Rollup merge of rust-lang#109408 - RalfJung:retags, r=compiler-errors

not *all* retags might be explicit in Runtime MIR

In rust-lang#105317 I made Miri treat `Rvalue::Ref/AddrOf` as implicit retagging sites. This updates the MIR docs accordingly.

For `Rvalue::Ref` I think this makes a lot more sense: creating a new reference is their entire point, so we can avoid bloating the MIR with retags. Also this seems to be the best way to handle cases like `*ptr = &[mut] ...`, where doing a retag is somewhat questionable since maybe `*ptr` points to another place now?

For `Rvalue::AddrOf`, Stacked Borrows needs this because even raw ptrs need some retagging, but Tree Borrows doesn't do ant retagging here and I hope we'll end up with a model where raw pointers don't get retagged.

compiler/rustc_middle/src/mir/syntax.rs

@@ -78,7 +78,8 @@ pub enum MirPhase {
     ///    MIR, this is UB.
     ///  - Retags: If `-Zmir-emit-retag` is enabled, analysis MIR has "implicit" retags in the same way
     ///    that Rust itself has them. Where exactly these are is generally subject to change, and so we
-    ///    don't document this here. Runtime MIR has all retags explicit.
+    ///    don't document this here. Runtime MIR has most retags explicit (though implicit retags
+    ///    can still occur at `Rvalue::{Ref,AddrOf}`).
     ///  - Generator bodies: In analysis MIR, locals may actually be behind a pointer that user code has
     ///    access to. This occurs in generator bodies. Such locals do not behave like other locals,
     ///    because they eg may be aliased in surprising ways. Runtime MIR has no such special locals -