mut i = 0; while i < candidates.len() { let is_dup = (0..candidates.len()) .filter(|&j| i != j) .any(|j| self.candidate_should_be_dropped_in_favor_of(&candidates[i], &candidates[j])); if is_dup { debug!("Dropping candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]); candidates.swap_remove(i); } else { debug!("Retaining candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]); i += 1; } } } // If there are *STILL* multiple candidates, give up and // report ambiguity. if candidates.len() > 1 { debug!("multiple matches, ambig"); return Ok(None); } // If there are *NO* candidates, that there are no impls -- // that we know of, anyway. Note that in the case where there // are unbound type variables within the obligation, it might // be the case that you could still satisfy the obligation // from another crate by instantiating the type variables with // a type from another crate that does have an impl. This case // is checked for in `evaluate_stack` (and hence users // who might care about this case, like coherence, should use // that function). if candidates.is_empty() { return Err(Unimplemented); } // Just one candidate left. let candidate = candidates.pop().unwrap(); match candidate { ImplCandidate(def_id) => { match self.tcx().trait_impl_polarity(def_id) { Some(ast::ImplPolarity::Negative) => return Err(Unimplemented), _ => {} } } _ => {} } Ok(Some(candidate)) } fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> bool { debug!("is_knowable(intercrate={})", self.intercrate); if !self.intercrate { return true; } let obligation = &stack.obligation; let predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate); // ok to skip binder because of the nature of the // trait-ref-is-knowable check, which does not care about // bound regions let trait_ref = &predicate.skip_binder().trait_ref; coherence::trait_ref_is_knowable(self.tcx(), trait_ref) } fn pick_candidate_cache(&self) -> &SelectionCache<'tcx> { // If there are any where-clauses in scope, then we always use // a cache local to this particular scope. Otherwise, we // switch to a global cache. We used to try and draw // finer-grained distinctions, but that led to a serious of // annoying and weird bugs like #22019 and #18290. This simple // rule seems to be pretty clearly safe and also still retains // a very high hit rate (~95% when compiling rustc). if !self.param_env().caller_bounds.is_empty() { return &self.param_env().selection_cache; } // Avoid using the master cache during coherence and just rely // on the local cache. This effectively disables caching // during coherence. It is really just a simplification to // avoid us having to fear that coherence results "pollute" // the master cache. Since coherence executes pretty quickly, // it's not worth going to more trouble to increase the // hit-rate I don't think. if self.intercrate { return &self.param_env().selection_cache; } // Otherwise, we can use the global cache. &self.tcx().selection_cache } fn check_candidate_cache(&mut self, cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>) -> Option>> { let cache = self.pick_candidate_cache(); let hashmap = cache.hashmap.borrow(); hashmap.get(&cache_fresh_trait_pred.0.trait_ref).cloned() } fn insert_candidate_cache(&mut self, cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>, candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>) { let cache = self.pick_candidate_cache(); let mut hashmap = cache.hashmap.borrow_mut(); hashmap.insert(cache_fresh_trait_pred.0.trait_ref.clone(), candidate); } fn should_update_candidate_cache(&mut self, cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>, candidate: &SelectionResult<'tcx, SelectionCandidate<'tcx>>) -> bool { // In general, it's a good idea to cache results, even // ambiguous ones, to save us some trouble later. But we have // to be careful not to cache results that could be // invalidated later by advances in inference. Normally, this // is not an issue, because any inference variables whose // types are not yet bound are "freshened" in the cache key, // which means that if we later get the same request once that // type variable IS bound, we'll have a different cache key. // For example, if we have `Vec<_#0t> : Foo`, and `_#0t` is // not yet known, we may cache the result as `None`. But if // later `_#0t` is bound to `Bar`, then when we freshen we'll // have `Vec : Foo` as the cache key. // // HOWEVER, it CAN happen that we get an ambiguity result in // one particular case around closures where the cache key // would not change. That is when the precise types of the // upvars that a closure references have not yet been figured // out (i.e., because it is not yet known if they are captured // by ref, and if by ref, what kind of ref). In these cases, // when matching a builtin bound, we will yield back an // ambiguous result. But the *cache key* is just the closure type, // it doesn't capture the state of the upvar computation. // // To avoid this trap, just don't cache ambiguous results if // the self-type contains no inference byproducts (that really // shouldn't happen in other circumstances anyway, given // coherence). match *candidate { Ok(Some(_)) | Err(_) => true, Ok(None) => { cache_fresh_trait_pred.0.input_types().has_infer_types() } } } fn assemble_candidates<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result, SelectionError<'tcx>> { let TraitObligationStack { obligation, .. } = *stack; let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false }; // Other bounds. Consider both in-scope bounds from fn decl // and applicable impls. There is a certain set of precedence rules here. match self.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) { Some(ty::BoundCopy) => { debug!("obligation self ty is {:?}", obligation.predicate.0.self_ty()); // User-defined copy impls are permitted, but only for // structs and enums. try!(self.assemble_candidates_from_impls(obligation, &mut candidates)); // For other types, we'll use the builtin rules. try!(self.assemble_builtin_bound_candidates(ty::BoundCopy, stack, &mut candidates)); } Some(bound @ ty::BoundSized) => { // Sized is never implementable by end-users, it is // always automatically computed. try!(self.assemble_builtin_bound_candidates(bound, stack, &mut candidates)); } None if self.tcx().lang_items.unsize_trait() == Some(obligation.predicate.def_id()) => { self.assemble_candidates_for_unsizing(obligation, &mut candidates); } Some(ty::BoundSend) | Some(ty::BoundSync) | None => { try!(self.assemble_closure_candidates(obligation, &mut candidates)); try!(self.assemble_fn_pointer_candidates(obligation, &mut candidates)); try!(self.assemble_candidates_from_impls(obligation, &mut candidates)); self.assemble_candidates_from_object_ty(obligation, &mut candidates); } } self.assemble_candidates_from_projected_tys(obligation, &mut candidates); try!(self.assemble_candidates_from_caller_bounds(stack, &mut candidates)); // Default implementations have lower priority, so we only // consider triggering a default if there is no other impl that can apply. if candidates.vec.is_empty() { try!(self.assemble_candidates_from_default_impls(obligation, &mut candidates)); } debug!("candidate list size: {}", candidates.vec.len()); Ok(candidates) } fn assemble_candidates_from_projected_tys(&mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) { let poly_trait_predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate); debug!("assemble_candidates_for_projected_tys({:?},{:?})", obligation, poly_trait_predicate); // FIXME(#20297) -- just examining the self-type is very simplistic // before we go into the whole skolemization thing, just // quickly check if the self-type is a projection at all. let trait_def_id = match poly_trait_predicate.0.trait_ref.self_ty().sty { ty::TyProjection(ref data) => data.trait_ref.def_id, ty::TyInfer(ty::TyVar(_)) => { // If the self-type is an inference variable, then it MAY wind up // being a projected type, so induce an ambiguity. // // FIXME(#20297) -- being strict about this can cause // inference failures with BorrowFrom, which is // unfortunate. Can we do better here? debug!("assemble_candidates_for_projected_tys: ambiguous self-type"); candidates.ambiguous = true; return; } _ => { return; } }; debug!("assemble_candidates_for_projected_tys: trait_def_id={:?}", trait_def_id); let result = self.infcx.probe(|snapshot| { self.match_projection_obligation_against_bounds_from_trait(obligation, snapshot) }); if result { candidates.vec.push(ProjectionCandidate); } } fn match_projection_obligation_against_bounds_from_trait( &mut self, obligation: &TraitObligation<'tcx>, snapshot: &infer::CombinedSnapshot) -> bool { let poly_trait_predicate = self.infcx().resolve_type_vars_if_possible(&obligation.predicate); let (skol_trait_predicate, skol_map) = self.infcx().skolemize_late_bound_regions(&poly_trait_predicate, snapshot); debug!("match_projection_obligation_against_bounds_from_trait: \ skol_trait_predicate={:?} skol_map={:?}", skol_trait_predicate, skol_map); let projection_trait_ref = match skol_trait_predicate.trait_ref.self_ty().sty { ty::TyProjection(ref data) => &data.trait_ref, _ => { self.tcx().sess.span_bug( obligation.cause.span, &format!("match_projection_obligation_against_bounds_from_trait() called \ but self-ty not a projection: {:?}", skol_trait_predicate.trait_ref.self_ty())); } }; debug!("match_projection_obligation_against_bounds_from_trait: \ projection_trait_ref={:?}", projection_trait_ref); let trait_predicates = self.tcx().lookup_predicates(projection_trait_ref.def_id); let bounds = trait_predicates.instantiate(self.tcx(), projection_trait_ref.substs); debug!("match_projection_obligation_against_bounds_from_trait: \ bounds={:?}", bounds); let matching_bound = util::elaborate_predicates(self.tcx(), bounds.predicates.into_vec()) .filter_to_traits() .find( |bound| self.infcx.probe( |_| self.match_projection(obligation, bound.clone(), skol_trait_predicate.trait_ref.clone(), &skol_map, snapshot))); debug!("match_projection_obligation_against_bounds_from_trait: \ matching_bound={:?}", matching_bound); match matching_bound { None => false, Some(bound) => { // Repeat the successful match, if any, this time outside of a probe. let result = self.match_projection(obligation, bound, skol_trait_predicate.trait_ref.clone(), &skol_map, snapshot); assert!(result); true } } } fn match_projection(&mut self, obligation: &TraitObligation<'tcx>, trait_bound: ty::PolyTraitRef<'tcx>, skol_trait_ref: ty::TraitRef<'tcx>, skol_map: &infer::SkolemizationMap, snapshot: &infer::CombinedSnapshot) -> bool { assert!(!skol_trait_ref.has_escaping_regions()); let origin = infer::RelateOutputImplTypes(obligation.cause.span); match self.infcx.sub_poly_trait_refs(false, origin, trait_bound.clone(), ty::Binder(skol_trait_ref.clone())) { Ok(()) => { } Err(_) => { return false; } } self.infcx.leak_check(skol_map, snapshot).is_ok() } /// Given an obligation like ``, search the obligations that the caller /// supplied to find out whether it is listed among them. /// /// Never affects inference environment. fn assemble_candidates_from_caller_bounds<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) -> Result<(),SelectionError<'tcx>> { debug!("assemble_candidates_from_caller_bounds({:?})", stack.obligation); let all_bounds = self.param_env().caller_bounds .iter() .filter_map(|o| o.to_opt_poly_trait_ref()); let matching_bounds = all_bounds.filter( |bound| self.evaluate_where_clause(stack, bound.clone()).may_apply()); let param_candidates = matching_bounds.map(|bound| ParamCandidate(bound)); candidates.vec.extend(param_candidates); Ok(()) } fn evaluate_where_clause<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>, where_clause_trait_ref: ty::PolyTraitRef<'tcx>) -> EvaluationResult<'tcx> { self.infcx().probe(move |_| { match self.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) { Ok(obligations) => { self.evaluate_predicates_recursively(stack.list(), obligations.iter()) } Err(()) => { EvaluatedToErr(Unimplemented) } } }) } /// Check for the artificial impl that the compiler will create for an obligation like `X : /// FnMut<..>` where `X` is a closure type. /// /// Note: the type parameters on a closure candidate are modeled as *output* type /// parameters and hence do not affect whether this trait is a match or not. They will be /// unified during the confirmation step. fn assemble_closure_candidates(&mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) -> Result<(),SelectionError<'tcx>> { let kind = match self.tcx().lang_items.fn_trait_kind(obligation.predicate.0.def_id()) { Some(k) => k, None => { return Ok(()); } }; // ok to skip binder because the substs on closure types never // touch bound regions, they just capture the in-scope // type/region parameters let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder()); let (closure_def_id, substs) = match self_ty.sty { ty::TyClosure(id, substs) => (id, substs), ty::TyInfer(ty::TyVar(_)) => { debug!("assemble_unboxed_closure_candidates: ambiguous self-type"); candidates.ambiguous = true; return Ok(()); } _ => { return Ok(()); } }; debug!("assemble_unboxed_candidates: self_ty={:?} kind={:?} obligation={:?}", self_ty, kind, obligation); match self.infcx.closure_kind(closure_def_id) { Some(closure_kind) => { debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind); if closure_kind.extends(kind) { candidates.vec.push(ClosureCandidate(closure_def_id, substs.clone())); } } None => { debug!("assemble_unboxed_candidates: closure_kind not yet known"); candidates.ambiguous = true; } } Ok(()) } /// Implement one of the `Fn()` family for a fn pointer. fn assemble_fn_pointer_candidates(&mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) -> Result<(),SelectionError<'tcx>> { // We provide impl of all fn traits for fn pointers. if self.tcx().lang_items.fn_trait_kind(obligation.predicate.def_id()).is_none() { return Ok(()); } // ok to skip binder because what we are inspecting doesn't involve bound regions let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder()); match self_ty.sty { ty::TyInfer(ty::TyVar(_)) => { debug!("assemble_fn_pointer_candidates: ambiguous self-type"); candidates.ambiguous = true; // could wind up being a fn() type } // provide an impl, but only for suitable `fn` pointers ty::TyBareFn(_, &ty::BareFnTy { unsafety: ast::Unsafety::Normal, abi: abi::Rust, sig: ty::Binder(ty::FnSig { inputs: _, output: ty::FnConverging(_), variadic: false }) }) => { candidates.vec.push(FnPointerCandidate); } _ => { } } Ok(()) } /// Search for impls that might apply to `obligation`. fn assemble_candidates_from_impls(&mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) -> Result<(), SelectionError<'tcx>> { debug!("assemble_candidates_from_impls(obligation={:?})", obligation); let def = self.tcx().lookup_trait_def(obligation.predicate.def_id()); def.for_each_relevant_impl( self.tcx(), obligation.predicate.0.trait_ref.self_ty(), |impl_def_id| { self.infcx.probe(|snapshot| { if let Ok(_) = self.match_impl(impl_def_id, obligation, snapshot) { candidates.vec.push(ImplCandidate(impl_def_id)); } }); } ); Ok(()) } fn assemble_candidates_from_default_impls(&mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) -> Result<(), SelectionError<'tcx>> { // OK to skip binder here because the tests we do below do not involve bound regions let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder()); debug!("assemble_candidates_from_default_impls(self_ty={:?})", self_ty); let def_id = obligation.predicate.def_id(); if self.tcx().trait_has_default_impl(def_id) { match self_ty.sty { ty::TyTrait(..) => { // For object types, we don't know what the closed // over types are. For most traits, this means we // conservatively say nothing; a candidate may be // added by `assemble_candidates_from_object_ty`. // However, for the kind of magic reflect trait, // we consider it to be implemented even for // object types, because it just lets you reflect // onto the object type, not into the object's // interior. if self.tcx().has_attr(def_id, "rustc_reflect_like") { candidates.vec.push(DefaultImplObjectCandidate(def_id)); } } ty::TyParam(..) | ty::TyProjection(..) => { // In these cases, we don't know what the actual // type is. Therefore, we cannot break it down // into its constituent types. So we don't // consider the `..` impl but instead just add no // candidates: this means that typeck will only // succeed if there is another reason to believe // that this obligation holds. That could be a // where-clause or, in the case of an object type, // it could be that the object type lists the // trait (e.g. `Foo+Send : Send`). See // `compile-fail/typeck-default-trait-impl-send-param.rs` // for an example of a test case that exercises // this path. } ty::TyInfer(ty::TyVar(_)) => { // the defaulted impl might apply, we don't know candidates.ambiguous = true; } _ => { if self.constituent_types_for_ty(self_ty).is_some() { candidates.vec.push(DefaultImplCandidate(def_id.clone())) } else { // We don't yet know what the constituent // types are. So call it ambiguous for now, // though this is a bit stronger than // necessary: that is, we know that the // defaulted impl applies, but we can't // process the confirmation step without // knowing the constituent types. (Anyway, in // the particular case of defaulted impls, it // doesn't really matter much either way, // since we won't be aiding inference by // processing the confirmation step.) candidates.ambiguous = true; } } } } Ok(()) } /// Search for impls that might apply to `obligation`. fn assemble_candidates_from_object_ty(&mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) { debug!("assemble_candidates_from_object_ty(self_ty={:?})", self.infcx.shallow_resolve(*obligation.self_ty().skip_binder())); // Object-safety candidates are only applicable to object-safe // traits. Including this check is useful because it helps // inference in cases of traits like `BorrowFrom`, which are // not object-safe, and which rely on being able to infer the // self-type from one of the other inputs. Without this check, // these cases wind up being considered ambiguous due to a // (spurious) ambiguity introduced here. let predicate_trait_ref = obligation.predicate.to_poly_trait_ref(); if !object_safety::is_object_safe(self.tcx(), predicate_trait_ref.def_id()) { return; } self.infcx.commit_if_ok(|snapshot| { let bound_self_ty = self.infcx.resolve_type_vars_if_possible(&obligation.self_ty()); let (self_ty, _) = self.infcx().skolemize_late_bound_regions(&bound_self_ty, snapshot); let poly_trait_ref = match self_ty.sty { ty::TyTrait(ref data) => { match self.tcx().lang_items.to_builtin_kind(obligation.predicate.def_id()) { Some(bound @ ty::BoundSend) | Some(bound @ ty::BoundSync) => { if data.bounds.builtin_bounds.contains(&bound) { debug!("assemble_candidates_from_object_ty: matched builtin bound, \ pushing candidate"); candidates.vec.push(BuiltinObjectCandidate); return Ok(()); } } _ => {} } data.principal_trait_ref_with_self_ty(self.tcx(), self_ty) } ty::TyInfer(ty::TyVar(_)) => { debug!("assemble_candidates_from_object_ty: ambiguous"); candidates.ambiguous = true; // could wind up being an object type return Ok(()); } _ => { return Ok(()); } }; debug!("assemble_candidates_from_object_ty: poly_trait_ref={:?}", poly_trait_ref); // Count only those upcast versions that match the trait-ref // we are looking for. Specifically, do not only check for the // correct trait, but also the correct type parameters. // For example, we may be trying to upcast `Foo` to `Bar`, // but `Foo` is declared as `trait Foo : Bar`. let upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref) .filter(|upcast_trait_ref| self.infcx.probe(|_| { let upcast_trait_ref = upcast_trait_ref.clone(); self.match_poly_trait_ref(obligation, upcast_trait_ref).is_ok() })).count(); if upcast_trait_refs > 1 { // can be upcast in many ways; need more type information candidates.ambiguous = true; } else if upcast_trait_refs == 1 { candidates.vec.push(ObjectCandidate); } Ok::<(),()>(()) }).unwrap(); } /// Search for unsizing that might apply to `obligation`. fn assemble_candidates_for_unsizing(&mut self, obligation: &TraitObligation<'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) { // We currently never consider higher-ranked obligations e.g. // `for<'a> &'a T: Unsize` to be implemented. This is not // because they are a priori invalid, and we could potentially add support // for them later, it's just that there isn't really a strong need for it. // A `T: Unsize` obligation is always used as part of a `T: CoerceUnsize` // impl, and those are generally applied to concrete types. // // That said, one might try to write a fn with a where clause like // for<'a> Foo<'a, T>: Unsize> // where the `'a` is kind of orthogonal to the relevant part of the `Unsize`. // Still, you'd be more likely to write that where clause as // T: Trait // so it seems ok if we (conservatively) fail to accept that `Unsize` // obligation above. Should be possible to extend this in the future. let self_ty = match self.tcx().no_late_bound_regions(&obligation.self_ty()) { Some(t) => t, None => { // Don't add any candidates if there are bound regions. return; } }; let source = self.infcx.shallow_resolve(self_ty); let target = self.infcx.shallow_resolve(obligation.predicate.0.input_types()[0]); debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})", source, target); let may_apply = match (&source.sty, &target.sty) { // Trait+Kx+'a -> Trait+Ky+'b (upcasts). (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => { // Upcasts permit two things: // // 1. Dropping builtin bounds, e.g. `Foo+Send` to `Foo` // 2. Tightening the region bound, e.g. `Foo+'a` to `Foo+'b` if `'a : 'b` // // Note that neither of these changes requires any // change at runtime. Eventually this will be // generalized. // // We always upcast when we can because of reason // #2 (region bounds). data_a.principal.def_id() == data_a.principal.def_id() && data_a.bounds.builtin_bounds.is_superset(&data_b.bounds.builtin_bounds) } // T -> Trait. (_, &ty::TyTrait(_)) => true, // Ambiguous handling is below T -> Trait, because inference // variables can still implement Unsize and nested // obligations will have the final say (likely deferred). (&ty::TyInfer(ty::TyVar(_)), _) | (_, &ty::TyInfer(ty::TyVar(_))) => { debug!("assemble_candidates_for_unsizing: ambiguous"); candidates.ambiguous = true; false } // [T; n] -> [T]. (&ty::TyArray(_, _), &ty::TySlice(_)) => true, // Struct -> Struct. (&ty::TyStruct(def_id_a, _), &ty::TyStruct(def_id_b, _)) => { def_id_a == def_id_b } _ => false }; if may_apply { candidates.vec.push(BuiltinUnsizeCandidate); } } /////////////////////////////////////////////////////////////////////////// // WINNOW // // Winnowing is the process of attempting to resolve ambiguity by // probing further. During the winnowing process, we unify all // type variables (ignoring skolemization) and then we also // attempt to evaluate recursive bounds to see if they are // satisfied. /// Further evaluate `candidate` to decide whether all type parameters match and whether nested /// obligations are met. Returns true if `candidate` remains viable after this further /// scrutiny. fn winnow_candidate<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>, candidate: &SelectionCandidate<'tcx>) -> EvaluationResult<'tcx> { debug!("winnow_candidate: candidate={:?}", candidate); let result = self.infcx.probe(|_| { let candidate = (*candidate).clone(); match self.confirm_candidate(stack.obligation, candidate) { Ok(selection) => self.winnow_selection(stack.list(), selection), Err(error) => EvaluatedToErr(error), } }); debug!("winnow_candidate depth={} result={:?}", stack.obligation.recursion_depth, result); result } fn winnow_selection<'o>(&mut self, stack: TraitObligationStackList<'o,'tcx>, selection: Selection<'tcx>) -> EvaluationResult<'tcx> { self.evaluate_predicates_recursively(stack, selection.nested_obligations().iter()) } /// Returns true if `candidate_i` should be dropped in favor of /// `candidate_j`. Generally speaking we will drop duplicate /// candidates and prefer where-clause candidates. /// Returns true if `victim` should be dropped in favor of /// `other`. Generally speaking we will drop duplicate /// candidates and prefer where-clause candidates. /// /// See the comment for "SelectionCandidate" for more details. fn candidate_should_be_dropped_in_favor_of<'o>(&mut self, victim: &SelectionCandidate<'tcx>, other: &SelectionCandidate<'tcx>) -> bool { if victim == other { return true; } match other { &ObjectCandidate(..) | &ParamCandidate(_) | &ProjectionCandidate => match victim { &DefaultImplCandidate(..) => { self.tcx().sess.bug( "default implementations shouldn't be recorded \ when there are other valid candidates"); } &PhantomFnCandidate => { self.tcx().sess.bug("PhantomFn didn't short-circuit selection"); } &ImplCandidate(..) | &ClosureCandidate(..) | &FnPointerCandidate(..) | &BuiltinObjectCandidate(..) | &BuiltinUnsizeCandidate(..) | &DefaultImplObjectCandidate(..) | &BuiltinCandidate(..) => { // We have a where-clause so don't go around looking // for impls. true } &ObjectCandidate(..) | &ProjectionCandidate => { // Arbitrarily give param candidates priority // over projection and object candidates. true }, &ParamCandidate(..) => false, &ErrorCandidate => false // propagate errors }, _ => false } } /////////////////////////////////////////////////////////////////////////// // BUILTIN BOUNDS // // These cover the traits that are built-in to the language // itself. This includes `Copy` and `Sized` for sure. For the // moment, it also includes `Send` / `Sync` and a few others, but // those will hopefully change to library-defined traits in the // future. fn assemble_builtin_bound_candidates<'o>(&mut self, bound: ty::BuiltinBound, stack: &TraitObligationStack<'o, 'tcx>, candidates: &mut SelectionCandidateSet<'tcx>) -> Result<(),SelectionError<'tcx>> { match self.builtin_bound(bound, stack.obligation) { Ok(If(..)) => { debug!("builtin_bound: bound={:?}", bound); candidates.vec.push(BuiltinCandidate(bound)); Ok(()) } Ok(ParameterBuiltin) => { Ok(()) } Ok(AmbiguousBuiltin) => { debug!("assemble_builtin_bound_candidates: ambiguous builtin"); Ok(candidates.ambiguous = true) } Err(e) => { Err(e) } } } fn builtin_bound(&mut self, bound: ty::BuiltinBound, obligation: &TraitObligation<'tcx>) -> Result,SelectionError<'tcx>> { // Note: these tests operate on types that may contain bound // regions. To be proper, we ought to skolemize here, but we // forego the skolemization and defer it until the // confirmation step. let self_ty = self.infcx.shallow_resolve(obligation.predicate.0.self_ty()); return match self_ty.sty { ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) | ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) | ty::TyBareFn(..) | ty::TyChar => { // safe for everything ok_if(Vec::new()) } ty::TyBox(_) => { // Box match bound { ty::BoundCopy => Err(Unimplemented), ty::BoundSized => ok_if(Vec::new()), ty::BoundSync | ty::BoundSend => { self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()"); } } } ty::TyRawPtr(..) => { // *const T, *mut T match bound { ty::BoundCopy | ty::BoundSized => ok_if(Vec::new()), ty::BoundSync | ty::BoundSend => { self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()"); } } } ty::TyTrait(ref data) => { match bound { ty::BoundSized => Err(Unimplemented), ty::BoundCopy => { if data.bounds.builtin_bounds.contains(&bound) { ok_if(Vec::new()) } else { // Recursively check all supertraits to find out if any further // bounds are required and thus we must fulfill. let principal = data.principal_trait_ref_with_self_ty(self.tcx(), self.tcx().types.err); let desired_def_id = obligation.predicate.def_id(); for tr in util::supertraits(self.tcx(), principal) { if tr.def_id() == desired_def_id { return ok_if(Vec::new()) } } Err(Unimplemented) } } ty::BoundSync | ty::BoundSend => { self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()"); } } } ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl }) => { // &mut T or &T match bound { ty::BoundCopy => { match mutbl { // &mut T is affine and hence never `Copy` ast::MutMutable => Err(Unimplemented), // &T is always copyable ast::MutImmutable => ok_if(Vec::new()), } } ty::BoundSized => ok_if(Vec::new()), ty::BoundSync | ty::BoundSend => { self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()"); } } } ty::TyArray(element_ty, _) => { // [T; n] match bound { ty::BoundCopy => ok_if(vec![element_ty]), ty::BoundSized => ok_if(Vec::new()), ty::BoundSync | ty::BoundSend => { self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()"); } } } ty::TyStr | ty::TySlice(_) => { match bound { ty::BoundSync | ty::BoundSend => { self.tcx().sess.bug("Send/Sync shouldn't occur in builtin_bounds()"); } ty::BoundCopy | ty::BoundSized => Err(Unimplemented), } } // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet ty::TyTuple(ref tys) => ok_if(tys.clone()), ty::TyClosure(def_id, substs) => { // FIXME -- This case is tricky. In the case of by-ref // closures particularly, we need the results of // inference to decide how to reflect the type of each // upvar (the upvar may have type `T`, but the runtime // type could be `&mut`, `&`, or just `T`). For now, // though, we'll do this unsoundly and assume that all // captures are by value. Really what we ought to do // is reserve judgement and then intertwine this // analysis with closure inference. assert_eq!(def_id.krate, ast::LOCAL_CRATE); // Unboxed closures shouldn't be // implicitly copyable if bound == ty::BoundCopy { return Ok(ParameterBuiltin); } // Upvars are always local variables or references to // local variables, and local variables cannot be // unsized, so the closure struct as a whole must be // Sized. if bound == ty::BoundSized { return ok_if(Vec::new()); } match self.infcx.closure_upvars(def_id, substs) { Some(upvars) => ok_if(upvars.iter().map(|c| c.ty).collect()), None => { debug!("assemble_builtin_bound_candidates: no upvar types available yet"); Ok(AmbiguousBuiltin) } } } ty::TyStruct(def_id, substs) => { let types: Vec = self.tcx().struct_fields(def_id, substs).iter() .map(|f| f.mt.ty) .collect(); nominal(bound, types) } ty::TyEnum(def_id, substs) => { let types: Vec = self.tcx().substd_enum_variants(def_id, substs) .iter() .flat_map(|variant| &variant.args) .cloned() .collect(); nominal(bound, types) } ty::TyProjection(_) | ty::TyParam(_) => { // Note: A type parameter is only considered to meet a // particular bound if there is a where clause telling // us that it does, and that case is handled by // `assemble_candidates_from_caller_bounds()`. Ok(ParameterBuiltin) } ty::TyInfer(ty::TyVar(_)) => { // Unbound type variable. Might or might not have // applicable impls and so forth, depending on what // those type variables wind up being bound to. debug!("assemble_builtin_bound_candidates: ambiguous builtin"); Ok(AmbiguousBuiltin) } ty::TyError => ok_if(Vec::new()), ty::TyInfer(ty::FreshTy(_)) | ty::TyInfer(ty::FreshIntTy(_)) | ty::TyInfer(ty::FreshFloatTy(_)) => { self.tcx().sess.bug( &format!( "asked to assemble builtin bounds of unexpected type: {:?}", self_ty)); } }; fn ok_if<'tcx>(v: Vec>) -> Result, SelectionError<'tcx>> { Ok(If(ty::Binder(v))) } fn nominal<'cx, 'tcx>(bound: ty::BuiltinBound, types: Vec>) -> Result, SelectionError<'tcx>> { // First check for markers and other nonsense. match bound { // Fallback to whatever user-defined impls exist in this case. ty::BoundCopy => Ok(ParameterBuiltin), // Sized if all the component types are sized. ty::BoundSized => ok_if(types), // Shouldn't be coming through here. ty::BoundSend | ty::BoundSync => unreachable!(), } } } /// For default impls, we need to break apart a type into its /// "constituent types" -- meaning, the types that it contains. /// /// Here are some (simple) examples: /// /// ``` /// (i32, u32) -> [i32, u32] /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32] /// Bar where struct Bar { x: T, y: u32 } -> [i32, u32] /// Zed where enum Zed { A(T), B(u32) } -> [i32, u32] /// ``` fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Option>> { match t.sty { ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) | ty::TyBareFn(..) | ty::TyStr | ty::TyError | ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) | ty::TyChar => { Some(Vec::new()) } ty::TyTrait(..) | ty::TyParam(..) | ty::TyProjection(..) | ty::TyInfer(ty::TyVar(_)) | ty::TyInfer(ty::FreshTy(_)) | ty::TyInfer(ty::FreshIntTy(_)) | ty::TyInfer(ty::FreshFloatTy(_)) => { self.tcx().sess.bug( &format!( "asked to assemble constituent types of unexpected type: {:?}", t)); } ty::TyBox(referent_ty) => { // Box Some(vec![referent_ty]) } ty::TyRawPtr(ty::TypeAndMut { ty: element_ty, ..}) | ty::TyRef(_, ty::TypeAndMut { ty: element_ty, ..}) => { Some(vec![element_ty]) }, ty::TyArray(element_ty, _) | ty::TySlice(element_ty) => { Some(vec![element_ty]) } ty::TyTuple(ref tys) => { // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet Some(tys.clone()) } ty::TyClosure(def_id, substs) => { assert_eq!(def_id.krate, ast::LOCAL_CRATE); match self.infcx.closure_upvars(def_id, substs) { Some(upvars) => { Some(upvars.iter().map(|c| c.ty).collect()) } None => { None } } } // for `PhantomData`, we pass `T` ty::TyStruct(def_id, substs) if Some(def_id) == self.tcx().lang_items.phantom_data() => { Some(substs.types.get_slice(TypeSpace).to_vec()) } ty::TyStruct(def_id, substs) => { Some(self.tcx().struct_fields(def_id, substs).iter() .map(|f| f.mt.ty) .collect()) } ty::TyEnum(def_id, substs) => { Some(self.tcx().substd_enum_variants(def_id, substs) .iter() .flat_map(|variant| &variant.args) .map(|&ty| ty) .collect()) } } } fn collect_predicates_for_types(&mut self, obligation: &TraitObligation<'tcx>, trait_def_id: ast::DefId, types: ty::Binder>>) -> Vec> { let derived_cause = match self.tcx().lang_items.to_builtin_kind(trait_def_id) { Some(_) => { self.derived_cause(obligation, BuiltinDerivedObligation) }, None => { self.derived_cause(obligation, ImplDerivedObligation) } }; // Because the types were potentially derived from // higher-ranked obligations they may reference late-bound // regions. For example, `for<'a> Foo<&'a int> : Copy` would // yield a type like `for<'a> &'a int`. In general, we // maintain the invariant that we never manipulate bound // regions, so we have to process these bound regions somehow. // // The strategy is to: // // 1. Instantiate those regions to skolemized regions (e.g., // `for<'a> &'a int` becomes `&0 int`. // 2. Produce something like `&'0 int : Copy` // 3. Re-bind the regions back to `for<'a> &'a int : Copy` // Move the binder into the individual types let bound_types: Vec>> = types.skip_binder() .iter() .map(|&nested_ty| ty::Binder(nested_ty)) .collect(); // For each type, produce a vector of resulting obligations let obligations: Result>, _> = bound_types.iter().map(|nested_ty| { self.infcx.commit_if_ok(|snapshot| { let (skol_ty, skol_map) = self.infcx().skolemize_late_bound_regions(nested_ty, snapshot); let Normalized { value: normalized_ty, mut obligations } = project::normalize_with_depth(self, obligation.cause.clone(), obligation.recursion_depth + 1, &skol_ty); let skol_obligation = util::predicate_for_trait_def(self.tcx(), derived_cause.clone(), trait_def_id, obligation.recursion_depth + 1, normalized_ty, vec![]); obligations.push(skol_obligation); Ok(self.infcx().plug_leaks(skol_map, snapshot, &obligations)) }) }).collect(); // Flatten those vectors (couldn't do it above due `collect`) match obligations { Ok(obligations) => obligations.into_iter().flat_map(|o| o).collect(), Err(ErrorReported) => Vec::new(), } } /////////////////////////////////////////////////////////////////////////// // CONFIRMATION // // Confirmation unifies the output type parameters of the trait // with the values found in the obligation, possibly yielding a // type error. See `README.md` for more details. fn confirm_candidate(&mut self, obligation: &TraitObligation<'tcx>, candidate: SelectionCandidate<'tcx>) -> Result,SelectionError<'tcx>> { debug!("confirm_candidate({:?}, {:?})", obligation, candidate); match candidate { BuiltinCandidate(builtin_bound) => { Ok(VtableBuiltin( try!(self.confirm_builtin_candidate(obligation, builtin_bound)))) } PhantomFnCandidate | ErrorCandidate => { Ok(VtableBuiltin(VtableBuiltinData { nested: vec![] })) } ParamCandidate(param) => { let obligations = self.confirm_param_candidate(obligation, param); Ok(VtableParam(obligations)) } DefaultImplCandidate(trait_def_id) => { let data = self.confirm_default_impl_candidate(obligation, trait_def_id); Ok(VtableDefaultImpl(data)) } DefaultImplObjectCandidate(trait_def_id) => { let data = self.confirm_default_impl_object_candidate(obligation, trait_def_id); Ok(VtableDefaultImpl(data)) } ImplCandidate(impl_def_id) => { let vtable_impl = try!(self.confirm_impl_candidate(obligation, impl_def_id)); Ok(VtableImpl(vtable_impl)) } ClosureCandidate(closure_def_id, substs) => { let vtable_closure = try!(self.confirm_closure_candidate(obligation, closure_def_id, &substs)); Ok(VtableClosure(vtable_closure)) } BuiltinObjectCandidate => { // This indicates something like `(Trait+Send) : // Send`. In this case, we know that this holds // because that's what the object type is telling us, // and there's really no additional obligations to // prove and no types in particular to unify etc. Ok(VtableParam(Vec::new())) } ObjectCandidate => { let data = self.confirm_object_candidate(obligation); Ok(VtableObject(data)) } FnPointerCandidate => { let fn_type = try!(self.confirm_fn_pointer_candidate(obligation)); Ok(VtableFnPointer(fn_type)) } ProjectionCandidate => { self.confirm_projection_candidate(obligation); Ok(VtableParam(Vec::new())) } BuiltinUnsizeCandidate => { let data = try!(self.confirm_builtin_unsize_candidate(obligation)); Ok(VtableBuiltin(data)) } } } fn confirm_projection_candidate(&mut self, obligation: &TraitObligation<'tcx>) { let _: Result<(),()> = self.infcx.commit_if_ok(|snapshot| { let result = self.match_projection_obligation_against_bounds_from_trait(obligation, snapshot); assert!(result); Ok(()) }); } fn confirm_param_candidate(&mut self, obligation: &TraitObligation<'tcx>, param: ty::PolyTraitRef<'tcx>) -> Vec> { debug!("confirm_param_candidate({:?},{:?})", obligation, param); // During evaluation, we already checked that this // where-clause trait-ref could be unified with the obligation // trait-ref. Repeat that unification now without any // transactional boundary; it should not fail. match self.match_where_clause_trait_ref(obligation, param.clone()) { Ok(obligations) => obligations, Err(()) => { self.tcx().sess.bug( &format!("Where clause `{:?}` was applicable to `{:?}` but now is not", param, obligation)); } } } fn confirm_builtin_candidate(&mut self, obligation: &TraitObligation<'tcx>, bound: ty::BuiltinBound) -> Result>, SelectionError<'tcx>> { debug!("confirm_builtin_candidate({:?})", obligation); match try!(self.builtin_bound(bound, obligation)) { If(nested) => Ok(self.vtable_builtin_data(obligation, bound, nested)), AmbiguousBuiltin | ParameterBuiltin => { self.tcx().sess.span_bug( obligation.cause.span, &format!("builtin bound for {:?} was ambig", obligation)); } } } fn vtable_builtin_data(&mut self, obligation: &TraitObligation<'tcx>, bound: ty::BuiltinBound, nested: ty::Binder>>) -> VtableBuiltinData> { let trait_def = match self.tcx().lang_items.from_builtin_kind(bound) { Ok(def_id) => def_id, Err(_) => { self.tcx().sess.bug("builtin trait definition not found"); } }; let obligations = self.collect_predicates_for_types(obligation, trait_def, nested); debug!("vtable_builtin_data: obligations={:?}", obligations); VtableBuiltinData { nested: obligations } } /// This handles the case where a `impl Foo for ..` impl is being used. /// The idea is that the impl applies to `X : Foo` if the following conditions are met: /// /// 1. For each constituent type `Y` in `X`, `Y : Foo` holds /// 2. For each where-clause `C` declared on `Foo`, `[Self => X] C` holds. fn confirm_default_impl_candidate(&mut self, obligation: &TraitObligation<'tcx>, trait_def_id: ast::DefId) -> VtableDefaultImplData> { debug!("confirm_default_impl_candidate({:?}, {:?})", obligation, trait_def_id); // binder is moved below let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty()); match self.constituent_types_for_ty(self_ty) { Some(types) => self.vtable_default_impl(obligation, trait_def_id, ty::Binder(types)), None => { self.tcx().sess.bug( &format!( "asked to confirm default implementation for ambiguous type: {:?}", self_ty)); } } } fn confirm_default_impl_object_candidate(&mut self, obligation: &TraitObligation<'tcx>, trait_def_id: ast::DefId) -> VtableDefaultImplData> { debug!("confirm_default_impl_object_candidate({:?}, {:?})", obligation, trait_def_id); assert!(self.tcx().has_attr(trait_def_id, "rustc_reflect_like")); // OK to skip binder, it is reintroduced below let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty()); match self_ty.sty { ty::TyTrait(ref data) => { // OK to skip the binder, it is reintroduced below let input_types = data.principal.skip_binder().substs.types.get_slice(TypeSpace); let assoc_types = data.bounds.projection_bounds .iter() .map(|pb| pb.skip_binder().ty); let all_types: Vec<_> = input_types.iter().cloned() .chain(assoc_types) .collect(); // reintroduce the two binding levels we skipped, then flatten into one let all_types = ty::Binder(ty::Binder(all_types)); let all_types = self.tcx().flatten_late_bound_regions(&all_types); self.vtable_default_impl(obligation, trait_def_id, all_types) } _ => { self.tcx().sess.bug( &format!( "asked to confirm default object implementation for non-object type: {:?}", self_ty)); } } } /// See `confirm_default_impl_candidate` fn vtable_default_impl(&mut self, obligation: &TraitObligation<'tcx>, trait_def_id: ast::DefId, nested: ty::Binder>>) -> VtableDefaultImplData> { debug!("vtable_default_impl_data: nested={:?}", nested); let mut obligations = self.collect_predicates_for_types(obligation, trait_def_id, nested); let trait_obligations: Result,()> = self.infcx.commit_if_ok(|snapshot| { let poly_trait_ref = obligation.predicate.to_poly_trait_ref(); let (trait_ref, skol_map) = self.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot); Ok(self.impl_or_trait_obligations(obligation.cause.clone(), obligation.recursion_depth + 1, trait_def_id, &trait_ref.substs, skol_map, snapshot)) }); // no Errors in that code above obligations.append(&mut trait_obligations.unwrap()); debug!("vtable_default_impl_data: obligations={:?}", obligations); VtableDefaultImplData { trait_def_id: trait_def_id, nested: obligations } } fn confirm_impl_candidate(&mut self, obligation: &TraitObligation<'tcx>, impl_def_id: ast::DefId) -> Result>, SelectionError<'tcx>> { debug!("confirm_impl_candidate({:?},{:?})", obligation, impl_def_id); // First, create the substitutions by matching the impl again, // this time not in a probe. self.infcx.commit_if_ok(|snapshot| { let (substs, skol_map) = self.rematch_impl(impl_def_id, obligation, snapshot); debug!("confirm_impl_candidate substs={:?}", substs); Ok(self.vtable_impl(impl_def_id, substs, obligation.cause.clone(), obligation.recursion_depth + 1, skol_map, snapshot)) }) } fn vtable_impl(&mut self, impl_def_id: ast::DefId, mut substs: Normalized<'tcx, Substs<'tcx>>, cause: ObligationCause<'tcx>, recursion_depth: usize, skol_map: infer::SkolemizationMap, snapshot: &infer::CombinedSnapshot) -> VtableImplData<'tcx, PredicateObligation<'tcx>> { debug!("vtable_impl(impl_def_id={:?}, substs={:?}, recursion_depth={}, skol_map={:?})", impl_def_id, substs, recursion_depth, skol_map); let mut impl_obligations = self.impl_or_trait_obligations(cause, recursion_depth, impl_def_id, &substs.value, skol_map, snapshot); debug!("vtable_impl: impl_def_id={:?} impl_obligations={:?}", impl_def_id, impl_obligations); impl_obligations.append(&mut substs.obligations); VtableImplData { impl_def_id: impl_def_id, substs: substs.value, nested: impl_obligations } } fn confirm_object_candidate(&mut self, obligation: &TraitObligation<'tcx>) -> VtableObjectData<'tcx> { debug!("confirm_object_candidate({:?})", obligation); // FIXME skipping binder here seems wrong -- we should // probably flatten the binder from the obligation and the // binder from the object. Have to try to make a broken test // case that results. -nmatsakis let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder()); let poly_trait_ref = match self_ty.sty { ty::TyTrait(ref data) => { data.principal_trait_ref_with_self_ty(self.tcx(), self_ty) } _ => { self.tcx().sess.span_bug(obligation.cause.span, "object candidate with non-object"); } }; // Upcast the object type to the obligation type. There must // be exactly one applicable trait-reference; if this were not // the case, we would have reported an ambiguity error rather // than successfully selecting one of the candidates. let mut upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref) .map(|upcast_trait_ref| { (upcast_trait_ref.clone(), self.infcx.probe(|_| { self.match_poly_trait_ref(obligation, upcast_trait_ref) }).is_ok()) }); let mut upcast_trait_ref = None; let mut vtable_base = 0; while let Some((supertrait, matches)) = upcast_trait_refs.next() { if matches { upcast_trait_ref = Some(supertrait); break; } vtable_base += util::count_own_vtable_entries(self.tcx(), supertrait); } assert!(upcast_trait_refs.all(|(_, matches)| !matches)); VtableObjectData { upcast_trait_ref: upcast_trait_ref.unwrap(), vtable_base: vtable_base } } fn confirm_fn_pointer_candidate(&mut self, obligation: &TraitObligation<'tcx>) -> Result,SelectionError<'tcx>> { debug!("confirm_fn_pointer_candidate({:?})", obligation); // ok to skip binder; it is reintroduced below let self_ty = self.infcx.shallow_resolve(*obligation.self_ty().skip_binder()); let sig = self_ty.fn_sig(); let trait_ref = util::closure_trait_ref_and_return_type(self.tcx(), obligation.predicate.def_id(), self_ty, sig, util::TupleArgumentsFlag::Yes) .map_bound(|(trait_ref, _)| trait_ref); try!(self.confirm_poly_trait_refs(obligation.cause.clone(), obligation.predicate.to_poly_trait_ref(), trait_ref)); Ok(self_ty) } fn confirm_closure_candidate(&mut self, obligation: &TraitObligation<'tcx>, closure_def_id: ast::DefId, substs: &Substs<'tcx>) -> Result>, SelectionError<'tcx>> { debug!("confirm_closure_candidate({:?},{:?},{:?})", obligation, closure_def_id, substs); let Normalized { value: trait_ref, obligations } = self.closure_trait_ref(obligation, closure_def_id, substs); debug!("confirm_closure_candidate(closure_def_id={:?}, trait_ref={:?}, obligations={:?})", closure_def_id, trait_ref, obligations); try!(self.confirm_poly_trait_refs(obligation.cause.clone(), obligation.predicate.to_poly_trait_ref(), trait_ref)); Ok(VtableClosureData { closure_def_id: closure_def_id, substs: substs.clone(), nested: obligations }) } /// In the case of closure types and fn pointers, /// we currently treat the input type parameters on the trait as /// outputs. This means that when we have a match we have only /// considered the self type, so we have to go back and make sure /// to relate the argument types too. This is kind of wrong, but /// since we control the full set of impls, also not that wrong, /// and it DOES yield better error messages (since we don't report /// errors as if there is no applicable impl, but rather report /// errors are about mismatched argument types. /// /// Here is an example. Imagine we have an closure expression /// and we desugared it so that the type of the expression is /// `Closure`, and `Closure` expects an int as argument. Then it /// is "as if" the compiler generated this impl: /// /// impl Fn(int) for Closure { ... } /// /// Now imagine our obligation is `Fn(usize) for Closure`. So far /// we have matched the self-type `Closure`. At this point we'll /// compare the `int` to `usize` and generate an error. /// /// Note that this checking occurs *after* the impl has selected, /// because these output type parameters should not affect the /// selection of the impl. Therefore, if there is a mismatch, we /// report an error to the user. fn confirm_poly_trait_refs(&mut self, obligation_cause: ObligationCause, obligation_trait_ref: ty::PolyTraitRef<'tcx>, expected_trait_ref: ty::PolyTraitRef<'tcx>) -> Result<(), SelectionError<'tcx>> { let origin = infer::RelateOutputImplTypes(obligation_cause.span); let obligation_trait_ref = obligation_trait_ref.clone(); match self.infcx.sub_poly_trait_refs(false, origin, expected_trait_ref.clone(), obligation_trait_ref.clone()) { Ok(()) => Ok(()), Err(e) => Err(OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e)) } } fn confirm_builtin_unsize_candidate(&mut self, obligation: &TraitObligation<'tcx>,) -> Result>, SelectionError<'tcx>> { let tcx = self.tcx(); // assemble_candidates_for_unsizing should ensure there are no late bound // regions here. See the comment there for more details. let source = self.infcx.shallow_resolve( tcx.no_late_bound_regions(&obligation.self_ty()).unwrap()); let target = self.infcx.shallow_resolve(obligation.predicate.0.input_types()[0]); debug!("confirm_builtin_unsize_candidate(source={:?}, target={:?})", source, target); let mut nested = vec![]; match (&source.sty, &target.sty) { // Trait+Kx+'a -> Trait+Ky+'b (upcasts). (&ty::TyTrait(ref data_a), &ty::TyTrait(ref data_b)) => { // See assemble_candidates_for_unsizing for more info. let bounds = ty::ExistentialBounds { region_bound: data_b.bounds.region_bound, builtin_bounds: data_b.bounds.builtin_bounds, projection_bounds: data_a.bounds.projection_bounds.clone(), }; let new_trait = tcx.mk_trait(data_a.principal.clone(), bounds); let origin = infer::Misc(obligation.cause.span); if self.infcx.sub_types(false, origin, new_trait, target).is_err() { return Err(Unimplemented); } // Register one obligation for 'a: 'b. let cause = ObligationCause::new(obligation.cause.span, obligation.cause.body_id, ObjectCastObligation(target)); let outlives = ty::OutlivesPredicate(data_a.bounds.region_bound, data_b.bounds.region_bound); nested.push(Obligation::with_depth(cause, obligation.recursion_depth + 1, ty::Binder(outlives).to_predicate())); } // T -> Trait. (_, &ty::TyTrait(ref data)) => { let object_did = data.principal_def_id(); if !object_safety::is_object_safe(tcx, object_did) { return Err(TraitNotObjectSafe(object_did)); } let cause = ObligationCause::new(obligation.cause.span, obligation.cause.body_id, ObjectCastObligation(target)); let mut push = |predicate| { nested.push(Obligation::with_depth(cause.clone(), obligation.recursion_depth + 1, predicate)); }; // Create the obligation for casting from T to Trait. push(data.principal_trait_ref_with_self_ty(tcx, source).to_predicate()); // We can only make objects from sized types. let mut builtin_bounds = data.bounds.builtin_bounds; builtin_bounds.insert(ty::BoundSized); // Create additional obligations for all the various builtin // bounds attached to the object cast. (In other words, if the // object type is Foo+Send, this would create an obligation // for the Send check.) for bound in &builtin_bounds { if let Ok(tr) = util::trait_ref_for_builtin_bound(tcx, bound, source) { push(tr.to_predicate()); } else { return Err(Unimplemented); } } // Create obligations for the projection predicates. for bound in data.projection_bounds_with_self_ty(tcx, source) { push(bound.to_predicate()); } // If the type is `Foo+'a`, ensures that the type // being cast to `Foo+'a` outlives `'a`: let outlives = ty::OutlivesPredicate(source, data.bounds.region_bound); push(ty::Binder(outlives).to_predicate()); } // [T; n] -> [T]. (&ty::TyArray(a, _), &ty::TySlice(b)) => { let origin = infer::Misc(obligation.cause.span); if self.infcx.sub_types(false, origin, a, b).is_err() { return Err(Unimplemented); } } // Struct -> Struct. (&ty::TyStruct(def_id, substs_a), &ty::TyStruct(_, substs_b)) => { let fields = tcx.lookup_struct_fields(def_id).iter().map(|f| { tcx.lookup_field_type_unsubstituted(def_id, f.id) }).collect::>(); // The last field of the structure has to exist and contain type parameters. let field = if let Some(&field) = fields.last() { field } else { return Err(Unimplemented); }; let mut ty_params = vec![]; for ty in field.walk() { if let ty::TyParam(p) = ty.sty { assert!(p.space == TypeSpace); let idx = p.idx as usize; if !ty_params.contains(&idx) { ty_params.push(idx); } } } if ty_params.is_empty() { return Err(Unimplemented); } // Replace type parameters used in unsizing with // TyError and ensure they do not affect any other fields. // This could be checked after type collection for any struct // with a potentially unsized trailing field. let mut new_substs = substs_a.clone(); for &i in &ty_params { new_substs.types.get_mut_slice(TypeSpace)[i] = tcx.types.err; } for &ty in fields.split_last().unwrap().1 { if ty.subst(tcx, &new_substs).references_error() { return Err(Unimplemented); } } // Extract Field and Field from Struct and Struct. let inner_source = field.subst(tcx, substs_a); let inner_target = field.subst(tcx, substs_b); // Check that the source structure with the target's // type parameters is a subtype of the target. for &i in &ty_params { let param_b = *substs_b.types.get(TypeSpace, i); new_substs.types.get_mut_slice(TypeSpace)[i] = param_b; } let new_struct = tcx.mk_struct(def_id, tcx.mk_substs(new_substs)); let origin = infer::Misc(obligation.cause.span); if self.infcx.sub_types(false, origin, new_struct, target).is_err() { return Err(Unimplemented); } // Construct the nested Field: Unsize> predicate. nested.push(util::predicate_for_trait_def(tcx, obligation.cause.clone(), obligation.predicate.def_id(), obligation.recursion_depth + 1, inner_source, vec![inner_target])); } _ => unreachable!() }; Ok(VtableBuiltinData { nested: nested }) } /////////////////////////////////////////////////////////////////////////// // Matching // // Matching is a common path used for both evaluation and // confirmation. It basically unifies types that appear in impls // and traits. This does affect the surrounding environment; // therefore, when used during evaluation, match routines must be // run inside of a `probe()` so that their side-effects are // contained. fn rematch_impl(&mut self, impl_def_id: ast::DefId, obligation: &TraitObligation<'tcx>, snapshot: &infer::CombinedSnapshot) -> (Normalized<'tcx, Substs<'tcx>>, infer::SkolemizationMap) { match self.match_impl(impl_def_id, obligation, snapshot) { Ok((substs, skol_map)) => (substs, skol_map), Err(()) => { self.tcx().sess.bug( &format!("Impl {:?} was matchable against {:?} but now is not", impl_def_id, obligation)); } } } fn match_impl(&mut self, impl_def_id: ast::DefId, obligation: &TraitObligation<'tcx>, snapshot: &infer::CombinedSnapshot) -> Result<(Normalized<'tcx, Substs<'tcx>>, infer::SkolemizationMap), ()> { let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap(); // Before we create the substitutions and everything, first // consider a "quick reject". This avoids creating more types // and so forth that we need to. if self.fast_reject_trait_refs(obligation, &impl_trait_ref) { return Err(()); } let (skol_obligation, skol_map) = self.infcx().skolemize_late_bound_regions( &obligation.predicate, snapshot); let skol_obligation_trait_ref = skol_obligation.trait_ref; let impl_substs = util::fresh_type_vars_for_impl(self.infcx, obligation.cause.span, impl_def_id); let impl_trait_ref = impl_trait_ref.subst(self.tcx(), &impl_substs); let impl_trait_ref = project::normalize_with_depth(self, obligation.cause.clone(), obligation.recursion_depth + 1, &impl_trait_ref); debug!("match_impl(impl_def_id={:?}, obligation={:?}, \ impl_trait_ref={:?}, skol_obligation_trait_ref={:?})", impl_def_id, obligation, impl_trait_ref, skol_obligation_trait_ref); let origin = infer::RelateOutputImplTypes(obligation.cause.span); if let Err(e) = self.infcx.sub_trait_refs(false, origin, impl_trait_ref.value.clone(), skol_obligation_trait_ref) { debug!("match_impl: failed sub_trait_refs due to `{}`", e); return Err(()); } if let Err(e) = self.infcx.leak_check(&skol_map, snapshot) { debug!("match_impl: failed leak check due to `{}`", e); return Err(()); } debug!("match_impl: success impl_substs={:?}", impl_substs); Ok((Normalized { value: impl_substs, obligations: impl_trait_ref.obligations }, skol_map)) } fn fast_reject_trait_refs(&mut self, obligation: &TraitObligation, impl_trait_ref: &ty::TraitRef) -> bool { // We can avoid creating type variables and doing the full // substitution if we find that any of the input types, when // simplified, do not match. obligation.predicate.0.input_types().iter() .zip(impl_trait_ref.input_types()) .any(|(&obligation_ty, &impl_ty)| { let simplified_obligation_ty = fast_reject::simplify_type(self.tcx(), obligation_ty, true); let simplified_impl_ty = fast_reject::simplify_type(self.tcx(), impl_ty, false); simplified_obligation_ty.is_some() && simplified_impl_ty.is_some() && simplified_obligation_ty != simplified_impl_ty }) } /// Normalize `where_clause_trait_ref` and try to match it against /// `obligation`. If successful, return any predicates that /// result from the normalization. Normalization is necessary /// because where-clauses are stored in the parameter environment /// unnormalized. fn match_where_clause_trait_ref(&mut self, obligation: &TraitObligation<'tcx>, where_clause_trait_ref: ty::PolyTraitRef<'tcx>) -> Result>,()> { try!(self.match_poly_trait_ref(obligation, where_clause_trait_ref)); Ok(Vec::new()) } /// Returns `Ok` if `poly_trait_ref` being true implies that the /// obligation is satisfied. fn match_poly_trait_ref(&self, obligation: &TraitObligation<'tcx>, poly_trait_ref: ty::PolyTraitRef<'tcx>) -> Result<(),()> { debug!("match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}", obligation, poly_trait_ref); let origin = infer::RelateOutputImplTypes(obligation.cause.span); match self.infcx.sub_poly_trait_refs(false, origin, poly_trait_ref, obligation.predicate.to_poly_trait_ref()) { Ok(()) => Ok(()), Err(_) => Err(()), } } /// Determines whether the self type declared against /// `impl_def_id` matches `obligation_self_ty`. If successful, /// returns the substitutions used to make them match. See /// `match_impl()`. For example, if `impl_def_id` is declared /// as: /// /// impl Foo for Box { ... } /// /// and `obligation_self_ty` is `int`, we'd get back an `Err(_)` /// result. But if `obligation_self_ty` were `Box`, we'd get /// back `Ok(T=int)`. fn match_inherent_impl(&mut self, impl_def_id: ast::DefId, obligation_cause: &ObligationCause, obligation_self_ty: Ty<'tcx>) -> Result,()> { // Create fresh type variables for each type parameter declared // on the impl etc. let impl_substs = util::fresh_type_vars_for_impl(self.infcx, obligation_cause.span, impl_def_id); // Find the self type for the impl. let impl_self_ty = self.tcx().lookup_item_type(impl_def_id).ty; let impl_self_ty = impl_self_ty.subst(self.tcx(), &impl_substs); debug!("match_impl_self_types(obligation_self_ty={:?}, impl_self_ty={:?})", obligation_self_ty, impl_self_ty); match self.match_self_types(obligation_cause, impl_self_ty, obligation_self_ty) { Ok(()) => { debug!("Matched impl_substs={:?}", impl_substs); Ok(impl_substs) } Err(()) => { debug!("NoMatch"); Err(()) } } } fn match_self_types(&mut self, cause: &ObligationCause, // The self type provided by the impl/caller-obligation: provided_self_ty: Ty<'tcx>, // The self type the obligation is for: required_self_ty: Ty<'tcx>) -> Result<(),()> { // FIXME(#5781) -- equating the types is stronger than // necessary. Should consider variance of trait w/r/t Self. let origin = infer::RelateSelfType(cause.span); match self.infcx.eq_types(false, origin, provided_self_ty, required_self_ty) { Ok(()) => Ok(()), Err(_) => Err(()), } } /////////////////////////////////////////////////////////////////////////// // Miscellany fn match_fresh_trait_refs(&self, previous: &ty::PolyTraitRef<'tcx>, current: &ty::PolyTraitRef<'tcx>) -> bool { let mut matcher = ty_match::Match::new(self.tcx()); matcher.relate(previous, current).is_ok() } fn push_stack<'o,'s:'o>(&mut self, previous_stack: TraitObligationStackList<'s, 'tcx>, obligation: &'o TraitObligation<'tcx>) -> TraitObligationStack<'o, 'tcx> { let fresh_trait_ref = obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener); TraitObligationStack { obligation: obligation, fresh_trait_ref: fresh_trait_ref, previous: previous_stack, } } fn closure_trait_ref_unnormalized(&mut self, obligation: &TraitObligation<'tcx>, closure_def_id: ast::DefId, substs: &Substs<'tcx>) -> ty::PolyTraitRef<'tcx> { let closure_type = self.infcx.closure_type(closure_def_id, substs); let ty::Binder((trait_ref, _)) = util::closure_trait_ref_and_return_type(self.tcx(), obligation.predicate.def_id(), obligation.predicate.0.self_ty(), // (1) &closure_type.sig, util::TupleArgumentsFlag::No); // (1) Feels icky to skip the binder here, but OTOH we know // that the self-type is an unboxed closure type and hence is // in fact unparameterized (or at least does not reference any // regions bound in the obligation). Still probably some // refactoring could make this nicer. ty::Binder(trait_ref) } fn closure_trait_ref(&mut self, obligation: &TraitObligation<'tcx>, closure_def_id: ast::DefId, substs: &Substs<'tcx>) -> Normalized<'tcx, ty::PolyTraitRef<'tcx>> { let trait_ref = self.closure_trait_ref_unnormalized( obligation, closure_def_id, substs); // A closure signature can contain associated types which // must be normalized. normalize_with_depth(self, obligation.cause.clone(), obligation.recursion_depth+1, &trait_ref) } /// Returns the obligations that are implied by instantiating an /// impl or trait. The obligations are substituted and fully /// normalized. This is used when confirming an impl or default /// impl. fn impl_or_trait_obligations(&mut self, cause: ObligationCause<'tcx>, recursion_depth: usize, def_id: ast::DefId, // of impl or trait substs: &Substs<'tcx>, // for impl or trait skol_map: infer::SkolemizationMap, snapshot: &infer::CombinedSnapshot) -> Vec> { debug!("impl_or_trait_obligations(def_id={:?})", def_id); let predicates = self.tcx().lookup_predicates(def_id); let predicates = predicates.instantiate(self.tcx(), substs); let predicates = normalize_with_depth(self, cause.clone(), recursion_depth, &predicates); let mut predicates = self.infcx().plug_leaks(skol_map, snapshot, &predicates); let mut obligations = util::predicates_for_generics(cause, recursion_depth, &predicates.value); obligations.append(&mut predicates.obligations); obligations } #[allow(unused_comparisons)] fn derived_cause(&self, obligation: &TraitObligation<'tcx>, variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>) -> ObligationCause<'tcx> { /*! * Creates a cause for obligations that are derived from * `obligation` by a recursive search (e.g., for a builtin * bound, or eventually a `impl Foo for ..`). If `obligation` * is itself a derived obligation, this is just a clone, but * otherwise we create a "derived obligation" cause so as to * keep track of the original root obligation for error * reporting. */ // NOTE(flaper87): As of now, it keeps track of the whole error // chain. Ideally, we should have a way to configure this either // by using -Z verbose or just a CLI argument. if obligation.recursion_depth >= 0 { let derived_cause = DerivedObligationCause { parent_trait_ref: obligation.predicate.to_poly_trait_ref(), parent_code: Rc::new(obligation.cause.code.clone()), }; ObligationCause::new(obligation.cause.span, obligation.cause.body_id, variant(derived_cause)) } else { obligation.cause.clone() } } } impl<'tcx> SelectionCache<'tcx> { pub fn new() -> SelectionCache<'tcx> { SelectionCache { hashmap: RefCell::new(FnvHashMap()) } } } impl<'o,'tcx> TraitObligationStack<'o,'tcx> { fn list(&'o self) -> TraitObligationStackList<'o,'tcx> { TraitObligationStackList::with(self) } fn iter(&'o self) -> TraitObligationStackList<'o,'tcx> { self.list() } } #[derive(Copy, Clone)] struct TraitObligationStackList<'o,'tcx:'o> { head: Option<&'o TraitObligationStack<'o,'tcx>> } impl<'o,'tcx> TraitObligationStackList<'o,'tcx> { fn empty() -> TraitObligationStackList<'o,'tcx> { TraitObligationStackList { head: None } } fn with(r: &'o TraitObligationStack<'o,'tcx>) -> TraitObligationStackList<'o,'tcx> { TraitObligationStackList { head: Some(r) } } } impl<'o,'tcx> Iterator for TraitObligationStackList<'o,'tcx>{ type Item = &'o TraitObligationStack<'o,'tcx>; fn next(&mut self) -> Option<&'o TraitObligationStack<'o,'tcx>> { match self.head { Some(o) => { *self = o.previous; Some(o) } None => None } } } impl<'o,'tcx> fmt::Debug for TraitObligationStack<'o,'tcx> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "TraitObligationStack({:?})", self.obligation) } } impl<'tcx> EvaluationResult<'tcx> { fn may_apply(&self) -> bool { match *self { EvaluatedToOk | EvaluatedToAmbig | EvaluatedToErr(OutputTypeParameterMismatch(..)) | EvaluatedToErr(TraitNotObjectSafe(_)) => true, EvaluatedToErr(Unimplemented) => false, } } } impl MethodMatchResult { pub fn may_apply(&self) -> bool { match *self { MethodMatched(_) => true, MethodAmbiguous(_) => true, MethodDidNotMatch => false, } } }