Add forget_ref tests.

Also rename drop_ref.rs to drop_forget_ref.rs in tests/compile-fail.

tests/compile-fail/drop_forget_ref.rs

@@ -0,0 +1,60 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+
+#![deny(drop_ref, forget_ref)]
+#![allow(toplevel_ref_arg, similar_names)]
+
+use std::mem::{drop, forget};
+
+struct SomeStruct;
+
+fn main() {
+    drop(&SomeStruct); //~ERROR call to `std::mem::drop` with a reference argument
+    forget(&SomeStruct); //~ERROR call to `std::mem::forget` with a reference argument
+
+    let mut owned1 = SomeStruct;
+    drop(&owned1); //~ERROR call to `std::mem::drop` with a reference argument
+    drop(&&owned1); //~ERROR call to `std::mem::drop` with a reference argument
+    drop(&mut owned1); //~ERROR call to `std::mem::drop` with a reference argument
+    drop(owned1); //OK
+    let mut owned2 = SomeStruct;
+    forget(&owned2); //~ERROR call to `std::mem::forget` with a reference argument
+    forget(&&owned2); //~ERROR call to `std::mem::forget` with a reference argument
+    forget(&mut owned2); //~ERROR call to `std::mem::forget` with a reference argument
+    forget(owned2); //OK
+
+    let reference1 = &SomeStruct;
+    drop(reference1); //~ERROR call to `std::mem::drop` with a reference argument
+    forget(&*reference1); //~ERROR call to `std::mem::forget` with a reference argument
+
+    let reference2 = &mut SomeStruct;
+    drop(reference2); //~ERROR call to `std::mem::drop` with a reference argument
+    let reference3 = &mut SomeStruct;
+    forget(reference3); //~ERROR call to `std::mem::forget` with a reference argument
+
+    let ref reference4 = SomeStruct;
+    drop(reference4); //~ERROR call to `std::mem::drop` with a reference argument
+    forget(reference4); //~ERROR call to `std::mem::forget` with a reference argument
+}
+
+#[allow(dead_code)]
+fn test_generic_fn_drop<T>(val: T) {
+    drop(&val); //~ERROR call to `std::mem::drop` with a reference argument
+    drop(val); //OK
+}
+
+#[allow(dead_code)]
+fn test_generic_fn_forget<T>(val: T) {
+    forget(&val); //~ERROR call to `std::mem::forget` with a reference argument
+    forget(val); //OK
+}
+
+#[allow(dead_code)]
+fn test_similarly_named_function() {
+    fn drop<T>(_val: T) {}
+    drop(&SomeStruct); //OK; call to unrelated function which happens to have the same name
+    std::mem::drop(&SomeStruct); //~ERROR call to `std::mem::drop` with a reference argument
+    fn forget<T>(_val: T) {}
+    forget(&SomeStruct); //OK; call to unrelated function which happens to have the same name
+    std::mem::forget(&SomeStruct); //~ERROR call to `std::mem::forget` with a reference argument
+}