/
select.rs
2400 lines (2135 loc) · 98.6 KB
/
select.rs
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// 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 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! See `doc.rs` 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::{DerivedObligationCause};
use super::{project};
use super::project::Normalized;
use super::{PredicateObligation, Obligation, TraitObligation, ObligationCause};
use super::{ObligationCauseCode, BuiltinDerivedObligation};
use super::{SelectionError, Unimplemented, Overflow, OutputTypeParameterMismatch};
use super::{Selection};
use super::{SelectionResult};
use super::{VtableBuiltin, VtableImpl, VtableParam, VtableUnboxedClosure,
VtableFnPointer, VtableObject};
use super::{VtableImplData, VtableObjectData, VtableBuiltinData};
use super::object_safety;
use super::{util};
use middle::fast_reject;
use middle::mem_categorization::Typer;
use middle::subst::{Subst, Substs, TypeSpace, VecPerParamSpace};
use middle::ty::{self, AsPredicate, RegionEscape, ToPolyTraitRef, Ty};
use middle::infer;
use middle::infer::{InferCtxt, TypeFreshener};
use middle::ty_fold::TypeFoldable;
use std::cell::RefCell;
use std::collections::hash_map::HashMap;
use std::rc::Rc;
use syntax::{abi, ast};
use util::common::ErrorReported;
use util::ppaux::Repr;
pub struct SelectionContext<'cx, 'tcx:'cx> {
infcx: &'cx InferCtxt<'cx, 'tcx>,
closure_typer: &'cx (ty::UnboxedClosureTyper<'tcx>+'cx),
/// 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: Option<&'prev TraitObligationStack<'prev, 'tcx>>
}
#[derive(Clone)]
pub struct SelectionCache<'tcx> {
hashmap: RefCell<HashMap<Rc<ty::TraitRef<'tcx>>,
SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
}
pub enum MethodMatchResult {
MethodMatched(MethodMatchedData),
MethodAmbiguous(/* list of impls that could apply */ Vec<ast::DefId>),
MethodDidNotMatch,
}
#[derive(Copy, Show)]
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 `uint` but the
/// obligation is for `int`. In that case, we drop the impl out of the
/// list. But the other cases are considered *candidates*.
///
/// Candidates can either be definitive or ambiguous. An ambiguous
/// candidate is one that might match or might not, depending on how
/// type variables wind up being resolved. This only occurs during inference.
///
/// For selection to succeed, there must be exactly one non-ambiguous
/// candidate. Usually, it is not possible to have more than one
/// definitive candidate, due to the coherence rules. However, there is
/// one case where it could occur: if there is a blanket impl for a
/// trait (that is, an impl applied to all T), and a type parameter
/// with a where clause. In that case, we can have a candidate from the
/// where clause and a second candidate from the impl. This is not a
/// problem because coherence guarantees us that the impl which would
/// be used to satisfy the where clause is the same one that we see
/// now. To resolve this issue, therefore, we ignore impls if we find a
/// matching where clause. Part of the reason for this is that where
/// clauses can give additional information (like, the types of output
/// parameters) that would have to be inferred from the impl.
#[derive(PartialEq,Eq,Show,Clone)]
enum SelectionCandidate<'tcx> {
BuiltinCandidate(ty::BuiltinBound),
ParamCandidate(ty::PolyTraitRef<'tcx>),
ImplCandidate(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.
UnboxedClosureCandidate(/* 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,
ErrorCandidate,
}
struct SelectionCandidateSet<'tcx> {
// a list of candidates that definitely apply to the current
// obligation (meaning: types unify).
vec: Vec<SelectionCandidate<'tcx>>,
// 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(Vec<Ty<'tcx>>),
ParameterBuiltin,
AmbiguousBuiltin
}
#[derive(Show)]
enum EvaluationResult<'tcx> {
EvaluatedToOk,
EvaluatedToAmbig,
EvaluatedToErr(SelectionError<'tcx>),
}
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>,
closure_typer: &'cx ty::UnboxedClosureTyper<'tcx>)
-> SelectionContext<'cx, 'tcx> {
SelectionContext {
infcx: infcx,
closure_typer: closure_typer,
freshener: infcx.freshener(),
intercrate: false,
}
}
pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>,
closure_typer: &'cx ty::UnboxedClosureTyper<'tcx>)
-> SelectionContext<'cx, 'tcx> {
SelectionContext {
infcx: infcx,
closure_typer: closure_typer,
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.closure_typer.param_env()
}
///////////////////////////////////////////////////////////////////////////
// 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<Foo>:Iterable<Bar>`, but the impl specifies
// `impl<T> Iterable<T> for Vec<T>`, than an error would result.
/// Evaluates whether the obligation can be satisfied. Returns an indication of whether the
/// obligation can be satisfied and, if so, by what means. Never affects surrounding typing
/// environment.
pub fn select(&mut self, obligation: &TraitObligation<'tcx>)
-> SelectionResult<'tcx, Selection<'tcx>> {
debug!("select({})", obligation.repr(self.tcx()));
assert!(!obligation.predicate.has_escaping_regions());
let stack = self.push_stack(None, obligation);
match try!(self.candidate_from_obligation(&stack)) {
None => Ok(None),
Some(candidate) => Ok(Some(try!(self.confirm_candidate(obligation, candidate)))),
}
}
///////////////////////////////////////////////////////////////////////////
// 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.repr(self.tcx()));
self.evaluate_predicate_recursively(None, 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(Some(previous_stack), &obligation)
}
Err(ErrorReported) => {
EvaluatedToOk
}
}
}
fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
stack: Option<&TraitObligationStack<'o, 'tcx>>,
mut predicates: I)
-> EvaluationResult<'tcx>
where I : Iterator<Item=&'a PredicateObligation<'tcx>>, '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: Option<&TraitObligationStack<'o, 'tcx>>,
obligation: &PredicateObligation<'tcx>)
-> EvaluationResult<'tcx>
{
debug!("evaluate_predicate_recursively({})",
obligation.repr(self.tcx()));
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: Option<&TraitObligationStack<'o, 'tcx>>,
obligation: &TraitObligation<'tcx>)
-> EvaluationResult<'tcx>
{
debug!("evaluate_obligation_recursively({})",
obligation.repr(self.tcx()));
let stack = self.push_stack(previous_stack.map(|x| x), 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<T:Eq> Eq for Vec<T>
//
// 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(|&t| ty::type_is_fresh(t));
if
unbound_input_types &&
(self.intercrate ||
stack.iter().skip(1).any(
|prev| stack.fresh_trait_ref.def_id() == prev.fresh_trait_ref.def_id()))
{
debug!("evaluate_stack({}) --> unbound argument, recursion --> ambiguous",
stack.fresh_trait_ref.repr(self.tcx()));
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<T> { data: T, next: Option<Box<List<T>>> {
//
// `Box<List<T>>` will be `Send` if `T` is `Send` and
// `Option<Box<List<T>>>` is `Send`, and in turn
// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` 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.repr(self.tcx()));
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.repr(self.tcx()),
obligation.repr(self.tcx()));
self.infcx.probe(|snapshot| {
let (skol_obligation_trait_ref, skol_map) =
self.infcx().skolemize_late_bound_regions(&obligation.predicate, snapshot);
match self.match_impl(impl_def_id, obligation, snapshot,
&skol_map, skol_obligation_trait_ref.trait_ref.clone()) {
Ok(substs) => {
let vtable_impl = self.vtable_impl(impl_def_id,
substs,
obligation.cause.clone(),
obligation.recursion_depth + 1,
skol_map,
snapshot);
self.winnow_selection(None, 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 `doc.rs` 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 {
debug!("{} --> overflow (limit={})",
stack.obligation.repr(self.tcx()),
recursion_limit);
return Err(Overflow)
}
// 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.repr(self.tcx()),
stack.repr(self.tcx()));
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.repr(self.tcx()),
c.repr(self.tcx()));
return c;
}
None => { }
}
// If no match, compute result and insert into cache.
let candidate = self.candidate_from_obligation_no_cache(stack);
debug!("CACHE MISS: cache_fresh_trait_pred={}, candidate={}",
cache_fresh_trait_pred.repr(self.tcx()), candidate.repr(self.tcx()));
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 ty::type_is_error(stack.obligation.predicate.0.self_ty()) {
return Ok(Some(ErrorCandidate));
}
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.repr(self.tcx()),
candidates.repr(self.tcx()));
// 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 ==
// uint, 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<T:Clone> Vec<T> { fn push_clone(...) { ... } }
//
// and we were to see some code `foo.push_clone()` where `boo`
// is a `Vec<Bar>` 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 =
range(0, candidates.len())
.filter(|&j| i != j)
.any(|j| self.candidate_should_be_dropped_in_favor_of(stack,
&candidates[i],
&candidates[j]));
if is_dup {
debug!("Dropping candidate #{}/{}: {}",
i, candidates.len(), candidates[i].repr(self.tcx()));
candidates.swap_remove(i);
} else {
debug!("Retaining candidate #{}/{}: {}",
i, candidates.len(), candidates[i].repr(self.tcx()));
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.len() == 0 {
return Err(Unimplemented);
}
// Just one candidate left.
let candidate = candidates.pop().unwrap();
match candidate {
ImplCandidate(def_id) => {
match ty::trait_impl_polarity(self.tcx(), def_id) {
Some(ast::ImplPolarity::Negative) => return Err(Unimplemented),
_ => {}
}
}
_ => {}
}
Ok(Some(candidate))
}
fn pick_candidate_cache(&self,
cache_fresh_trait_pred: &ty::PolyTraitPredicate<'tcx>)
-> &SelectionCache<'tcx>
{
// High-level idea: we have to decide whether to consult the
// cache that is specific to this scope, or to consult the
// global cache. We want the cache that is specific to this
// scope whenever where clauses might affect the result.
// 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;
}
// If the trait refers to any parameters in scope, then use
// the cache of the param-environment.
if
cache_fresh_trait_pred.0.input_types().iter().any(
|&t| ty::type_has_self(t) || ty::type_has_params(t))
{
return &self.param_env().selection_cache;
}
// If the trait refers to unbound type variables, and there
// are where clauses in scope, then use the local environment.
// If there are no where clauses in scope, which is a very
// common case, then we can use the global environment.
// See the discussion in doc.rs for more details.
if
!self.param_env().caller_bounds.is_empty() &&
cache_fresh_trait_pred.0.input_types().iter().any(
|&t| ty::type_has_ty_infer(t))
{
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<SelectionResult<'tcx, SelectionCandidate<'tcx>>>
{
let cache = self.pick_candidate_cache(cache_fresh_trait_pred);
let hashmap = cache.hashmap.borrow();
hashmap.get(&cache_fresh_trait_pred.0.trait_ref).map(|c| (*c).clone())
}
fn insert_candidate_cache(&mut self,
cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>)
{
let cache = self.pick_candidate_cache(&cache_fresh_trait_pred);
let mut hashmap = cache.hashmap.borrow_mut();
hashmap.insert(cache_fresh_trait_pred.0.trait_ref.clone(), candidate);
}
fn assemble_candidates<'o>(&mut self,
stack: &TraitObligationStack<'o, 'tcx>)
-> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>>
{
// Check for overflow.
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().repr(self.tcx()));
try!(self.assemble_candidates_from_impls(obligation, &mut candidates));
try!(self.assemble_builtin_bound_candidates(ty::BoundCopy,
stack,
&mut candidates));
}
Some(bound @ ty::BoundSend) |
Some(bound @ ty::BoundSync) => {
try!(self.assemble_candidates_from_impls(obligation, &mut candidates));
// No explicit impls were declared for this type, consider the fallback rules.
if candidates.vec.is_empty() && !candidates.ambiguous {
try!(self.assemble_builtin_bound_candidates(bound, stack, &mut candidates));
}
}
Some(bound @ ty::BoundSized) => {
// Sized and Copy are always automatically computed.
try!(self.assemble_builtin_bound_candidates(bound, stack, &mut candidates));
}
None => {
// For the time being, we ignore user-defined impls for builtin-bounds, other than
// `Copy`.
// (And unboxed candidates only apply to the Fn/FnMut/etc traits.)
try!(self.assemble_unboxed_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));
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.repr(self.tcx()),
poly_trait_predicate.repr(self.tcx()));
// 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::ty_projection(ref data) => data.trait_ref.def_id,
ty::ty_infer(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?
candidates.ambiguous = true;
return;
}
_ => { return; }
};
debug!("assemble_candidates_for_projected_tys: trait_def_id={}",
trait_def_id.repr(self.tcx()));
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.repr(self.tcx()),
skol_map.repr(self.tcx()));
let projection_trait_ref = match skol_trait_predicate.trait_ref.self_ty().sty {
ty::ty_projection(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().repr(self.tcx())).as_slice());
}
};
debug!("match_projection_obligation_against_bounds_from_trait: \
projection_trait_ref={}",
projection_trait_ref.repr(self.tcx()));
let trait_def = ty::lookup_trait_def(self.tcx(), projection_trait_ref.def_id);
let bounds = trait_def.generics.to_bounds(self.tcx(), projection_trait_ref.substs);
debug!("match_projection_obligation_against_bounds_from_trait: \
bounds={}",
bounds.repr(self.tcx()));
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.repr(self.tcx()));
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: Rc<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 `<SomeTrait for T>`, 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.repr(self.tcx()));
let caller_trait_refs: Vec<_> =
self.param_env().caller_bounds.predicates.iter()
.filter_map(|o| o.to_opt_poly_trait_ref())
.collect();
let all_bounds =
util::transitive_bounds(
self.tcx(), &caller_trait_refs[]);
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(Some(stack), obligations.iter())
}
Err(()) => {
EvaluatedToErr(Unimplemented)
}
}
})
}
/// Check for the artificial impl that the compiler will create for an obligation like `X :
/// FnMut<..>` where `X` is an unboxed closure type.
///
/// Note: the type parameters on an unboxed 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_unboxed_closure_candidates(&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>)
-> Result<(),SelectionError<'tcx>>
{
let kind = match self.fn_family_trait_kind(obligation.predicate.0.def_id()) {
Some(k) => k,
None => { return Ok(()); }
};
let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
let (closure_def_id, substs) = match self_ty.sty {
ty::ty_unboxed_closure(id, _, ref substs) => (id, substs.clone()),
ty::ty_infer(ty::TyVar(_)) => {
candidates.ambiguous = true;
return Ok(());
}
_ => { return Ok(()); }
};
debug!("assemble_unboxed_candidates: self_ty={} kind={:?} obligation={}",
self_ty.repr(self.tcx()),
kind,
obligation.repr(self.tcx()));
let closure_kind = self.closure_typer.unboxed_closure_kind(closure_def_id);
debug!("closure_kind = {:?}", closure_kind);
if closure_kind == kind {
candidates.vec.push(UnboxedClosureCandidate(closure_def_id, substs.clone()));
}
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 a `Fn` impl for fn pointers. There is no need to provide
// the other traits (e.g. `FnMut`) since those are provided by blanket
// impls.
if Some(obligation.predicate.def_id()) != self.tcx().lang_items.fn_trait() {
return Ok(());
}
let self_ty = self.infcx.shallow_resolve(obligation.self_ty());