Add unit tests for the diagnostic

Author: Orion Gonzalez

tests/ui/c-variadic/fn-item-diagnostic-issue-69232.rs

@@ -0,0 +1,13 @@
+// https://github.com/rust-lang/rust/issues/69232
+
+extern "C" {
+    fn foo(x: usize, ...);
+}
+
+fn test() -> u8 {
+    127
+}
+
+fn main() {
+    unsafe { foo(1, test) }; //~ ERROR can't pass a function item to a variadic function
+}

tests/ui/c-variadic/fn-item-diagnostic-issue-69232.stderr

@@ -0,0 +1,12 @@
+error[E0617]: can't pass a function item to a variadic function
+  --> $DIR/issue-69232.rs:12:21
+   |
+LL |     unsafe { foo(1, test) };
+   |                     ^^^^ help: use a function pointer instead: `&test`
+   |
+   = help: a function item is zero-sized and needs to be casted into a pointer-sized function pointer to be used in FFI
+   = note: for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0617`.