test: cross-edition metavar fragment specifiers

There's a subtle interaction between macros with metavar expressions and the
edition-dependent fragment matching behavior. This test illustrates the current
behavior when using macro-generating-macros across crate boundaries with
different editions.

Co-Authored-By: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Co-Authored-By: Eric Holk <eric@theincredibleholk.org>

tests/ui/macros/auxiliary/metavar_2021.rs

@@ -0,0 +1,12 @@
+//@ edition: 2021
+#[macro_export]
+macro_rules! make_matcher {
+    ($name:ident, $fragment_type:ident, $d:tt) => {
+        #[macro_export]
+        macro_rules! $name {
+            ($d _:$fragment_type) => { true };
+            (const { 0 }) => { false };
+        }
+    };
+}
+make_matcher!(is_expr_from_2021, expr, $);

tests/ui/macros/expr_2021_with_metavar_expr.rs

@@ -0,0 +1,28 @@
+//@ compile-flags: --edition=2024 -Z unstable-options
+//@ aux-build: metavar_2021.rs
+//@ run-pass
+
+// This test captures the behavior of macro-generating-macros with fragment
+// specifiers across edition boundaries.
+
+#![feature(expr_fragment_specifier_2024)]
+#![feature(macro_metavar_expr)]
+#![allow(incomplete_features)]
+
+extern crate metavar_2021;
+
+use metavar_2021::{is_expr_from_2021, make_matcher};
+
+make_matcher!(is_expr_from_2024, expr, $);
+
+fn main() {
+    let from_2021 = is_expr_from_2021!(const { 0 });
+    dbg!(from_2021);
+    let from_2024 = is_expr_from_2024!(const { 0 });
+    dbg!(from_2024);
+
+    // These capture the current, empirically determined behavior.
+    // It's not clear whether this is the desired behavior.
+    assert!(!from_2021);
+    assert!(!from_2024);
+}