//! [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::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, TypeFoldable};
use rustc_target::spec::abi::Abi;
+use crate::traits::coherence::Conflict;
use crate::traits::{util, SelectionResult};
+use crate::traits::{Overflow, Unimplemented};
use super::BuiltinImplConditions;
+use super::IntercrateAmbiguityCause;
+use super::OverflowError;
use super::SelectionCandidate::{self, *};
-use super::{SelectionCandidateSet, SelectionContext, TraitObligationStack};
+use super::{EvaluatedCandidate, SelectionCandidateSet, SelectionContext, TraitObligationStack};
impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
pub(super) fn candidate_from_obligation<'o>(
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!("evaluate_stack: pushing cause = {:?}", cause);
+ self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
+ }
+ }
+ }
+ return Ok(None);
+ }
+
+ let candidate_set = self.assemble_candidates(stack)?;
+
+ if candidate_set.ambiguous {
+ debug!("candidate set contains ambig");
+ return Ok(None);
+ }
+
+ let mut candidates = candidate_set.vec;
+
+ debug!("assembled {} candidates for {:?}: {:?}", candidates.len(), stack, candidates);
+
+ // At this point, we know that each of the entries in the
+ // candidate set is *individually* applicable. Now we have to
+ // figure out if they contain mutual incompatibilities. This
+ // frequently arises if we have an unconstrained input type --
+ // for example, we are looking for `$0: Eq` where `$0` is some
+ // unconstrained type variable. In that case, we'll get a
+ // candidate which assumes $0 == int, one that assumes `$0 ==
+ // usize`, etc. This spells an ambiguity.
+
+ // If there is more than one candidate, first winnow them down
+ // by considering extra conditions (nested obligations and so
+ // forth). We don't winnow if there is exactly one
+ // candidate. This is a relatively minor distinction but it
+ // can lead to better inference and error-reporting. An
+ // example would be if there was an impl:
+ //
+ // impl<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_negative_and_reservation_impls(candidates.pop().unwrap());
+ }
+
+ // 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) => Err(Overflow),
+ })
+ .flat_map(Result::transpose)
+ .collect::<Result<Vec<_>, _>>()?;
+
+ debug!("winnowed to {} candidates for {:?}: {:?}", candidates.len(), stack, candidates);
+
+ let needs_infer = stack.obligation.predicate.needs_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!("Dropping candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]);
+ candidates.swap_remove(i);
+ } else {
+ debug!("Retaining candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]);
+ i += 1;
+
+ // If there are *STILL* multiple candidates, give up
+ // and report ambiguity.
+ if 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.references_error() {
+ debug!("no results for error type, treating as ambiguous");
+ return Ok(None);
+ }
+ return Err(Unimplemented);
+ }
+
+ // Just one candidate left.
+ self.filter_negative_and_reservation_impls(candidates.pop().unwrap().candidate)
+ }
+
pub(super) fn assemble_candidates<'o>(
&mut self,
stack: &TraitObligationStack<'o, 'tcx>,
Ok(Some(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!("evaluate_stack: pushing cause = {:?}", cause);
- self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
- }
- }
- }
- return Ok(None);
- }
-
- let candidate_set = self.assemble_candidates(stack)?;
-
- if candidate_set.ambiguous {
- debug!("candidate set contains ambig");
- return Ok(None);
- }
-
- let mut candidates = candidate_set.vec;
-
- debug!("assembled {} candidates for {:?}: {:?}", candidates.len(), stack, candidates);
-
- // At this point, we know that each of the entries in the
- // candidate set is *individually* applicable. Now we have to
- // figure out if they contain mutual incompatibilities. This
- // frequently arises if we have an unconstrained input type --
- // for example, we are looking for `$0: Eq` where `$0` is some
- // unconstrained type variable. In that case, we'll get a
- // candidate which assumes $0 == int, one that assumes `$0 ==
- // usize`, etc. This spells an ambiguity.
-
- // If there is more than one candidate, first winnow them down
- // by considering extra conditions (nested obligations and so
- // forth). We don't winnow if there is exactly one
- // candidate. This is a relatively minor distinction but it
- // can lead to better inference and error-reporting. An
- // example would be if there was an impl:
- //
- // impl<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_negative_and_reservation_impls(candidates.pop().unwrap());
- }
-
- // 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) => Err(Overflow),
- })
- .flat_map(Result::transpose)
- .collect::<Result<Vec<_>, _>>()?;
-
- debug!("winnowed to {} candidates for {:?}: {:?}", candidates.len(), stack, candidates);
-
- let needs_infer = stack.obligation.predicate.needs_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!("Dropping candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]);
- candidates.swap_remove(i);
- } else {
- debug!("Retaining candidate #{}/{}: {:?}", i, candidates.len(), candidates[i]);
- i += 1;
-
- // If there are *STILL* multiple candidates, give up
- // and report ambiguity.
- if 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.references_error() {
- debug!("no results for error type, treating as ambiguous");
- return Ok(None);
- }
- return Err(Unimplemented);
- }
-
- // Just one candidate left.
- self.filter_negative_and_reservation_impls(candidates.pop().unwrap().candidate)
- }
-
fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
debug!("is_knowable(intercrate={:?})", self.intercrate);
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