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Rollup merge of rust-lang#77305 - lcnr:candidate_from_obligation, r=d…
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…avidtwco

move candidate_from_obligation_no_cache

It's only called from `candidate_from_obligation` which is already in this file.
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@Dylan-DPC
Dylan-DPC authored Sep 30, 2020
2 parents 7d37b6d + db5b70f commit bdab60f
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162 changes: 161 additions & 1 deletion compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -7,14 +7,19 @@
//! [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>(
Expand Down Expand Up @@ -62,6 +67,161 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
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>,
Expand Down
155 changes: 0 additions & 155 deletions compiler/rustc_trait_selection/src/traits/select/mod.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1029,161 +1029,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, '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);

Expand Down

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