add test for rust-lang#52334

Fixes rust-lang#52334

tests/ui/extern/extern-C-non-FFI-safe-arg-ice-52334.rs

@@ -0,0 +1,13 @@
+// test for #52334 ICE when casting extern "C" fn when it has a non-FFI-safe argument
+//@ check-pass
+
+type Foo = extern "C" fn(::std::ffi::CStr);
+//~^ WARN `extern` fn uses type `[i8]`, which is not FFI-safe
+extern "C" {
+    fn meh(blah: Foo);
+    //~^ WARN `extern` block uses type `[i8]`, which is not FFI-safe
+}
+
+fn main() {
+    meh as usize;
+}

tests/ui/extern/extern-C-non-FFI-safe-arg-ice-52334.stderr

@@ -0,0 +1,22 @@
+warning: `extern` fn uses type `[i8]`, which is not FFI-safe
+  --> $DIR/extern-C-non-FFI-safe-arg-ice-52334.rs:4:12
+   |
+LL | type Foo = extern "C" fn(::std::ffi::CStr);
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
+   |
+   = help: consider using a raw pointer instead
+   = note: slices have no C equivalent
+   = note: `#[warn(improper_ctypes_definitions)]` on by default
+
+warning: `extern` block uses type `[i8]`, which is not FFI-safe
+  --> $DIR/extern-C-non-FFI-safe-arg-ice-52334.rs:7:18
+   |
+LL |     fn meh(blah: Foo);
+   |                  ^^^ not FFI-safe
+   |
+   = help: consider using a raw pointer instead
+   = note: slices have no C equivalent
+   = note: `#[warn(improper_ctypes)]` on by default
+
+warning: 2 warnings emitted
+