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candidate_assembly.rs
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candidate_assembly.rs
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//! 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 hir::LangItem;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_infer::traits::TraitEngine;
use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
use rustc_lint_defs::builtin::DEREF_INTO_DYN_SUPERTRAIT;
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, ToPredicate, Ty, TypeVisitable};
use rustc_target::spec::abi::Abi;
use crate::traits;
use crate::traits::coherence::Conflict;
use crate::traits::query::evaluate_obligation::InferCtxtExt;
use crate::traits::{util, SelectionResult};
use crate::traits::{Ambiguous, ErrorReporting, Overflow, Unimplemented};
use super::BuiltinImplConditions;
use super::IntercrateAmbiguityCause;
use super::OverflowError;
use super::SelectionCandidate::{self, *};
use super::{EvaluatedCandidate, SelectionCandidateSet, SelectionContext, TraitObligationStack};
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
#[instrument(level = "debug", skip(self))]
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!(?cache_fresh_trait_pred);
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!(candidate = ?c, "CACHE HIT");
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!(?candidate, "CACHE MISS");
self.insert_candidate_cache(
stack.obligation.param_env,
cache_fresh_trait_pred,
dep_node,
candidate.clone(),
);
candidate
}
fn candidate_from_obligation_no_cache<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
if let Some(conflict) = self.is_knowable(stack) {
debug!("coherence stage: not knowable");
if self.intercrate_ambiguity_causes.is_some() {
debug!("evaluate_stack: intercrate_ambiguity_causes is some");
// Heuristics: show the diagnostics when there are no candidates in crate.
if let Ok(candidate_set) = self.assemble_candidates(stack) {
let mut no_candidates_apply = true;
for c in candidate_set.vec.iter() {
if self.evaluate_candidate(stack, &c)?.may_apply() {
no_candidates_apply = false;
break;
}
}
if !candidate_set.ambiguous && no_candidates_apply {
let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
let self_ty = trait_ref.self_ty();
let (trait_desc, self_desc) = with_no_trimmed_paths!({
let trait_desc = trait_ref.print_only_trait_path().to_string();
let self_desc = if self_ty.has_concrete_skeleton() {
Some(self_ty.to_string())
} else {
None
};
(trait_desc, self_desc)
});
let cause = if let Conflict::Upstream = conflict {
IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc }
} else {
IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
};
debug!(?cause, "evaluate_stack: pushing cause");
self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
}
}
}
return Ok(None);
}
let candidate_set = self.assemble_candidates(stack)?;
if candidate_set.ambiguous {
debug!("candidate set contains ambig");
return Ok(None);
}
let candidates = candidate_set.vec;
debug!(?stack, ?candidates, "assembled {} candidates", candidates.len());
// 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.
let mut candidates = self.filter_impls(candidates, stack.obligation);
// 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 {
return self.filter_reservation_impls(candidates.pop().unwrap(), stack.obligation);
}
// Winnow, but record the exact outcome of evaluation, which
// is needed for specialization. Propagate overflow if it occurs.
let mut candidates = candidates
.into_iter()
.map(|c| match self.evaluate_candidate(stack, &c) {
Ok(eval) if eval.may_apply() => {
Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
}
Ok(_) => Ok(None),
Err(OverflowError::Canonical) => Err(Overflow(OverflowError::Canonical)),
Err(OverflowError::ErrorReporting) => Err(ErrorReporting),
Err(OverflowError::Error(e)) => Err(Overflow(OverflowError::Error(e))),
})
.flat_map(Result::transpose)
.collect::<Result<Vec<_>, _>>()?;
debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
let needs_infer = stack.obligation.predicate.has_infer_types_or_consts();
// If there are STILL multiple candidates, we can further
// reduce the list by dropping duplicates -- including
// resolving specializations.
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],
needs_infer,
)
});
if is_dup {
debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
candidates.swap_remove(i);
} else {
debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
i += 1;
// If there are *STILL* multiple candidates, give up
// and report ambiguity.
if i > 1 {
debug!("multiple matches, ambig");
return Err(Ambiguous(
candidates
.into_iter()
.filter_map(|c| match c.candidate {
SelectionCandidate::ImplCandidate(def_id) => Some(def_id),
_ => None,
})
.collect(),
));
}
}
}
}
// If there are *NO* candidates, then 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() {
// If there's an error type, 'downgrade' our result from
// `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
// emitting additional spurious errors, since we're guaranteed
// to have emitted at least one.
if stack.obligation.predicate.references_error() {
debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous");
return Ok(None);
}
return Err(Unimplemented);
}
// Just one candidate left.
self.filter_reservation_impls(candidates.pop().unwrap().candidate, stack.obligation)
}
#[instrument(skip(self, stack), level = "debug")]
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() {
debug!(ty = ?obligation.predicate.skip_binder().self_ty(), "ambiguous inference var or opaque type");
// 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 };
// The only way to prove a NotImplemented(T: Foo) predicate is via a negative impl.
// There are no compiler built-in rules for this.
if obligation.polarity() == ty::ImplPolarity::Negative {
self.assemble_candidates_for_trait_alias(obligation, &mut candidates);
self.assemble_candidates_from_impls(obligation, &mut candidates);
} else {
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 = ?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.pointee_trait() == Some(def_id) {
// `Pointee` is automatically implemented for every type.
candidates.vec.push(PointeeCandidate);
} 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.destruct_trait() == Some(def_id) {
self.assemble_const_destruct_candidates(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)
}
#[tracing::instrument(level = "debug", skip(self, candidates))]
fn assemble_candidates_from_projected_tys(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// 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(|_| self.match_projection_obligation_against_definition_bounds(obligation));
candidates.vec.extend(result.into_iter().map(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.
#[tracing::instrument(level = "debug", skip(self, stack, candidates))]
fn assemble_candidates_from_caller_bounds<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) -> Result<(), SelectionError<'tcx>> {
debug!(?stack.obligation);
let all_bounds = stack
.obligation
.param_env
.caller_bounds()
.iter()
.filter_map(|o| o.to_opt_poly_trait_pred());
// 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.
for bound in matching_bounds {
// FIXME(oli-obk): it is suspicious that we are dropping the constness and
// polarity here.
let wc = self.where_clause_may_apply(stack, bound.map_bound(|t| t.trait_ref))?;
if wc.may_apply() {
candidates.vec.push(ParamCandidate(bound));
}
}
Ok(())
}
fn assemble_generator_candidates(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
if self.tcx().lang_items().gen_trait() != Some(obligation.predicate.def_id()) {
return;
}
// 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!(?self_ty, ?obligation, "assemble_generator_candidates",);
candidates.vec.push(GeneratorCandidate);
}
ty::Infer(ty::TyVar(_)) => {
debug!("assemble_generator_candidates: ambiguous self-type");
candidates.ambiguous = true;
}
_ => {}
}
}
/// 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>,
) {
let Some(kind) = self.tcx().fn_trait_kind_from_lang_item(obligation.predicate.def_id()) else {
return;
};
// 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!(?kind, ?obligation, "assemble_unboxed_candidates");
match self.infcx.closure_kind(closure_substs) {
Some(closure_kind) => {
debug!(?closure_kind, "assemble_unboxed_candidates");
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;
}
_ => {}
}
}
/// Implements one of the `Fn()` family for a fn pointer.
fn assemble_fn_pointer_candidates(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'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;
}
// 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::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 { is_const: false });
}
}
// Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396).
ty::FnDef(def_id, _) => {
if let ty::FnSig {
unsafety: hir::Unsafety::Normal,
abi: Abi::Rust,
c_variadic: false,
..
} = self_ty.fn_sig(self.tcx()).skip_binder()
{
if self.tcx().codegen_fn_attrs(def_id).target_features.is_empty() {
candidates
.vec
.push(FnPointerCandidate { is_const: self.tcx().is_const_fn(def_id) });
}
}
}
_ => {}
}
}
/// Searches for impls that might apply to `obligation`.
fn assemble_candidates_from_impls(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
debug!(?obligation, "assemble_candidates_from_impls");
// Essentially any user-written impl will match with an error type,
// so creating `ImplCandidates` isn't useful. However, we might
// end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized)
// This helps us avoid overflow: see issue #72839
// Since compilation is already guaranteed to fail, this is just
// to try to show the 'nicest' possible errors to the user.
// We don't check for errors in the `ParamEnv` - in practice,
// it seems to cause us to be overly aggressive in deciding
// to give up searching for candidates, leading to spurious errors.
if obligation.predicate.references_error() {
return;
}
self.tcx().for_each_relevant_impl(
obligation.predicate.def_id(),
obligation.predicate.skip_binder().trait_ref.self_ty(),
|impl_def_id| {
// Before we create the substitutions and everything, first
// consider a "quick reject". This avoids creating more types
// and so forth that we need to.
let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) {
return;
}
self.infcx.probe(|_| {
if let Ok(_substs) = self.match_impl(impl_def_id, impl_trait_ref, obligation) {
candidates.vec.push(ImplCandidate(impl_def_id));
}
});
},
);
}
fn assemble_candidates_from_auto_impls(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'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!(?self_ty, "assemble_candidates_from_auto_impls");
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
// `ui/typeck/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)),
}
}
}
/// Searches for impls that might apply to `obligation`.
fn assemble_candidates_from_object_ty(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
debug!(
self_ty = ?obligation.self_ty().skip_binder(),
"assemble_candidates_from_object_ty",
);
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!(?poly_trait_ref, "assemble_candidates_from_object_ty");
let poly_trait_predicate = self.infcx().resolve_vars_if_possible(obligation.predicate);
let placeholder_trait_predicate =
self.infcx().replace_bound_vars_with_placeholders(poly_trait_predicate);
// 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 candidate_supertraits = util::supertraits(self.tcx(), poly_trait_ref)
.enumerate()
.filter(|&(_, upcast_trait_ref)| {
self.infcx.probe(|_| {
self.match_normalize_trait_ref(
obligation,
upcast_trait_ref,
placeholder_trait_predicate.trait_ref,
)
.is_ok()
})
})
.map(|(idx, _)| ObjectCandidate(idx));
candidates.vec.extend(candidate_supertraits);
})
}
/// Temporary migration for #89190
fn need_migrate_deref_output_trait_object(
&mut self,
ty: Ty<'tcx>,
cause: &traits::ObligationCause<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> Option<(Ty<'tcx>, DefId)> {
let tcx = self.tcx();
if tcx.features().trait_upcasting {
return None;
}
// <ty as Deref>
let trait_ref = ty::TraitRef {
def_id: tcx.lang_items().deref_trait()?,
substs: tcx.mk_substs_trait(ty, &[]),
};
let obligation = traits::Obligation::new(
cause.clone(),
param_env,
ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx),
);
if !self.infcx.predicate_may_hold(&obligation) {
return None;
}
let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot();
let normalized_ty = fulfillcx.normalize_projection_type(
&self.infcx,
param_env,
ty::ProjectionTy {
item_def_id: tcx.lang_items().deref_target()?,
substs: trait_ref.substs,
},
cause.clone(),
);
let ty::Dynamic(data, ..) = normalized_ty.kind() else {
return None;
};
let def_id = data.principal_def_id()?;
return Some((normalized_ty, def_id));
}
/// 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 Some(source) = obligation.self_ty().no_bound_vars() else {
// Don't add any candidates if there are bound regions.
return;
};
let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
debug!(?source, ?target, "assemble_candidates_for_unsizing");
match (source.kind(), target.kind()) {
// Trait+Kx+'a -> Trait+Ky+'b (upcasts).
(&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
// Upcast coercions permit several 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`
// 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar`
//
// Note that neither of the first two of these changes requires any
// change at runtime. The third needs to change pointer metadata at runtime.
//
// We always perform upcasting coercions when we can because of reason
// #2 (region bounds).
let auto_traits_compatible = 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));
if auto_traits_compatible {
let principal_def_id_a = data_a.principal_def_id();
let principal_def_id_b = data_b.principal_def_id();
if principal_def_id_a == principal_def_id_b {
// no cyclic
candidates.vec.push(BuiltinUnsizeCandidate);
} else if principal_def_id_a.is_some() && principal_def_id_b.is_some() {
// not casual unsizing, now check whether this is trait upcasting coercion.
let principal_a = data_a.principal().unwrap();
let target_trait_did = principal_def_id_b.unwrap();
let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
if let Some((deref_output_ty, deref_output_trait_did)) = self
.need_migrate_deref_output_trait_object(
source,
&obligation.cause,
obligation.param_env,
)
{
if deref_output_trait_did == target_trait_did {
self.tcx().struct_span_lint_hir(
DEREF_INTO_DYN_SUPERTRAIT,
obligation.cause.body_id,
obligation.cause.span,
|lint| {
lint.build(&format!(
"`{}` implements `Deref` with supertrait `{}` as output",
source,
deref_output_ty
)).emit();
},
);
return;
}
}
for (idx, upcast_trait_ref) in
util::supertraits(self.tcx(), source_trait_ref).enumerate()
{
if upcast_trait_ref.def_id() == target_trait_did {
candidates.vec.push(TraitUpcastingUnsizeCandidate(idx));
}
}
}
}
}
// `T` -> `Trait`
(_, &ty::Dynamic(..)) => {
candidates.vec.push(BuiltinUnsizeCandidate);
}
// 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;
}
// `[T; n]` -> `[T]`
(&ty::Array(..), &ty::Slice(_)) => {
candidates.vec.push(BuiltinUnsizeCandidate);
}
// `Struct<T>` -> `Struct<U>`
(&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
if def_id_a == def_id_b {
candidates.vec.push(BuiltinUnsizeCandidate);
}
}
// `(.., T)` -> `(.., U)`
(&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
if tys_a.len() == tys_b.len() {
candidates.vec.push(BuiltinUnsizeCandidate);
}
}
_ => {}
};
}
#[tracing::instrument(level = "debug", skip(self, obligation, candidates))]
fn assemble_candidates_for_trait_alias(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'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!(?self_ty);
let def_id = obligation.predicate.def_id();
if self.tcx().is_trait_alias(def_id) {
candidates.vec.push(TraitAliasCandidate(def_id));
}
}
/// Assembles the trait which are built-in to the language itself:
/// `Copy`, `Clone` and `Sized`.
#[tracing::instrument(level = "debug", skip(self, candidates))]
fn assemble_builtin_bound_candidates(
&mut self,
conditions: BuiltinImplConditions<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
match conditions {
BuiltinImplConditions::Where(nested) => {
candidates
.vec
.push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
}
BuiltinImplConditions::None => {}
BuiltinImplConditions::Ambiguous => {
candidates.ambiguous = true;
}
}
}
fn assemble_const_destruct_candidates(
&mut self,
obligation: &TraitObligation<'tcx>,
candidates: &mut SelectionCandidateSet<'tcx>,
) {
// If the predicate is `~const Destruct` in a non-const environment, we don't actually need
// to check anything. We'll short-circuit checking any obligations in confirmation, too.
if !obligation.is_const() {
candidates.vec.push(ConstDestructCandidate(None));
return;
}
let self_ty = self.infcx().shallow_resolve(obligation.self_ty());
match self_ty.skip_binder().kind() {
ty::Opaque(..)
| ty::Dynamic(..)
| ty::Error(_)
| ty::Bound(..)
| ty::Param(_)
| ty::Placeholder(_)
| ty::Projection(_) => {
// We don't know if these are `~const Destruct`, at least
// not structurally... so don't push a candidate.
}
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Infer(ty::IntVar(_))
| ty::Infer(ty::FloatVar(_))
| ty::Str
| ty::RawPtr(_)
| ty::Ref(..)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Never
| ty::Foreign(_)
| ty::Array(..)
| ty::Slice(_)
| ty::Closure(..)
| ty::Generator(..)
| ty::Tuple(_)
| ty::GeneratorWitness(_) => {
// These are built-in, and cannot have a custom `impl const Destruct`.
candidates.vec.push(ConstDestructCandidate(None));
}
ty::Adt(..) => {
// Find a custom `impl Drop` impl, if it exists
let relevant_impl = self.tcx().find_map_relevant_impl(
self.tcx().require_lang_item(LangItem::Drop, None),
obligation.predicate.skip_binder().trait_ref.self_ty(),
Some,
);
if let Some(impl_def_id) = relevant_impl {
// Check that `impl Drop` is actually const, if there is a custom impl
if self.tcx().constness(impl_def_id) == hir::Constness::Const {
candidates.vec.push(ConstDestructCandidate(Some(impl_def_id)));
}
} else {
// Otherwise check the ADT like a built-in type (structurally)
candidates.vec.push(ConstDestructCandidate(None));
}
}
ty::Infer(_) => {
candidates.ambiguous = true;
}
}
}
}