Auto merge of #122243 - RalfJung:local-place-sanity-check, r=oli-obk

interpret: ensure that Place is never used for a different frame

We store the address where the stack frame stores its `locals`. The idea is that even if we pop and push, or switch to a different thread with a larger number of frames, then the `locals` address will most likely change so we'll notice that problem. This is made possible by some recent changes by `@WaffleLapkin,` where we no longer use `Place` across things that change the number of stack frames.

I made these debug assertions for now, just to make sure this can't cost us any perf.

The first commit is unrelated but it's a one-line comment change so it didn't warrant a separate PR...

r? `@oli-obk`

compiler/rustc_mir_transform/src/add_retag.rs

@@ -118,7 +118,7 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
         }
 
         // PART 3
-        // Add retag after assignments where data "enters" this function: the RHS is behind a deref and the LHS is not.
+        // Add retag after assignments.
         for block_data in basic_blocks {
             // We want to insert statements as we iterate. To this end, we
             // iterate backwards using indices.