Add regression test for #4968

Author: JohnTitor

tests/ui/crashes/ice-4968.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+// Test for https://github.com/rust-lang/rust-clippy/issues/4968
+
+trait Trait {
+    type Assoc;
+}
+
+use std::mem::{self, ManuallyDrop};
+
+#[allow(unused)]
+fn func<T: Trait>(slice: Vec<T::Assoc>) {
+    unsafe {
+        let _: Vec<ManuallyDrop<T::Assoc>> = mem::transmute(slice);
+    }
+}
+
+fn main() {}