Add regression test for issue 398

    error[E0425]: cannot find value `_0` in this scope
       --> tests/test_display.rs:308:17
        |
    308 |         #[error("{0}")]
        |                 ^^^^^ not found in this scope

    error[E0425]: cannot find value `__display_x` in this scope
       --> tests/test_display.rs:310:17
        |
    310 |         #[error("{x}")]
        |                 ^^^^^ not found in this scope

tests/test_display.rs

@@ -266,7 +266,7 @@ fn test_pointer() {
 }
 
 #[test]
-fn test_macro_rules() {
+fn test_macro_rules_variant_from_call_site() {
     // Regression test for https://github.com/dtolnay/thiserror/issues/86
 
     macro_rules! decl_error {
@@ -291,6 +291,30 @@ fn test_macro_rules() {
     assert("0", Error1::Repro(0));
 }
 
+#[test]
+fn test_macro_rules_message_from_call_site() {
+    // Regression test for https://github.com/dtolnay/thiserror/issues/398
+
+    macro_rules! decl_error {
+        ($($errors:tt)*) => {
+            #[derive(Error, Debug)]
+            pub enum Error {
+                $($errors)*
+            }
+        };
+    }
+
+    decl_error! {
+        #[error("{0}")]
+        Unnamed(u8),
+        #[error("{x}")]
+        Named { x: u8 },
+    }
+
+    assert("0", Error::Unnamed(0));
+    assert("0", Error::Named { x: 0 });
+}
+
 #[test]
 fn test_raw() {
     #[derive(Error, Debug)]