interpret: avoid forcing all integer newtypes into memory during clear_provenance

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -175,6 +175,16 @@ impl<Prov: Provenance> Immediate<Prov> {
         }
         interp_ok(())
     }
+
+    pub fn has_provenance(&self) -> bool {
+        match self {
+            Immediate::Scalar(scalar) => matches!(scalar, Scalar::Ptr { .. }),
+            Immediate::ScalarPair(s1, s2) => {
+                matches!(s1, Scalar::Ptr { .. }) || matches!(s2, Scalar::Ptr { .. })
+            }
+            Immediate::Uninit => false,
+        }
+    }
 }
 
 // ScalarPair needs a type to interpret, so we often have an immediate and a type together

compiler/rustc_const_eval/src/interpret/place.rs

@@ -759,6 +759,13 @@ where
         &mut self,
         dest: &impl Writeable<'tcx, M::Provenance>,
     ) -> InterpResult<'tcx> {
+        // If this is an efficiently represented local variable without provenance, skip the
+        // `as_mplace_or_mutable_local` that would otherwise force this local into memory.
+        if let Right(imm) = dest.to_op(self)?.as_mplace_or_imm() {
+            if !imm.has_provenance() {
+                return interp_ok(());
+            }
+        }
         match self.as_mplace_or_mutable_local(&dest.to_place())? {
             Right((local_val, _local_layout, local)) => {
                 local_val.clear_provenance()?;