interpret: fix mir::UnOp layout computation

compiler/rustc_const_eval/src/interpret/step.rs

@@ -190,8 +190,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             }
 
             UnaryOp(un_op, ref operand) => {
-                // The operand always has the same type as the result.
-                let val = self.read_immediate(&self.eval_operand(operand, Some(dest.layout))?)?;
+                let layout = util::unop_homogeneous(un_op).then_some(dest.layout);
+                let val = self.read_immediate(&self.eval_operand(operand, layout)?)?;
                 let result = self.unary_op(un_op, &val)?;
                 assert_eq!(result.layout, dest.layout, "layout mismatch for result of {un_op:?}");
                 self.write_immediate(*result, &dest)?;

compiler/rustc_const_eval/src/util/mod.rs

@@ -38,3 +38,13 @@ pub fn binop_right_homogeneous(op: mir::BinOp) -> bool {
         Offset | Shl | ShlUnchecked | Shr | ShrUnchecked => false,
     }
 }
+
+/// Classify whether an operator is "homogeneous", i.e., the operand has the
+/// same type as the result.
+#[inline]
+pub fn unop_homogeneous(op: mir::UnOp) -> bool {
+    match op {
+        mir::UnOp::Not | mir::UnOp::Neg => true,
+        mir::UnOp::PtrMetadata => false,
+    }
+}