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Auto merge of #132625 - compiler-errors:cache-only-if-opaque, r=lcnr

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Only disable cache if predicate has opaques within it

This is an alternative to https://github.com/rust-lang/rust/pull/132075.

This refines the check implemented in https://github.com/rust-lang/rust/pull/126024 to only disable the global cache if the predicate being considered has opaques in it. This is still theoretically unsound, since goals can indirectly rely on opaques in the defining scope, but we're much less likely to hit it.

It doesn't totally fix https://github.com/rust-lang/rust/issues/132064: for example, `lemmy` goes from 1:29 (on rust 1.81) to 9:53 (on nightly) to 4:07 (after this PR). But I think it's at least *more* sound than a total revert :/

r? lcnr
This commit is contained in:
bors 2024-11-06 21:22:14 +00:00
commit c07aa1e171
1 changed files with 18 additions and 6 deletions
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24
compiler/rustc_trait_selection/src/traits/select/mod.rs
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@ -1310,7 +1310,7 @@ fn check_evaluation_cache(
trait_pred: ty::PolyTraitPredicate<'tcx>, trait_pred: ty::PolyTraitPredicate<'tcx>,
) -> Option<EvaluationResult> { ) -> Option<EvaluationResult> {
let tcx = self.tcx(); let tcx = self.tcx();
if self.can_use_global_caches(param_env) { if self.can_use_global_caches(param_env, trait_pred) {
if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) { if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) {
return Some(res); return Some(res);
} }
@ -1331,7 +1331,7 @@ fn insert_evaluation_cache(
return; return;
} }
if self.can_use_global_caches(param_env) && !trait_pred.has_infer() { if self.can_use_global_caches(param_env, trait_pred) && !trait_pred.has_infer() {
debug!(?trait_pred, ?result, "insert_evaluation_cache global"); debug!(?trait_pred, ?result, "insert_evaluation_cache global");
// This may overwrite the cache with the same value // This may overwrite the cache with the same value
// FIXME: Due to #50507 this overwrites the different values // FIXME: Due to #50507 this overwrites the different values
@ -1476,7 +1476,11 @@ fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<
} }
/// Returns `true` if the global caches can be used. /// Returns `true` if the global caches can be used.
fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool { fn can_use_global_caches(
&self,
param_env: ty::ParamEnv<'tcx>,
pred: ty::PolyTraitPredicate<'tcx>,
) -> bool {
// If there are any inference variables in the `ParamEnv`, then we // If there are any inference variables in the `ParamEnv`, then we
// always use a cache local to this particular scope. Otherwise, we // always use a cache local to this particular scope. Otherwise, we
// switch to a global cache. // switch to a global cache.
@ -1494,7 +1498,15 @@ fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
TypingMode::Coherence => false, TypingMode::Coherence => false,
// Avoid using the global cache when we're defining opaque types // Avoid using the global cache when we're defining opaque types
// as their hidden type may impact the result of candidate selection. // as their hidden type may impact the result of candidate selection.
TypingMode::Analysis { defining_opaque_types } => defining_opaque_types.is_empty(), //
// HACK: This is still theoretically unsound. Goals can indirectly rely
// on opaques in the defining scope, and it's easier to do so with TAIT.
// However, if we disqualify *all* goals from being cached, perf suffers.
// This is likely fixed by better caching in general in the new solver.
// See: <https://github.com/rust-lang/rust/issues/132064>.
TypingMode::Analysis { defining_opaque_types } => {
defining_opaque_types.is_empty() || !pred.has_opaque_types()
}
// The global cache is only used if there are no opaque types in // The global cache is only used if there are no opaque types in
// the defining scope or we're outside of analysis. // the defining scope or we're outside of analysis.
// //
@ -1512,7 +1524,7 @@ fn check_candidate_cache(
let tcx = self.tcx(); let tcx = self.tcx();
let pred = cache_fresh_trait_pred.skip_binder(); let pred = cache_fresh_trait_pred.skip_binder();
if self.can_use_global_caches(param_env) { if self.can_use_global_caches(param_env, cache_fresh_trait_pred) {
if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) { if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
return Some(res); return Some(res);
} }
@ -1562,7 +1574,7 @@ fn insert_candidate_cache(
return; return;
} }
if self.can_use_global_caches(param_env) { if self.can_use_global_caches(param_env, cache_fresh_trait_pred) {
if let Err(Overflow(OverflowError::Canonical)) = candidate { if let Err(Overflow(OverflowError::Canonical)) = candidate {
// Don't cache overflow globally; we only produce this in certain modes. // Don't cache overflow globally; we only produce this in certain modes.
} else if !pred.has_infer() && !candidate.has_infer() { } else if !pred.has_infer() && !candidate.has_infer() {