Fix test case and issue number

Author: JohnTitor

src/test/ui/unboxed-closures/issue-30904.rs

@@ -0,0 +1,18 @@
+#![feature(fn_traits, unboxed_closures)]
+
+fn test<F: for<'x> FnOnce<(&'x str,)>>(_: F) {}
+
+struct Compose<F,G>(F,G);
+impl<T,F,G> FnOnce<(T,)> for Compose<F,G>
+where F: FnOnce<(T,)>, G: FnOnce<(F::Output,)> {
+    type Output = G::Output;
+    extern "rust-call" fn call_once(self, (x,): (T,)) -> G::Output {
+        (self.1)((self.0)(x))
+    }
+}
+
+fn bad<T>(f: fn(&'static str) -> T) {
+    test(Compose(f, |_| {})); //~ ERROR: mismatched types
+}
+
+fn main() {}

src/test/ui/unboxed-closures/issue-30904.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-30906.rs:15:5
+   |
+LL |     test(Compose(f, |_| {}));
+   |     ^^^^ one type is more general than the other
+   |
+   = note: expected type `std::ops::FnOnce<(&'x str,)>`
+              found type `std::ops::FnOnce<(&str,)>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.