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`

src/helpers.rs

@@ -413,7 +413,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 .ok_or_else(|| err_ub_format!("callee has fewer arguments than expected"))?;
             // Make the local live, and insert the initial value.
             this.storage_live(local)?;
-            let callee_arg = this.local_to_place(this.frame_idx(), local)?;
+            let callee_arg = this.local_to_place(local)?;
             this.write_immediate(*arg, &callee_arg)?;
         }
         if callee_args.next().is_some() {

src/machine.rs

@@ -1488,14 +1488,13 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
 
     fn after_local_allocated(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        frame: usize,
         local: mir::Local,
         mplace: &MPlaceTy<'tcx, Provenance>,
     ) -> InterpResult<'tcx> {
         let Some(Provenance::Concrete { alloc_id, .. }) = mplace.ptr().provenance else {
             panic!("after_local_allocated should only be called on fresh allocations");
         };
-        let local_decl = &ecx.active_thread_stack()[frame].body.local_decls[local];
+        let local_decl = &ecx.frame().body.local_decls[local];
         let span = local_decl.source_info.span;
         ecx.machine.allocation_spans.borrow_mut().insert(alloc_id, (span, None));
         Ok(())