check types in debug mode in impl Debug for Expr, helping to catch bugs

crates/fayalite/src/expr.rs

@@ -274,6 +274,17 @@ pub struct Expr<T: Type> {
 
 impl<T: Type + fmt::Debug> fmt::Debug for Expr<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        #[cfg(debug_assertions)]
+        {
+            let Self {
+                __enum,
+                __ty,
+                __flow,
+            } = self;
+            let expr_ty = __ty.canonical();
+            let enum_ty = __enum.to_expr().__ty;
+            assert_eq!(expr_ty, enum_ty, "expr ty mismatch:\nExpr {{\n__enum: {__enum:?},\n__ty: {__ty:?},\n__flow: {__flow:?}\n}}");
+        }
         self.__enum.fmt(f)
     }
 }