Rollup merge of rust-lang#105207 - RalfJung:interpret-clobber-return,…

… r=oli-obk

interpret: clobber return place when calling function

Makes sure the callee cannot observe the previous contents of the return place, and the caller cannot read any of the old return place contents even if the function unwinds.

I don't think we can test for this though, that would require some strange hand-written MIR.

r? ````@oli-obk````

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -676,6 +676,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         return_to_block: StackPopCleanup,
     ) -> InterpResult<'tcx> {
         trace!("body: {:#?}", body);
+        // Clobber previous return place contents, nobody is supposed to be able to see them any more
+        // This also checks dereferenceable, but not align. We rely on all constructed places being
+        // sufficiently aligned (in particular we rely on `deref_operand` checking alignment).
+        self.write_uninit(return_place)?;
         // first push a stack frame so we have access to the local substs
         let pre_frame = Frame {
             body,