use rustc_hir as hir;
use rustc_infer::traits::ObligationCause;
use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
-use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, 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::{ErrorReporting, Overflow, Unimplemented};
+use crate::traits::util;
use super::BuiltinImplConditions;
-use super::IntercrateAmbiguityCause;
-use super::OverflowError;
-use super::SelectionCandidate::{self, *};
-use super::{EvaluatedCandidate, SelectionCandidateSet, SelectionContext, TraitObligationStack};
+use super::SelectionCandidate::*;
+use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack};
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
- #[instrument(level = "debug", skip(self), ret)]
- 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!("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!("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 Err(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_non_region_infer();
-
- // 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 Ok(None);
- }
- }
- }
- }
-
- // 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,
// type/region parameters.
let self_ty = obligation.self_ty().skip_binder();
match self_ty.kind() {
- ty::Generator(..) => {
+ // async constructs get lowered to a special kind of generator that
+ // should *not* `impl Generator`.
+ ty::Generator(did, ..) if !self.tcx().generator_is_async(*did) => {
debug!(?self_ty, ?obligation, "assemble_generator_candidates",);
candidates.vec.push(GeneratorCandidate);
) {
let self_ty = obligation.self_ty().skip_binder();
if let ty::Generator(did, ..) = self_ty.kind() {
+ // async constructs get lowered to a special kind of generator that
+ // should directly `impl Future`.
if self.tcx().generator_is_async(*did) {
debug!(?self_ty, ?obligation, "assemble_future_candidates",);
projects / rust.git / blobdiff