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Remove this check for object-safety during selection of trait object candidates

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I don't really understand what it's for, but see the comment here:
https://github.com/rust-lang/rust/pull/50173#discussion_r204222336

where arielb1 said

> Does this check do anything these days? I think `$0: Trait` is always considered ambiguous

and nikomatsakis agreed we may be able to get rid of it
This commit is contained in:
Michael Hewson 2018-10-17 18:54:13 -04:00
parent eb997d76d5
commit 3db22039dc
1 changed files with 0 additions and 11 deletions
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11
src/librustc/traits/select.rs
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@ -2111,17 +2111,6 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
data.principal().with_self_ty(this.tcx(), self_ty)
}
ty::Infer(ty::TyVar(_)) => {
// Object-safety candidates are only applicable to object-safe
// traits. Including this check is useful because it helps
// inference in cases of traits like `BorrowFrom`, which are
// not object-safe, and which rely on being able to infer the
// self-type from one of the other inputs. Without this check,
// these cases wind up being considered ambiguous due to a
// (spurious) ambiguity introduced here.
let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
if !this.tcx().is_object_safe(predicate_trait_ref.def_id()) {
return;
}
debug!("assemble_candidates_from_object_ty: ambiguous");
candidates.ambiguous = true; // could wind up being an object type
return;