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Don't assemble non-env/bound candidates if projection is rigid

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This commit is contained in:
Michael Goulet
2025-04-17 19:06:46 +00:00
parent 1f76d219c9
commit e882ff4e7e
6 changed files with 107 additions and 56 deletions
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87
compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
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@@ -288,6 +288,21 @@ where
) -> Vec<Candidate<I>>;
}
/// Allows callers of `assemble_and_evaluate_candidates` to choose whether to limit
/// candidate assembly to param-env and alias-bound candidates.
///
/// On top of being a micro-optimization, as it avoids doing unnecessary work when
/// a param-env trait bound candidate shadows impls for normalization, this is also
/// required to prevent query cycles due to RPITIT inference. See the issue at:
/// <https://github.com/rust-lang/trait-system-refactor-initiative/issues/173>.
pub(super) enum AssembleCandidatesFrom {
All,
/// Only assemble candidates from the environment and alias bounds, ignoring
/// user-written and built-in impls. We only expect `ParamEnv` and `AliasBound`
/// candidates to be assembled.
EnvAndBounds,
}
impl<D, I> EvalCtxt<'_, D>
where
D: SolverDelegate<Interner = I>,
@@ -296,6 +311,7 @@ where
pub(super) fn assemble_and_evaluate_candidates<G: GoalKind<D>>(
&mut self,
goal: Goal<I, G>,
assemble_from: AssembleCandidatesFrom,
) -> Vec<Candidate<I>> {
let Ok(normalized_self_ty) =
self.structurally_normalize_ty(goal.param_env, goal.predicate.self_ty())
@@ -322,16 +338,18 @@ where
}
}
self.assemble_impl_candidates(goal, &mut candidates);
self.assemble_builtin_impl_candidates(goal, &mut candidates);
self.assemble_alias_bound_candidates(goal, &mut candidates);
self.assemble_object_bound_candidates(goal, &mut candidates);
self.assemble_param_env_candidates(goal, &mut candidates);
match assemble_from {
AssembleCandidatesFrom::All => {
self.assemble_impl_candidates(goal, &mut candidates);
self.assemble_builtin_impl_candidates(goal, &mut candidates);
self.assemble_object_bound_candidates(goal, &mut candidates);
}
AssembleCandidatesFrom::EnvAndBounds => {}
}
candidates
}
@@ -754,6 +772,9 @@ where
})
}
/// Assemble and merge candidates for goals which are related to an underlying trait
/// goal. Right now, this is normalizes-to and host effect goals.
///
/// We sadly can't simply take all possible candidates for normalization goals
/// and check whether they result in the same constraints. We want to make sure
/// that trying to normalize an alias doesn't result in constraints which aren't
@@ -782,47 +803,44 @@ where
///
/// See trait-system-refactor-initiative#124 for more details.
#[instrument(level = "debug", skip(self, inject_normalize_to_rigid_candidate), ret)]
pub(super) fn merge_candidates(
pub(super) fn assemble_and_merge_candidates<G: GoalKind<D>>(
&mut self,
proven_via: Option<TraitGoalProvenVia>,
candidates: Vec<Candidate<I>>,
goal: Goal<I, G>,
inject_normalize_to_rigid_candidate: impl FnOnce(&mut EvalCtxt<'_, D>) -> QueryResult<I>,
) -> QueryResult<I> {
let Some(proven_via) = proven_via else {
// We don't care about overflow. If proving the trait goal overflowed, then
// it's enough to report an overflow error for that, we don't also have to
// overflow during normalization.
return Ok(self.make_ambiguous_response_no_constraints(MaybeCause::Ambiguity));
//
// We use `forced_ambiguity` here over `make_ambiguous_response_no_constraints`
// because the former will also record a built-in candidate in the inspector.
return self.forced_ambiguity(MaybeCause::Ambiguity).map(|cand| cand.result);
};
match proven_via {
TraitGoalProvenVia::ParamEnv | TraitGoalProvenVia::AliasBound => {
let mut considered_candidates = Vec::new();
considered_candidates.extend(
candidates
.iter()
.filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
.map(|c| c.result),
);
// Even when a trait bound has been proven using a where-bound, we
// still need to consider alias-bounds for normalization, see
// tests/ui/next-solver/alias-bound-shadowed-by-env.rs.
//
// `tests/ui/next-solver/alias-bound-shadowed-by-env.rs`.
let candidates_from_env_and_bounds: Vec<_> = self
.assemble_and_evaluate_candidates(goal, AssembleCandidatesFrom::EnvAndBounds);
// We still need to prefer where-bounds over alias-bounds however.
// See tests/ui/winnowing/norm-where-bound-gt-alias-bound.rs.
//
// FIXME(const_trait_impl): should this behavior also be used by
// constness checking. Doing so is *at least theoretically* breaking,
// see github.com/rust-lang/rust/issues/133044#issuecomment-2500709754
if considered_candidates.is_empty() {
considered_candidates.extend(
candidates
.iter()
.filter(|c| matches!(c.source, CandidateSource::AliasBound))
.map(|c| c.result),
);
}
// See `tests/ui/winnowing/norm-where-bound-gt-alias-bound.rs`.
let mut considered_candidates: Vec<_> = if candidates_from_env_and_bounds
.iter()
.any(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
{
candidates_from_env_and_bounds
.into_iter()
.filter(|c| matches!(c.source, CandidateSource::ParamEnv(_)))
.map(|c| c.result)
.collect()
} else {
candidates_from_env_and_bounds.into_iter().map(|c| c.result).collect()
};
// If the trait goal has been proven by using the environment, we want to treat
// aliases as rigid if there are no applicable projection bounds in the environment.
@@ -839,6 +857,9 @@ where
}
}
TraitGoalProvenVia::Misc => {
let candidates =
self.assemble_and_evaluate_candidates(goal, AssembleCandidatesFrom::All);
// Prefer "orphaned" param-env normalization predicates, which are used
// (for example, and ideally only) when proving item bounds for an impl.
let candidates_from_env: Vec<_> = candidates