take back half-baked noaliasing check in Assignment

Author: RalfJung

compiler/rustc_const_eval/src/interpret/place.rs

@@ -869,8 +869,6 @@ where
             Ok(src_val) => {
                 assert!(!src.layout.is_unsized(), "cannot have unsized immediates");
                 // Yay, we got a value that we can write directly.
-                // FIXME: Add a check to make sure that if `src` is indirect,
-                // it does not overlap with `dest`.
                 return self.write_immediate_no_validate(*src_val, dest);
             }
             Err(mplace) => mplace,
@@ -890,7 +888,7 @@ where
         });
         assert_eq!(src.meta, dest.meta, "Can only copy between equally-sized instances");
 
-        self.mem_copy(src.ptr, src.align, dest.ptr, dest.align, size, /*nonoverlapping*/ true)
+        self.mem_copy(src.ptr, src.align, dest.ptr, dest.align, size, /*nonoverlapping*/ false)
     }
 
     /// Copies the data from an operand to a place. The layouts may disagree, but they must

compiler/rustc_const_eval/src/interpret/step.rs

@@ -158,6 +158,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         place: mir::Place<'tcx>,
     ) -> InterpResult<'tcx> {
         let dest = self.eval_place(place)?;
+        // FIXME: ensure some kind of non-aliasing between LHS and RHS?
+        // Also see https://github.com/rust-lang/rust/issues/68364.
 
         use rustc_middle::mir::Rvalue::*;
         match *rvalue {