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Rollup merge of #65850 - mikeyhew:patch-1, r=nikomatsakis

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Update comments re type parameter hack in object safety

To check if a method's receiver type is object safe, we create a new receiver type by substituting in a bogus type parameter (let's call it `U`) for `Self`, and checking that the unmodified receiver type implements `DispatchFromDyn<receiver type with Self = U>`. It would be better to use `dyn Trait` directly, and the only reason we don't is because it triggers another check that `Trait` is object safe, resulting in a query cycle. Once the feature `object_safe_for_dispatch` (tracking issue https://github.com/rust-lang/rust/issues/43561) is stabilized, this will no longer be the case, and we'll be able to use `dyn Trait` as the unsized `Self` type. I've updated the comments in object_safety.rs accordingly.

cc @Centril @nikomatsakis @bovinebuddha
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Mazdak Farrokhzad 2019-10-31 02:54:07 +01:00 • committed by GitHub
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12
src/librustc/traits/object_safety.rs
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@ -520,9 +520,11 @@ impl<'tcx> TyCtxt<'tcx> {
/// a pointer.
///
/// In practice, we cannot use `dyn Trait` explicitly in the obligation because it would result
/// in a new check that `Trait` is object safe, creating a cycle. So instead, we fudge a little
/// by introducing a new type parameter `U` such that `Self: Unsize<U>` and `U: Trait + ?Sized`,
/// and use `U` in place of `dyn Trait`. Written as a chalk-style query:
/// in a new check that `Trait` is object safe, creating a cycle (until object_safe_for_dispatch
/// is stabilized, see tracking issue https://github.com/rust-lang/rust/issues/43561).
/// Instead, we fudge a little by introducing a new type parameter `U` such that
/// `Self: Unsize<U>` and `U: Trait + ?Sized`, and use `U` in place of `dyn Trait`.
/// Written as a chalk-style query:
///
/// forall (U: Trait + ?Sized) {
/// if (Self: Unsize<U>) {
@ -556,8 +558,8 @@ impl<'tcx> TyCtxt<'tcx> {
// the type `U` in the query
// use a bogus type parameter to mimick a forall(U) query using u32::MAX for now.
// FIXME(mikeyhew) this is a total hack, and we should replace it when real forall queries
// are implemented
// FIXME(mikeyhew) this is a total hack. Once object_safe_for_dispatch is stabilized, we can
// replace this with `dyn Trait`
let unsized_self_ty: Ty<'tcx> = self.mk_ty_param(
::std::u32::MAX,
Symbol::intern("RustaceansAreAwesome"),