Add test that expr_2021 only works on 2024 edition

Co-authored-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Author: eholk

tests/ui/macros/expr_2021_old_edition.rs

@@ -0,0 +1,13 @@
+//@  compile-flags: --edition=2021
+
+// This test ensures that expr_2021 is not allowed on pre-2024 editions
+
+macro_rules! m {
+    ($e:expr_2021) => { //~ ERROR: invalid fragment specifier `expr_2021`
+        $e
+    };
+}
+
+fn main() {
+    m!(()); //~ ERROR: no rules expected the token `(`
+}

tests/ui/macros/expr_2021_old_edition.stderr

@@ -0,0 +1,25 @@
+error: invalid fragment specifier `expr_2021`
+  --> $DIR/expr_2021_old_edition.rs:6:6
+   |
+LL |     ($e:expr_2021) => {
+   |      ^^^^^^^^^^^^
+   |
+   = help: valid fragment specifiers are `ident`, `block`, `stmt`, `expr`, `pat`, `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, `item` and `vis`
+
+error: no rules expected the token `(`
+  --> $DIR/expr_2021_old_edition.rs:12:8
+   |
+LL | macro_rules! m {
+   | -------------- when calling this macro
+...
+LL |     m!(());
+   |        ^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$e:ident`
+  --> $DIR/expr_2021_old_edition.rs:6:6
+   |
+LL |     ($e:expr_2021) => {
+   |      ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+