Add a test against Result<(), ()>

Author: MasterAwesome

tests/ui/lint/lint-ctypes-enum.rs

@@ -171,7 +171,8 @@ extern "C" {
     //~^ ERROR `extern` block uses type
     fn result_cascading_e(x: Result<(), Result<(), num::NonZero<u8>>>);
     //~^ ERROR `extern` block uses type
-
+    fn result_unit_t_e(x: Result<(), ()>);
+    //~^ ERROR `extern` block uses type
 }
 
 pub fn main() {}

tests/ui/lint/lint-ctypes-enum.stderr

@@ -237,5 +237,14 @@ LL |     fn result_cascading_e(x: Result<(), Result<(), num::NonZero<u8>>>);
    = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
    = note: enum has no representation hint
 
-error: aborting due to 25 previous errors
+error: `extern` block uses type `Result<(), ()>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-enum.rs:174:27
+   |
+LL |     fn result_unit_t_e(x: Result<(), ()>);
+   |                           ^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider adding a `#[repr(C)]`, `#[repr(transparent)]`, or integer `#[repr(...)]` attribute to this enum
+   = note: enum has no representation hint
+
+error: aborting due to 26 previous errors