rust-lang · bors · Jun 9, 2025 · Jun 7, 2025
diff --git a/tests/ui/unboxed-closures/fn-traits-hrtb-coercion.rs b/tests/ui/unboxed-closures/fn-traits-hrtb-coercion.rs
@@ -0,0 +1,39 @@
+//! Test for issue <github.com/rust-lang/rust/issues/30904>
+//! Related to higher-ranked lifetime inference with unboxed closures and FnOnce.
+
+#![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))
+    }
+}
+
+struct Str<'a>(&'a str);
+
+fn mk_str<'a>(s: &'a str) -> Str<'a> {
+    Str(s)
+}
+
+fn main() {
+    let _: for<'a> fn(&'a str) -> Str<'a> = mk_str;
+    let _: for<'a> fn(&'a str) -> Str<'a> = Str;
+    //~^ ERROR: mismatched types
+
+    test(|_: &str| {});
+    test(mk_str);
+    test(Str);
+
+    test(Compose(|_: &str| {}, |_| {}));
+    test(Compose(mk_str, |_| {}));
+    test(Compose(Str, |_| {}));
+}
diff --git a/tests/ui/unboxed-closures/fn-traits-hrtb-coercion.stderr b/tests/ui/unboxed-closures/fn-traits-hrtb-coercion.stderr
@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/fn-traits-hrtb-coercion.rs:29:45
+   |
+LL |     let _: for<'a> fn(&'a str) -> Str<'a> = Str;
+   |            ------------------------------   ^^^ one type is more general than the other
+   |            |
+   |            expected due to this
+   |
+   = note:      expected fn pointer `for<'a> fn(&'a _) -> Str<'a>`
+           found struct constructor `fn(&_) -> Str<'_> {Str::<'_>}`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.