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Preserve most sub-obligations in the projection cache

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Aaron Hill 2021-05-31 13:22:40 -05:00
parent 64929313f5
commit f2d9ee9c34
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GPG key ID: B4087E510E98B164
6 changed files with 41 additions and 82 deletions
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71
compiler/rustc_trait_selection/src/traits/project.rs
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@ -10,6 +10,7 @@ use super::PredicateObligation;
use super::Selection;
use super::SelectionContext;
use super::SelectionError;
use super::TraitQueryMode;
use super::{
ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
ImplSourceGeneratorData, ImplSourcePointeeData, ImplSourceUserDefinedData,
@ -18,7 +19,7 @@ use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
use crate::traits::error_reporting::InferCtxtExt;
use crate::traits::error_reporting::InferCtxtExt as _;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_errors::ErrorReported;
use rustc_hir::def_id::DefId;
@ -912,6 +913,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
}
let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
match project_type(selcx, &obligation) {
Ok(ProjectedTy::Progress(Progress {
ty: projected_ty,
@ -925,7 +927,7 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
let projected_ty = selcx.infcx().resolve_vars_if_possible(projected_ty);
debug!(?projected_ty, ?depth, ?projected_obligations);
let result = if projected_ty.has_projections() {
let mut result = if projected_ty.has_projections() {
let mut normalizer = AssocTypeNormalizer::new(
selcx,
param_env,
@ -942,8 +944,26 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
Normalized { value: projected_ty, obligations: projected_obligations }
};
let cache_value = prune_cache_value_obligations(infcx, &result);
infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, cache_value);
let mut canonical =
SelectionContext::with_query_mode(selcx.infcx(), TraitQueryMode::Canonical);
result.obligations.drain_filter(|projected_obligation| {
// If any global obligations always apply, considering regions, then we don't
// need to include them. The `is_global` check rules out inference variables,
// so there's no need for the caller of `opt_normalize_projection_type`
// to evaluate them.
// Note that we do *not* discard obligations that evaluate to
// `EvaluatedtoOkModuloRegions`. Evaluating these obligations
// inside of a query (e.g. `evaluate_obligation`) can change
// the result to `EvaluatedToOkModuloRegions`, while an
// `EvaluatedToOk` obligation will never change the result.
// See #85360 for more details
projected_obligation.is_global(canonical.tcx())
&& canonical
.evaluate_root_obligation(projected_obligation)
.map_or(false, |res| res.must_apply_considering_regions())
});
infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
obligations.extend(result.obligations);
Ok(Some(result.value))
}
@ -974,49 +994,6 @@ fn opt_normalize_projection_type<'a, 'b, 'tcx>(
}
}
/// If there are unresolved type variables, then we need to include
/// any subobligations that bind them, at least until those type
/// variables are fully resolved.
fn prune_cache_value_obligations<'a, 'tcx>(
infcx: &'a InferCtxt<'a, 'tcx>,
result: &NormalizedTy<'tcx>,
) -> NormalizedTy<'tcx> {
if infcx.unresolved_type_vars(&result.value).is_none() {
return NormalizedTy { value: result.value, obligations: vec![] };
}
let mut obligations: Vec<_> = result
.obligations
.iter()
.filter(|obligation| {
let bound_predicate = obligation.predicate.kind();
match bound_predicate.skip_binder() {
// We found a `T: Foo<X = U>` predicate, let's check
// if `U` references any unresolved type
// variables. In principle, we only care if this
// projection can help resolve any of the type
// variables found in `result.value` -- but we just
// check for any type variables here, for fear of
// indirect obligations (e.g., we project to `?0`,
// but we have `T: Foo<X = ?1>` and `?1: Bar<X =
// ?0>`).
ty::PredicateKind::Projection(data) => {
infcx.unresolved_type_vars(&bound_predicate.rebind(data.ty)).is_some()
}
// We are only interested in `T: Foo<X = U>` predicates, whre
// `U` references one of `unresolved_type_vars`. =)
_ => false,
}
})
.cloned()
.collect();
obligations.shrink_to_fit();
NormalizedTy { value: result.value, obligations }
}
/// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
/// hold. In various error cases, we cannot generate a valid
/// normalized projection. Therefore, we create an inference variable