Don't lint ptr_arg on &mut Cow<_>

Author: Jarcho

clippy_lints/src/ptr.rs

@@ -436,7 +436,7 @@ fn check_fn_args<'cx, 'tcx: 'cx>(
                             DerefTy::Path,
                             None,
                         ),
-                        Some(sym::Cow) => {
+                        Some(sym::Cow) if mutability == Mutability::Not => {
                             let ty_name = name.args
                                 .and_then(|args| {
                                     args.args.iter().find_map(|a| match a {

tests/ui/ptr_arg.rs

@@ -194,3 +194,10 @@ fn two_vecs(a: &mut Vec<u32>, b: &mut Vec<u32>) {
     a.push(0);
     b.push(1);
 }
+
+// Issue #8495
+fn cow_conditional_to_mut(a: &mut Cow<str>) {
+    if a.is_empty() {
+        a.to_mut().push_str("foo");
+    }
+}