Add regression test for #134060

Mostly just to check that the lint impl doesn't ICE from an easy case.

tests/ui/lint/improper_ctypes_definitions_ice_134060.rs

@@ -0,0 +1,15 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/134060> due to impl bug from
+//! <https://github.com/rust-lang/rust/pull/131669>. This test should be adjusted in favor of more
+//! comprehensive coverage when the changes are to be relanded, as this is a basic sanity check to
+//! check that the fuzzed example from #134060 doesn't ICE.
+
+//@ check-pass
+
+#![crate_type = "lib"]
+
+pub trait Foo {
+    extern "C" fn foo_(&self, _: ()) -> i64 {
+        //~^ WARN `extern` fn uses type `()`, which is not FFI-safe
+        0
+    }
+}

tests/ui/lint/improper_ctypes_definitions_ice_134060.stderr

@@ -0,0 +1,12 @@
+warning: `extern` fn uses type `()`, which is not FFI-safe
+  --> $DIR/improper_ctypes_definitions_ice_134060.rs:11:34
+   |
+LL |     extern "C" fn foo_(&self, _: ()) -> i64 {
+   |                                  ^^ not FFI-safe
+   |
+   = help: consider using a struct instead
+   = note: tuples have unspecified layout
+   = note: `#[warn(improper_ctypes_definitions)]` on by default
+
+warning: 1 warning emitted
+