Add a test demonstrating the problem

tests/ui/macros/auxiliary/expr_2021_implicit.rs

@@ -0,0 +1,9 @@
+//@ edition:2021
+
+#[macro_export]
+macro_rules! m {
+    ($expr:expr) => {
+        compile_error!("did not expect an expression to be parsed");
+    };
+    (const { }) => {};
+}

tests/ui/macros/expr_2021_implicit_in_2024.rs

@@ -0,0 +1,10 @@
+//@ compile-flags: --edition=2024 -Zunstable-options
+//@ aux-build:expr_2021_implicit.rs
+
+extern crate expr_2021_implicit;
+
+// Makes sure that a `:expr` fragment matcher defined in a edition 2021 crate
+// still parses like an `expr_2021` fragment matcher in a 2024 user crate.
+expr_2021_implicit::m!(const {});
+
+fn main() {}

tests/ui/macros/expr_2021_implicit_in_2024.stderr

@@ -0,0 +1,10 @@
+error: did not expect an expression to be parsed
+  --> $DIR/expr_2021_implicit_in_2024.rs:8:1
+   |
+LL | expr_2021_implicit::m!(const {});
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: this error originates in the macro `expr_2021_implicit::m` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+