Add a regression test for issue rust-lang#63460

Author: petrochenkov

src/test/ui/hygiene/eager-from-opaque-2.rs

@@ -0,0 +1,22 @@
+// Regression test for the issue #63460.
+
+// check-pass
+
+#[macro_export]
+macro_rules! separator {
+    () => { "/" };
+}
+
+#[macro_export]
+macro_rules! concat_separator {
+    ( $e:literal, $($other:literal),+ ) => {
+        concat!($e, $crate::separator!(), $crate::concat_separator!($($other),+))
+    };
+    ( $e:literal ) => {
+        $e
+    }
+}
+
+fn main() {
+    println!("{}", concat_separator!(2, 3, 4))
+}