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.

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

@@ -0,0 +1,13 @@
+#![feature(negative_impls)]
+#![feature(auto_traits)]
+
+auto trait AutoTrait {}
+
+impl<T> !AutoTrait for [T] {}
+
+fn needs_auto_trait<T: AutoTrait + ?Sized>() {}
+
+fn main() {
+  needs_auto_trait::<str>();
+  //~^ ERROR the trait bound `[u8]: AutoTrait` is not satisfied in `str`
+}

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

@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `[u8]: AutoTrait` is not satisfied in `str`
+  --> $DIR/str-contains-slice-conceptually.rs:11:22
+   |
+LL |   needs_auto_trait::<str>();
+   |                      ^^^ within `str`, the trait `AutoTrait` is not implemented for `[u8]`
+   |
+   = 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
+   |
+LL | fn needs_auto_trait<T: AutoTrait + ?Sized>() {}
+   |                        ^^^^^^^^^ required by this bound in `needs_auto_trait`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.