Special note for str in auto traits

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -3107,6 +3107,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 self.tcx.def_span(def_id),
                                 "required because it's used within this closure",
                             ),
+                            ty::Str => err.note("`str` is considered to contain a `[u8]` slice for auto trait purposes"),
                             _ => err.note(&msg),
                         };
                     }

tests/ui/auto-traits/str-contains-slice-conceptually.stderr

@@ -4,7 +4,7 @@ error[E0277]: the trait bound `[u8]: AutoTrait` is not satisfied in `str`
 LL |   needs_auto_trait::<str>();
    |                      ^^^ within `str`, the trait `AutoTrait` is not implemented for `[u8]`
    |
-   = note: required because it appears within the type `str`
+   = note: `str` is considered to contain a `[u8]` slice for auto trait purposes
 note: required by a bound in `needs_auto_trait`
   --> $DIR/str-contains-slice-conceptually.rs:8:24
    |