Add a test for an opaque macro eagerly expanding its arguments

Author: petrochenkov

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

@@ -0,0 +1,18 @@
+// Opaque macro can eagerly expand its input without breaking its resolution.
+// Regression test for issue #63685.
+
+macro_rules! foo {
+    () => {
+        "foo"
+    };
+}
+
+macro_rules! bar {
+    () => {
+        foo!() //~ ERROR cannot find macro `foo!` in this scope
+    };
+}
+
+fn main() {
+    format_args!(bar!());
+}

src/test/ui/hygiene/eager-from-opaque.stderr

@@ -0,0 +1,8 @@
+error: cannot find macro `foo!` in this scope
+  --> $DIR/eager-from-opaque.rs:12:9
+   |
+LL |         foo!()
+   |         ^^^
+
+error: aborting due to previous error
+