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move candidate assembly into a submodule

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Bastian Kauschke 2020-05-25 22:08:30 +02:00
parent 634977f8f2
commit b8172ec405
2 changed files with 613 additions and 607 deletions
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611
src/librustc_trait_selection/traits/select/candidate_assembly.rs Normal file
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@ -0,0 +1,611 @@
//! Candidate assembly.
//!
//! The selection process begins by examining all in-scope impls,
//! caller obligations, and so forth and assembling a list of
//! candidates. See the [rustc dev guide] for more details.
//!
//! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
use rustc_hir as hir;
use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
use rustc_middle::ty::{self, TypeFoldable};
use rustc_target::spec::abi::Abi;
use crate::traits::{util, SelectionResult};
use super::BuiltinImplConditions;
use super::SelectionCandidate::{self, *};
use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack};
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub(super) 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.
self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
// Check the cache. Note that we freshen the trait-ref
// separately rather than using `stack.fresh_trait_ref` --
// this is because we want the unbound variables to be
// replaced with fresh types starting from index 0.
let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
debug!(
"candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
cache_fresh_trait_pred, stack
);
debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
if let Some(c) =
self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
{
debug!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c);
return c;
}
// If no match, compute result and insert into cache.
//
// FIXME(nikomatsakis) -- this cache is not taking into
// account cycles that may have occurred in forming the
// candidate. I don't know of any specific problems that
// result but it seems awfully suspicious.
let (candidate, dep_node) =
self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
debug!("CACHE MISS: SELECT({:?})={:?}", cache_fresh_trait_pred, candidate);
self.insert_candidate_cache(
stack.obligation.param_env,
cache_fresh_trait_pred,
dep_node,
candidate.clone(),
);
candidate
}
pub(super) fn assemble_candidates<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
let TraitObligationStack { obligation, .. } = *stack;
let obligation = &Obligation {
param_env: obligation.param_env,
cause: obligation.cause.clone(),
recursion_depth: obligation.recursion_depth,
predicate: self.infcx().resolve_vars_if_possible(&obligation.predicate),
};
if obligation.predicate.skip_binder().self_ty().is_ty_var() {
// Self is a type variable (e.g., `_: AsRef<str>`).
//
// This is somewhat problematic, as the current scheme can't really
// handle it turning to be a projection. This does end up as truly
// ambiguous in most cases anyway.
//
// Take the fast path out - this also improves
// performance by preventing assemble_candidates_from_impls from
// matching every impl for this trait.
return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
}
let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
self.assemble_candidates_for_trait_alias(obligation, &mut candidates)?;
// Other bounds. Consider both in-scope bounds from fn decl
// and applicable impls. There is a certain set of precedence rules here.
let def_id = obligation.predicate.def_id();
let lang_items = self.tcx().lang_items();
if lang_items.copy_trait() == Some(def_id) {
debug!("obligation self ty is {:?}", obligation.predicate.skip_binder().self_ty());
// User-defined copy impls are permitted, but only for
// structs and enums.
self.assemble_candidates_from_impls(obligation, &mut candidates)?;
// For other types, we'll use the builtin rules.
let copy_conditions = self.copy_clone_conditions(obligation);
self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
} else if lang_items.discriminant_kind_trait() == Some(def_id) {
// `DiscriminantKind` is automatically implemented for every type.
candidates.vec.push(DiscriminantKindCandidate);
} else if lang_items.sized_trait() == Some(def_id) {
// Sized is never implementable by end-users, it is
// always automatically computed.
let sized_conditions = self.sized_conditions(obligation);
self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates)?;
} else if lang_items.unsize_trait() == Some(def_id) {
self.assemble_candidates_for_unsizing(obligation, &mut candidates);
} else {
if lang_items.clone_trait() == Some(def_id) {
// Same builtin conditions as `Copy`, i.e., every type which has builtin support
// for `Copy` also has builtin support for `Clone`, and tuples/arrays of `Clone`
// types have builtin support for `Clone`.
let clone_conditions = self.copy_clone_conditions(obligation);
self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
}
self.assemble_generator_candidates(obligation, &mut candidates)?;
self.assemble_closure_candidates(obligation, &mut candidates)?;
self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
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);
self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
// Auto 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() {
self.assemble_candidates_from_auto_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>,
) {
debug!("assemble_candidates_for_projected_tys({:?})", obligation);
// Before we go into the whole placeholder thing, just
// quickly check if the self-type is a projection at all.
match obligation.predicate.skip_binder().trait_ref.self_ty().kind {
ty::Projection(_) | ty::Opaque(..) => {}
ty::Infer(ty::TyVar(_)) => {
span_bug!(
obligation.cause.span,
"Self=_ should have been handled by assemble_candidates"
);
}
_ => return,
}
let result = self.infcx.probe(|snapshot| {
self.match_projection_obligation_against_definition_bounds(obligation, snapshot)
});
if result {
candidates.vec.push(ProjectionCandidate);
}
}
/// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
/// supplied to find out whether it is listed among them.
///
/// Never affects the 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 = stack
.obligation
.param_env
.caller_bounds
.iter()
.filter_map(|o| o.to_opt_poly_trait_ref());
// Micro-optimization: filter out predicates relating to different traits.
let matching_bounds =
all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
// Keep only those bounds which may apply, and propagate overflow if it occurs.
let mut param_candidates = vec![];
for bound in matching_bounds {
let wc = self.evaluate_where_clause(stack, bound)?;
if wc.may_apply() {
param_candidates.push(ParamCandidate(bound));
}
}
candidates.vec.extend(param_candidates);
Ok(())
}
fn assemble_generator_candidates(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
return Ok(());
}
// Okay to skip binder because the substs on generator types never
// touch bound regions, they just capture the in-scope
// type/region parameters.
let self_ty = *obligation.self_ty().skip_binder();
match self_ty.kind {
ty::Generator(..) => {
debug!(
"assemble_generator_candidates: self_ty={:?} obligation={:?}",
self_ty, obligation
);
candidates.vec.push(GeneratorCandidate);
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_generator_candidates: ambiguous self-type");
candidates.ambiguous = true;
}
_ => {}
}
Ok(())
}
/// Checks 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().fn_trait_kind_from_lang_item(obligation.predicate.def_id()) {
Some(k) => k,
None => {
return Ok(());
}
};
// Okay to skip binder because the substs on closure types never
// touch bound regions, they just capture the in-scope
// type/region parameters
match obligation.self_ty().skip_binder().kind {
ty::Closure(_, closure_substs) => {
debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}", kind, obligation);
match self.infcx.closure_kind(closure_substs) {
Some(closure_kind) => {
debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
if closure_kind.extends(kind) {
candidates.vec.push(ClosureCandidate);
}
}
None => {
debug!("assemble_unboxed_candidates: closure_kind not yet known");
candidates.vec.push(ClosureCandidate);
}
}
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
candidates.ambiguous = true;
}
_ => {}
}
Ok(())
}
/// Implements 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().fn_trait_kind_from_lang_item(obligation.predicate.def_id()).is_none() {
return Ok(());
}
// Okay to skip binder because what we are inspecting doesn't involve bound regions.
let self_ty = *obligation.self_ty().skip_binder();
match self_ty.kind {
ty::Infer(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::FnDef(..) | ty::FnPtr(_) => {
if let ty::FnSig {
unsafety: hir::Unsafety::Normal,
abi: Abi::Rust,
c_variadic: false,
..
} = self_ty.fn_sig(self.tcx()).skip_binder()
{
candidates.vec.push(FnPointerCandidate);
}
}
_ => {}
}
Ok(())
}
/// Searches 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);
self.tcx().for_each_relevant_impl(
obligation.predicate.def_id(),
obligation.predicate.skip_binder().trait_ref.self_ty(),
|impl_def_id| {
self.infcx.probe(|snapshot| {
if let Ok(_substs) = self.match_impl(impl_def_id, obligation, snapshot) {
candidates.vec.push(ImplCandidate(impl_def_id));
}
});
},
);
Ok(())
}
fn assemble_candidates_from_auto_impls(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
// Okay to skip binder here because the tests we do below do not involve bound regions.
let self_ty = *obligation.self_ty().skip_binder();
debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
let def_id = obligation.predicate.def_id();
if self.tcx().trait_is_auto(def_id) {
match self_ty.kind {
ty::Dynamic(..) => {
// For object types, we don't know what the closed
// over types are. This means we conservatively
// say nothing; a candidate may be added by
// `assemble_candidates_from_object_ty`.
}
ty::Foreign(..) => {
// Since the contents of foreign types is unknown,
// we don't add any `..` impl. Default traits could
// still be provided by a manual implementation for
// this trait and type.
}
ty::Param(..) | ty::Projection(..) => {
// 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::Infer(ty::TyVar(_)) => {
// The auto impl might apply; we don't know.
candidates.ambiguous = true;
}
ty::Generator(_, _, movability)
if self.tcx().lang_items().unpin_trait() == Some(def_id) =>
{
match movability {
hir::Movability::Static => {
// Immovable generators are never `Unpin`, so
// suppress the normal auto-impl candidate for it.
}
hir::Movability::Movable => {
// Movable generators are always `Unpin`, so add an
// unconditional builtin candidate.
candidates.vec.push(BuiltinCandidate { has_nested: false });
}
}
}
_ => candidates.vec.push(AutoImplCandidate(def_id)),
}
}
Ok(())
}
/// Searches 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={:?})",
obligation.self_ty().skip_binder()
);
self.infcx.probe(|_snapshot| {
// The code below doesn't care about regions, and the
// self-ty here doesn't escape this probe, so just erase
// any LBR.
let self_ty = self.tcx().erase_late_bound_regions(&obligation.self_ty());
let poly_trait_ref = match self_ty.kind {
ty::Dynamic(ref data, ..) => {
if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
debug!(
"assemble_candidates_from_object_ty: matched builtin bound, \
pushing candidate"
);
candidates.vec.push(BuiltinObjectCandidate);
return;
}
if let Some(principal) = data.principal() {
if !self.infcx.tcx.features().object_safe_for_dispatch {
principal.with_self_ty(self.tcx(), self_ty)
} else if self.tcx().is_object_safe(principal.def_id()) {
principal.with_self_ty(self.tcx(), self_ty)
} else {
return;
}
} else {
// Only auto trait bounds exist.
return;
}
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_candidates_from_object_ty: ambiguous");
candidates.ambiguous = true; // could wind up being an object type
return;
}
_ => return,
};
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<i32>`,
// but `Foo` is declared as `trait Foo: Bar<u32>`.
let upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref)
.filter(|upcast_trait_ref| {
self.infcx
.probe(|_| 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);
}
})
}
/// Searches 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<Trait+'a>` 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<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
// 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<Foo<'a, Trait>>
// 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 source = match obligation.self_ty().no_bound_vars() {
Some(t) => t,
None => {
// Don't add any candidates if there are bound regions.
return;
}
};
let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})", source, target);
let may_apply = match (&source.kind, &target.kind) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
// Upcasts permit two things:
//
// 1. Dropping auto traits, 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_b.principal_def_id()
&& data_b
.auto_traits()
// All of a's auto traits need to be in b's auto traits.
.all(|b| data_a.auto_traits().any(|a| a == b))
}
// `T` -> `Trait`
(_, &ty::Dynamic(..)) => true,
// Ambiguous handling is below `T` -> `Trait`, because inference
// variables can still implement `Unsize<Trait>` and nested
// obligations will have the final say (likely deferred).
(&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
debug!("assemble_candidates_for_unsizing: ambiguous");
candidates.ambiguous = true;
false
}
// `[T; n]` -> `[T]`
(&ty::Array(..), &ty::Slice(_)) => true,
// `Struct<T>` -> `Struct<U>`
(&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
def_id_a == def_id_b
}
// `(.., T)` -> `(.., U)`
(&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
_ => false,
};
if may_apply {
candidates.vec.push(BuiltinUnsizeCandidate);
}
}
fn assemble_candidates_for_trait_alias(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
// Okay to skip binder here because the tests we do below do not involve bound regions.
let self_ty = *obligation.self_ty().skip_binder();
debug!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty);
let def_id = obligation.predicate.def_id();
if self.tcx().is_trait_alias(def_id) {
candidates.vec.push(TraitAliasCandidate(def_id));
}
Ok(())
}
/// Assembles the trait which are built-in to the language itself:
/// `Copy`, `Clone` and `Sized`.
fn assemble_builtin_bound_candidates(
&mut self,
conditions: BuiltinImplConditions<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
match conditions {
BuiltinImplConditions::Where(nested) => {
debug!("builtin_bound: nested={:?}", nested);
candidates
.vec
.push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
}
BuiltinImplConditions::None => {}
BuiltinImplConditions::Ambiguous => {
debug!("assemble_builtin_bound_candidates: ambiguous builtin");
candidates.ambiguous = true;
}
}
Ok(())
}
}

609
src/librustc_trait_selection/traits/select/mod.rs
View File

@ -53,7 +53,6 @@ use rustc_middle::ty::{
self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
};
use rustc_span::symbol::sym;
use rustc_target::spec::abi::Abi;
use std::cell::{Cell, RefCell};
use std::cmp;
@ -63,6 +62,8 @@ use std::rc::Rc;
pub use rustc_middle::traits::select::*;
mod candidate_assembly;
pub struct SelectionContext<'cx, 'tcx> {
infcx: &'cx InferCtxt<'cx, 'tcx>,
@ -932,61 +933,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
Ok(())
}
///////////////////////////////////////////////////////////////////////////
// CANDIDATE ASSEMBLY
//
// The selection process begins by examining all in-scope impls,
// caller obligations, and so forth and assembling a list of
// candidates. See the [rustc dev guide] for more details.
//
// [rustc dev guide]:
// https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
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.
self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
// Check the cache. Note that we freshen the trait-ref
// separately rather than using `stack.fresh_trait_ref` --
// this is because we want the unbound variables to be
// replaced with fresh types starting from index 0.
let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
debug!(
"candidate_from_obligation(cache_fresh_trait_pred={:?}, obligation={:?})",
cache_fresh_trait_pred, stack
);
debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
if let Some(c) =
self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
{
debug!("CACHE HIT: SELECT({:?})={:?}", cache_fresh_trait_pred, c);
return c;
}
// If no match, compute result and insert into cache.
//
// FIXME(nikomatsakis) -- this cache is not taking into
// account cycles that may have occurred in forming the
// candidate. I don't know of any specific problems that
// result but it seems awfully suspicious.
let (candidate, dep_node) =
self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
debug!("CACHE MISS: SELECT({:?})={:?}", cache_fresh_trait_pred, candidate);
self.insert_candidate_cache(
stack.obligation.param_env,
cache_fresh_trait_pred,
dep_node,
candidate.clone(),
);
candidate
}
fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
where
OP: FnOnce(&mut Self) -> R,
@ -1320,116 +1266,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
.insert(param_env.and(trait_ref), WithDepNode::new(dep_node, candidate));
}
fn assemble_candidates<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> Result<SelectionCandidateSet<'tcx>, SelectionError<'tcx>> {
let TraitObligationStack { obligation, .. } = *stack;
let obligation = &Obligation {
param_env: obligation.param_env,
cause: obligation.cause.clone(),
recursion_depth: obligation.recursion_depth,
predicate: self.infcx().resolve_vars_if_possible(&obligation.predicate),
};
if obligation.predicate.skip_binder().self_ty().is_ty_var() {
// Self is a type variable (e.g., `_: AsRef<str>`).
//
// This is somewhat problematic, as the current scheme can't really
// handle it turning to be a projection. This does end up as truly
// ambiguous in most cases anyway.
//
// Take the fast path out - this also improves
// performance by preventing assemble_candidates_from_impls from
// matching every impl for this trait.
return Ok(SelectionCandidateSet { vec: vec![], ambiguous: true });
}
let mut candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
self.assemble_candidates_for_trait_alias(obligation, &mut candidates)?;
// Other bounds. Consider both in-scope bounds from fn decl
// and applicable impls. There is a certain set of precedence rules here.
let def_id = obligation.predicate.def_id();
let lang_items = self.tcx().lang_items();
if lang_items.copy_trait() == Some(def_id) {
debug!("obligation self ty is {:?}", obligation.predicate.skip_binder().self_ty());
// User-defined copy impls are permitted, but only for
// structs and enums.
self.assemble_candidates_from_impls(obligation, &mut candidates)?;
// For other types, we'll use the builtin rules.
let copy_conditions = self.copy_clone_conditions(obligation);
self.assemble_builtin_bound_candidates(copy_conditions, &mut candidates)?;
} else if lang_items.discriminant_kind_trait() == Some(def_id) {
// `DiscriminantKind` is automatically implemented for every type.
candidates.vec.push(DiscriminantKindCandidate);
} else if lang_items.sized_trait() == Some(def_id) {
// Sized is never implementable by end-users, it is
// always automatically computed.
let sized_conditions = self.sized_conditions(obligation);
self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates)?;
} else if lang_items.unsize_trait() == Some(def_id) {
self.assemble_candidates_for_unsizing(obligation, &mut candidates);
} else {
if lang_items.clone_trait() == Some(def_id) {
// Same builtin conditions as `Copy`, i.e., every type which has builtin support
// for `Copy` also has builtin support for `Clone`, and tuples/arrays of `Clone`
// types have builtin support for `Clone`.
let clone_conditions = self.copy_clone_conditions(obligation);
self.assemble_builtin_bound_candidates(clone_conditions, &mut candidates)?;
}
self.assemble_generator_candidates(obligation, &mut candidates)?;
self.assemble_closure_candidates(obligation, &mut candidates)?;
self.assemble_fn_pointer_candidates(obligation, &mut candidates)?;
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);
self.assemble_candidates_from_caller_bounds(stack, &mut candidates)?;
// Auto 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() {
self.assemble_candidates_from_auto_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>,
) {
debug!("assemble_candidates_for_projected_tys({:?})", obligation);
// Before we go into the whole placeholder thing, just
// quickly check if the self-type is a projection at all.
match obligation.predicate.skip_binder().trait_ref.self_ty().kind {
ty::Projection(_) | ty::Opaque(..) => {}
ty::Infer(ty::TyVar(_)) => {
span_bug!(
obligation.cause.span,
"Self=_ should have been handled by assemble_candidates"
);
}
_ => return,
}
let result = self.infcx.probe(|snapshot| {
self.match_projection_obligation_against_definition_bounds(obligation, snapshot)
});
if result {
candidates.vec.push(ProjectionCandidate);
}
}
fn match_projection_obligation_against_definition_bounds(
&mut self,
obligation: &TraitObligation<'tcx>,
@ -1523,42 +1359,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
&& self.infcx.leak_check(false, placeholder_map, snapshot).is_ok()
}
/// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
/// supplied to find out whether it is listed among them.
///
/// Never affects the 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 = stack
.obligation
.param_env
.caller_bounds
.iter()
.filter_map(|o| o.to_opt_poly_trait_ref());
// Micro-optimization: filter out predicates relating to different traits.
let matching_bounds =
all_bounds.filter(|p| p.def_id() == stack.obligation.predicate.def_id());
// Keep only those bounds which may apply, and propagate overflow if it occurs.
let mut param_candidates = vec![];
for bound in matching_bounds {
let wc = self.evaluate_where_clause(stack, bound)?;
if wc.may_apply() {
param_candidates.push(ParamCandidate(bound));
}
}
candidates.vec.extend(param_candidates);
Ok(())
}
fn evaluate_where_clause<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
@ -1574,383 +1374,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
})
}
fn assemble_generator_candidates(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
return Ok(());
}
// Okay to skip binder because the substs on generator types never
// touch bound regions, they just capture the in-scope
// type/region parameters.
let self_ty = *obligation.self_ty().skip_binder();
match self_ty.kind {
ty::Generator(..) => {
debug!(
"assemble_generator_candidates: self_ty={:?} obligation={:?}",
self_ty, obligation
);
candidates.vec.push(GeneratorCandidate);
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_generator_candidates: ambiguous self-type");
candidates.ambiguous = true;
}
_ => {}
}
Ok(())
}
/// Checks 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().fn_trait_kind_from_lang_item(obligation.predicate.def_id()) {
Some(k) => k,
None => {
return Ok(());
}
};
// Okay to skip binder because the substs on closure types never
// touch bound regions, they just capture the in-scope
// type/region parameters
match obligation.self_ty().skip_binder().kind {
ty::Closure(_, closure_substs) => {
debug!("assemble_unboxed_candidates: kind={:?} obligation={:?}", kind, obligation);
match self.infcx.closure_kind(closure_substs) {
Some(closure_kind) => {
debug!("assemble_unboxed_candidates: closure_kind = {:?}", closure_kind);
if closure_kind.extends(kind) {
candidates.vec.push(ClosureCandidate);
}
}
None => {
debug!("assemble_unboxed_candidates: closure_kind not yet known");
candidates.vec.push(ClosureCandidate);
}
}
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_unboxed_closure_candidates: ambiguous self-type");
candidates.ambiguous = true;
}
_ => {}
}
Ok(())
}
/// Implements 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().fn_trait_kind_from_lang_item(obligation.predicate.def_id()).is_none() {
return Ok(());
}
// Okay to skip binder because what we are inspecting doesn't involve bound regions.
let self_ty = *obligation.self_ty().skip_binder();
match self_ty.kind {
ty::Infer(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::FnDef(..) | ty::FnPtr(_) => {
if let ty::FnSig {
unsafety: hir::Unsafety::Normal,
abi: Abi::Rust,
c_variadic: false,
..
} = self_ty.fn_sig(self.tcx()).skip_binder()
{
candidates.vec.push(FnPointerCandidate);
}
}
_ => {}
}
Ok(())
}
/// Searches 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);
self.tcx().for_each_relevant_impl(
obligation.predicate.def_id(),
obligation.predicate.skip_binder().trait_ref.self_ty(),
|impl_def_id| {
self.infcx.probe(|snapshot| {
if let Ok(_substs) = self.match_impl(impl_def_id, obligation, snapshot) {
candidates.vec.push(ImplCandidate(impl_def_id));
}
});
},
);
Ok(())
}
fn assemble_candidates_from_auto_impls(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
// Okay to skip binder here because the tests we do below do not involve bound regions.
let self_ty = *obligation.self_ty().skip_binder();
debug!("assemble_candidates_from_auto_impls(self_ty={:?})", self_ty);
let def_id = obligation.predicate.def_id();
if self.tcx().trait_is_auto(def_id) {
match self_ty.kind {
ty::Dynamic(..) => {
// For object types, we don't know what the closed
// over types are. This means we conservatively
// say nothing; a candidate may be added by
// `assemble_candidates_from_object_ty`.
}
ty::Foreign(..) => {
// Since the contents of foreign types is unknown,
// we don't add any `..` impl. Default traits could
// still be provided by a manual implementation for
// this trait and type.
}
ty::Param(..) | ty::Projection(..) => {
// 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::Infer(ty::TyVar(_)) => {
// The auto impl might apply; we don't know.
candidates.ambiguous = true;
}
ty::Generator(_, _, movability)
if self.tcx().lang_items().unpin_trait() == Some(def_id) =>
{
match movability {
hir::Movability::Static => {
// Immovable generators are never `Unpin`, so
// suppress the normal auto-impl candidate for it.
}
hir::Movability::Movable => {
// Movable generators are always `Unpin`, so add an
// unconditional builtin candidate.
candidates.vec.push(BuiltinCandidate { has_nested: false });
}
}
}
_ => candidates.vec.push(AutoImplCandidate(def_id)),
}
}
Ok(())
}
/// Searches 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={:?})",
obligation.self_ty().skip_binder()
);
self.infcx.probe(|_snapshot| {
// The code below doesn't care about regions, and the
// self-ty here doesn't escape this probe, so just erase
// any LBR.
let self_ty = self.tcx().erase_late_bound_regions(&obligation.self_ty());
let poly_trait_ref = match self_ty.kind {
ty::Dynamic(ref data, ..) => {
if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
debug!(
"assemble_candidates_from_object_ty: matched builtin bound, \
pushing candidate"
);
candidates.vec.push(BuiltinObjectCandidate);
return;
}
if let Some(principal) = data.principal() {
if !self.infcx.tcx.features().object_safe_for_dispatch {
principal.with_self_ty(self.tcx(), self_ty)
} else if self.tcx().is_object_safe(principal.def_id()) {
principal.with_self_ty(self.tcx(), self_ty)
} else {
return;
}
} else {
// Only auto trait bounds exist.
return;
}
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_candidates_from_object_ty: ambiguous");
candidates.ambiguous = true; // could wind up being an object type
return;
}
_ => return,
};
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<i32>`,
// but `Foo` is declared as `trait Foo: Bar<u32>`.
let upcast_trait_refs = util::supertraits(self.tcx(), poly_trait_ref)
.filter(|upcast_trait_ref| {
self.infcx
.probe(|_| 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);
}
})
}
/// Searches 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<Trait+'a>` 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<U>` obligation is always used as part of a `T: CoerceUnsize<U>`
// 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<Foo<'a, Trait>>
// 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 source = match obligation.self_ty().no_bound_vars() {
Some(t) => t,
None => {
// Don't add any candidates if there are bound regions.
return;
}
};
let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
debug!("assemble_candidates_for_unsizing(source={:?}, target={:?})", source, target);
let may_apply = match (&source.kind, &target.kind) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
// Upcasts permit two things:
//
// 1. Dropping auto traits, 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_b.principal_def_id()
&& data_b
.auto_traits()
// All of a's auto traits need to be in b's auto traits.
.all(|b| data_a.auto_traits().any(|a| a == b))
}
// `T` -> `Trait`
(_, &ty::Dynamic(..)) => true,
// Ambiguous handling is below `T` -> `Trait`, because inference
// variables can still implement `Unsize<Trait>` and nested
// obligations will have the final say (likely deferred).
(&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
debug!("assemble_candidates_for_unsizing: ambiguous");
candidates.ambiguous = true;
false
}
// `[T; n]` -> `[T]`
(&ty::Array(..), &ty::Slice(_)) => true,
// `Struct<T>` -> `Struct<U>`
(&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
def_id_a == def_id_b
}
// `(.., T)` -> `(.., U)`
(&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
_ => false,
};
if may_apply {
candidates.vec.push(BuiltinUnsizeCandidate);
}
}
fn assemble_candidates_for_trait_alias(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
// Okay to skip binder here because the tests we do below do not involve bound regions.
let self_ty = *obligation.self_ty().skip_binder();
debug!("assemble_candidates_for_trait_alias(self_ty={:?})", self_ty);
let def_id = obligation.predicate.def_id();
if self.tcx().is_trait_alias(def_id) {
candidates.vec.push(TraitAliasCandidate(def_id));
}
Ok(())
}
///////////////////////////////////////////////////////////////////////////
// WINNOW
//
@ -2128,34 +1551,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}
}
///////////////////////////////////////////////////////////////////////////
// BUILTIN BOUNDS
//
// These cover the traits that are built-in to the language
// itself: `Copy`, `Clone` and `Sized`.
fn assemble_builtin_bound_candidates(
&mut self,
conditions: BuiltinImplConditions<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
match conditions {
BuiltinImplConditions::Where(nested) => {
debug!("builtin_bound: nested={:?}", nested);
candidates
.vec
.push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
}
BuiltinImplConditions::None => {}
BuiltinImplConditions::Ambiguous => {
debug!("assemble_builtin_bound_candidates: ambiguous builtin");
candidates.ambiguous = true;
}
}
Ok(())
}
fn sized_conditions(
&mut self,
obligation: &TraitObligation<'tcx>,