Rollup merge of #122076 - WaffleLapkin:mplace-args, r=RalfJung

Tweak the way we protect in-place function arguments in interpreters

Use `MPlaceTy` instead of `PlaceTy` in `FnArg` and ignore (copy) locals in an earlier step ("Locals that don't have their address taken are as protected as they can ever be").

This seems to be crucial for tail call support (as they can't refer to caller's locals which are killed when replacing the stack frame).

r? `@RalfJung`
cc `@oli-obk`
see rust-lang/rust#121273 (comment)

Author: matthiaskrgr

src/machine.rs

@@ -8,7 +8,6 @@ use std::fmt;
 use std::path::Path;
 use std::process;
 
-use either::Either;
 use rand::rngs::StdRng;
 use rand::Rng;
 use rand::SeedableRng;
@@ -962,7 +961,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
             // to run extra MIR), and Ok(Some(body)) if we found MIR to run for the
             // foreign function
             // Any needed call to `goto_block` will be performed by `emulate_foreign_item`.
-            let args = ecx.copy_fn_args(args)?; // FIXME: Should `InPlace` arguments be reset to uninit?
+            let args = ecx.copy_fn_args(args); // FIXME: Should `InPlace` arguments be reset to uninit?
             let link_name = ecx.item_link_name(instance.def_id());
             return ecx.emulate_foreign_item(link_name, abi, &args, dest, ret, unwind);
         }
@@ -981,7 +980,7 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
         ret: Option<mir::BasicBlock>,
         unwind: mir::UnwindAction,
     ) -> InterpResult<'tcx> {
-        let args = ecx.copy_fn_args(args)?; // FIXME: Should `InPlace` arguments be reset to uninit?
+        let args = ecx.copy_fn_args(args); // FIXME: Should `InPlace` arguments be reset to uninit?
         ecx.emulate_dyn_sym(fn_val, abi, &args, dest, ret, unwind)
     }
 
@@ -1334,18 +1333,12 @@ impl<'mir, 'tcx> Machine<'mir, 'tcx> for MiriMachine<'mir, 'tcx> {
 
     fn protect_in_place_function_argument(
         ecx: &mut InterpCx<'mir, 'tcx, Self>,
-        place: &PlaceTy<'tcx, Provenance>,
+        place: &MPlaceTy<'tcx, Provenance>,
     ) -> InterpResult<'tcx> {
         // If we have a borrow tracker, we also have it set up protection so that all reads *and
         // writes* during this call are insta-UB.
         let protected_place = if ecx.machine.borrow_tracker.is_some() {
-            // Have to do `to_op` first because a `Place::Local` doesn't imply the local doesn't have an address.
-            if let Either::Left(place) = ecx.place_to_op(place)?.as_mplace_or_imm() {
-                ecx.protect_place(&place)?.into()
-            } else {
-                // Locals that don't have their address taken are as protected as they can ever be.
-                place.clone()
-            }
+            ecx.protect_place(&place)?.into()
         } else {
             // No borrow tracker.
             place.clone()