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_const_eval/src/interpret/machine.rs

@@ -260,24 +260,6 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
         F2::NAN
     }
 
-    /// Called before writing the specified `local` of the `frame`.
-    /// Since writing a ZST is not actually accessing memory or locals, this is never invoked
-    /// for ZST reads.
-    ///
-    /// Due to borrow checker trouble, we indicate the `frame` as an index rather than an `&mut
-    /// Frame`.
-    #[inline(always)]
-    fn before_access_local_mut<'a>(
-        _ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
-        _frame: usize,
-        _local: mir::Local,
-    ) -> InterpResult<'tcx>
-    where
-        'tcx: 'mir,
-    {
-        Ok(())
-    }
-
     /// Called before a basic block terminator is executed.
     #[inline]
     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
@@ -531,7 +513,6 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
     #[inline(always)]
     fn after_local_allocated(
         _ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        _frame: usize,
         _local: mir::Local,
         _mplace: &MPlaceTy<'tcx, Self::Provenance>,
     ) -> InterpResult<'tcx> {