Add UI test for needless_pass_by_ref_mut

tests/ui/needless_pass_by_ref_mut.rs

@@ -0,0 +1,47 @@
+#![allow(unused)]
+
+// Should only warn for `s`.
+fn foo(s: &mut Vec<u32>, b: &u32, x: &mut u32) {
+    *x += *b + s.len() as u32;
+}
+
+// Should not warn.
+fn foo2(s: &mut Vec<u32>) {
+    s.push(8);
+}
+
+// Should not warn because we return it.
+fn foo3(s: &mut Vec<u32>) -> &mut Vec<u32> {
+    s
+}
+
+// Should not warn because `s` is used as mutable.
+fn foo4(s: &mut Vec<u32>) {
+    Vec::push(s, 4);
+}
+
+struct Bar;
+
+impl Bar {
+    // Should not warn on `&mut self`.
+    fn bar(&mut self) {}
+}
+
+trait Babar {
+    // Should not warn here since it's a trait method.
+    fn method(arg: &mut u32);
+}
+
+impl Babar for Bar {
+    // Should not warn here since it's a trait method.
+    fn method(a: &mut u32) {}
+}
+
+fn main() {
+    let mut u = 0;
+    let mut v = vec![0];
+    foo(&mut v, &0, &mut u);
+    foo2(&mut v);
+    foo3(&mut v);
+    println!("{u}");
+}

tests/ui/needless_pass_by_ref_mut.stderr

@@ -0,0 +1,10 @@
+error: this argument is a mutable reference, but not used mutably
+  --> $DIR/needless_pass_by_ref_mut.rs:4:11
+   |
+LL | fn foo(s: &mut Vec<u32>, b: &u32, x: &mut u32) {
+   |           ^^^^^^^^^^^^^ help: consider changing to &Vec<u32>
+   |
+   = note: `-D clippy::needless-pass-by-ref-mut` implied by `-D warnings`
+
+error: aborting due to previous error
+