interpret: always enable write_immediate sanity checks

Author: RalfJung

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -118,6 +118,7 @@ impl<Prov: Provenance> Immediate<Prov> {
             (Immediate::Scalar(scalar), Abi::Scalar(s)) => {
                 assert_eq!(scalar.size(), s.size(cx));
                 if !matches!(s.primitive(), abi::Pointer(..)) {
+                    // This is not a pointer, it should not carry provenance.
                     assert!(matches!(scalar, Scalar::Int(..)));
                 }
             }

compiler/rustc_const_eval/src/interpret/place.rs

@@ -651,10 +651,10 @@ where
                 if !self.validation_in_progress() {
                     M::after_local_write(self, local, /*storage_live*/ false)?;
                 }
-                // Double-check that the value we are storing and the local fit to each other.
-                if cfg!(debug_assertions) {
-                    src.assert_matches_abi(local_layout.abi, self);
-                }
+                // Double-check that the value we are storing and the local fit to each other. We do
+                // this even with debug assertions as interpreter behavior can get really strange
+                // when this is violated.
+                src.assert_matches_abi(local_layout.abi, self);
             }
             Left(mplace) => {
                 self.write_immediate_to_mplace_no_validate(src, mplace.layout, mplace.mplace)?;
@@ -672,6 +672,7 @@ where
         layout: TyAndLayout<'tcx>,
         dest: MemPlace<M::Provenance>,
     ) -> InterpResult<'tcx> {
+        // Ensure the value matches the type of the place it is written to.
         if cfg!(debug_assertions) {
             value.assert_matches_abi(layout.abi, self);
         }