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move candidate_from_obligation out of assembly
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it doesn't belong there as it also does winnowing
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@lcnr
lcnr committed Nov 29, 2022
1 parent e0098a5 commit bb982df
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213 changes: 3 additions & 210 deletions compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,225 +9,18 @@ use hir::LangItem;
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,
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