add test to guard against inaccurate diagnostic

Author: tshepang

tests/ui/macros/invalid-fragment-specifier.rs

@@ -0,0 +1,10 @@
+macro_rules! test {
+    ($wrong:id) => {};
+} //~^ ERROR: invalid fragment specifier `id`
+
+// guard against breaking raw identifier diagnostic
+macro_rules! test_raw_identifer {
+    ($wrong:r#if) => {};
+} //~^ ERROR: invalid fragment specifier `r#if`
+
+fn main() {}

tests/ui/macros/invalid-fragment-specifier.stderr

@@ -0,0 +1,18 @@
+error: invalid fragment specifier `id`
+  --> $DIR/invalid-fragment-specifier.rs:2:6
+   |
+LL |     ($wrong:id) => {};
+   |      ^^^^^^^^^
+   |
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`
+
+error: invalid fragment specifier `r#if`
+  --> $DIR/invalid-fragment-specifier.rs:7:6
+   |
+LL |     ($wrong:r#if) => {};
+   |      ^^^^^^^^^^^
+   |
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`
+
+error: aborting due to 2 previous errors
+