macros: add test for special return types (#7857)

tests-build/tests/macros.rs

@@ -12,6 +12,9 @@ fn compile_fail_full() {
     #[cfg(feature = "full")]
     t.pass("tests/pass/macros_main_loop.rs");
 
+    #[cfg(feature = "full")]
+    t.pass("tests/pass/impl_trait.rs");
+
     #[cfg(feature = "full")]
     t.compile_fail("tests/fail/macros_invalid_input.rs");
 

tests-build/tests/pass/impl_trait.rs

@@ -0,0 +1,17 @@
+use tests_build::tokio;
+
+#[tokio::main]
+async fn never() -> ! {
+    loop {}
+}
+
+#[tokio::main]
+async fn impl_trait() -> impl Iterator<Item = impl core::fmt::Debug> {
+    [()].into_iter()
+}
+
+fn main() {
+    if impl_trait().count() == 10 {
+        never();
+    }
+}