improper_ctypes: add test for #66202

This commit adds a test of the improper ctypes lint, checking that
return type are normalized bethat return types are normalized before
being checked for FFI-safety, and that transparent newtype wrappers
are FFI-safe if the type being wrapped is FFI-safe.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/lint/lint-ctypes-66202.rs

@@ -0,0 +1,17 @@
+#![deny(improper_ctypes)]
+
+// This test checks that return types are normalized before being checked for FFI-safety, and that
+// transparent newtype wrappers are FFI-safe if the type being wrapped is FFI-safe.
+
+#[repr(transparent)]
+pub struct W<T>(T);
+
+extern "C" {
+    pub fn bare() -> ();
+    pub fn normalize() -> <() as ToOwned>::Owned;
+    //~^ ERROR uses type `()`
+    pub fn transparent() -> W<()>;
+    //~^ ERROR uses type `W<()>`
+}
+
+fn main() {}

src/test/ui/lint/lint-ctypes-66202.stderr

@@ -0,0 +1,29 @@
+error: `extern` block uses type `()`, which is not FFI-safe
+  --> $DIR/lint-ctypes-66202.rs:11:27
+   |
+LL |     pub fn normalize() -> <() as ToOwned>::Owned;
+   |                           ^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+note: the lint level is defined here
+  --> $DIR/lint-ctypes-66202.rs:1:9
+   |
+LL | #![deny(improper_ctypes)]
+   |         ^^^^^^^^^^^^^^^
+   = help: consider using a struct instead
+   = note: tuples have unspecified layout
+
+error: `extern` block uses type `W<()>`, which is not FFI-safe
+  --> $DIR/lint-ctypes-66202.rs:13:29
+   |
+LL |     pub fn transparent() -> W<()>;
+   |                             ^^^^^ not FFI-safe
+   |
+   = note: composed only of `PhantomData`
+note: the type is defined here
+  --> $DIR/lint-ctypes-66202.rs:7:1
+   |
+LL | pub struct W<T>(T);
+   | ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+