Rollup merge of #107941 - compiler-errors:str-has-u8-slice-for-auto, r=lcnr

Treat `str` as containing `[u8]` for auto trait purposes

Wanted to gauge ``@rust-lang/lang`` and ``@rust-lang/types`` teams' thoughts on treating `str` as "containing" a `[u8]` slice for auto-trait purposes.

``@dtolnay`` brought this up in https://github.com/rust-lang/rust/issues/13231#issuecomment-1399386472 as a blocker for future `str` type librarification, and I think it's both a valid concern and very easy to fix. I'm interested in actually doing that `str` type librarification (#107939), but this probably should be considered in the mean time regardless of that PR.

r? types for the impl, though this definitely needs an FCP.

compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs

@@ -21,12 +21,14 @@ pub(super) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
         | ty::Float(_)
         | ty::FnDef(..)
         | ty::FnPtr(_)
-        | ty::Str
         | ty::Error(_)
         | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
         | ty::Never
         | ty::Char => Ok(vec![]),
 
+        // Treat this like `struct str([u8]);`
+        ty::Str => Ok(vec![tcx.mk_slice(tcx.types.u8)]),
+
         ty::Dynamic(..)
         | ty::Param(..)
         | ty::Foreign(..)