Add test of unsupported elided lifetime in extern Rust sig

Author: dtolnay

tests/ui/unsupported_elided.rs

@@ -0,0 +1,20 @@
+use std::marker::PhantomData;
+
+#[cxx::bridge]
+mod ffi {
+    extern "Rust" {
+        type T;
+
+        fn f(t: &T) -> &str;
+    }
+}
+
+pub struct T<'a> {
+    _lifetime: PhantomData<&'a ()>,
+}
+
+fn f<'a>(_t: &T<'a>) -> &'a str {
+    ""
+}
+
+fn main() {}