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Rollup merge of #80732 - spastorino:trait-inheritance-self2, r=nikomatsakis

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Allow Trait inheritance with cycles on associated types take 2

This reverts the revert of #79209 and fixes the ICEs that's occasioned by that PR exposing some problems that are addressed in #80648 and #79811.
For easier review I'd say, check only the last commit, the first one is just a revert of the revert of #79209 which was already approved.

This also could be considered part or the actual fix of #79560 but I guess for that to be closed and fixed completely we would need to land #80648 and #79811 too.

r? `@nikomatsakis`
cc `@Aaron1011`
This commit is contained in:
Dylan DPC 2021-02-09 02:39:50 +01:00 • committed by GitHub
commit 44e526b2c3
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30 changed files with 583 additions and 152 deletions
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41
compiler/rustc_infer/src/traits/util.rs
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@ -1,9 +1,10 @@
use smallvec::smallvec;
use crate::traits::{Obligation, ObligationCause, PredicateObligation};
use rustc_data_structures::fx::FxHashSet;
use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
use rustc_middle::ty::outlives::Component;
use rustc_middle::ty::{self, ToPredicate, TyCtxt, WithConstness};
use rustc_span::symbol::Ident;
pub fn anonymize_predicate<'tcx>(
tcx: TyCtxt<'tcx>,
@ -282,6 +283,44 @@ pub fn transitive_bounds<'tcx>(
elaborate_trait_refs(tcx, bounds).filter_to_traits()
}
/// A specialized variant of `elaborate_trait_refs` that only elaborates trait references that may
/// define the given associated type `assoc_name`. It uses the
/// `super_predicates_that_define_assoc_type` query to avoid enumerating super-predicates that
/// aren't related to `assoc_item`. This is used when resolving types like `Self::Item` or
/// `T::Item` and helps to avoid cycle errors (see e.g. #35237).
pub fn transitive_bounds_that_define_assoc_type<'tcx>(
tcx: TyCtxt<'tcx>,
bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
assoc_name: Ident,
) -> impl Iterator<Item = ty::PolyTraitRef<'tcx>> {
let mut stack: Vec<_> = bounds.collect();
let mut visited = FxIndexSet::default();
std::iter::from_fn(move || {
while let Some(trait_ref) = stack.pop() {
let anon_trait_ref = tcx.anonymize_late_bound_regions(trait_ref);
if visited.insert(anon_trait_ref) {
let super_predicates = tcx.super_predicates_that_define_assoc_type((
trait_ref.def_id(),
Some(assoc_name),
));
for (super_predicate, _) in super_predicates.predicates {
let bound_predicate = super_predicate.kind();
let subst_predicate = super_predicate
.subst_supertrait(tcx, &bound_predicate.rebind(trait_ref.skip_binder()));
if let Some(binder) = subst_predicate.to_opt_poly_trait_ref() {
stack.push(binder.value);
}
}
return Some(trait_ref);
}
}
return None;
})
}
///////////////////////////////////////////////////////////////////////////
// Other
///////////////////////////////////////////////////////////////////////////