Add needless_pass_by_ref_mut edition 2021 specific test

There is a specific test that `unsafe fn` are not lint, and that
`unsafe` blocks are not lint either. Since in edition 2024 `unsafe`
blocks are required inside `unsafe fn` to do unsafe things, set up a
specific test for edition 2021.

Author: samueltardieu

tests/ui/needless_pass_by_ref_mut.rs

@@ -301,7 +301,7 @@ struct Data<T: ?Sized> {
 }
 // Unsafe functions should not warn.
 unsafe fn get_mut_unchecked<T>(ptr: &mut NonNull<Data<T>>) -> &mut T {
-    &mut (*ptr.as_ptr()).value
+    unsafe { &mut (*ptr.as_ptr()).value }
 }
 // Unsafe blocks should not warn.
 fn get_mut_unchecked2<T>(ptr: &mut NonNull<Data<T>>) -> &mut T {

tests/ui/needless_pass_by_ref_mut_2021.rs

@@ -0,0 +1,12 @@
+//@edition: 2021
+//@check-pass
+#![warn(clippy::needless_pass_by_ref_mut)]
+
+struct Data<T: ?Sized> {
+    value: T,
+}
+
+// Unsafe functions should not warn.
+unsafe fn get_mut_unchecked<T>(ptr: &mut std::ptr::NonNull<Data<T>>) -> &mut T {
+    &mut (*ptr.as_ptr()).value
+}