Lint passing Cow by reference

Add lint for reference to Cow to the same place in the code where
lint for reference to String lives.

https://github.com/rust-lang-nursery/rust-clippy/issues/2405

clippy_lints/src/ptr.rs

@@ -213,6 +213,21 @@ fn check_fn(cx: &LateContext, decl: &FnDecl, fn_id: NodeId, opt_body_id: Option<
                         },
                     );
                 }
+            } else if match_type(cx, ty, &paths::COW) {
+                let as_str = format!("{}", snippet_opt(cx, arg.span).unwrap());
+                let mut cc = as_str.chars();
+                cc.next();
+                let replacement: String = cc.collect();
+
+                span_lint_and_then(
+                    cx,
+                    PTR_ARG,
+                    arg.span,
+                    "using a reference to `Cow` is not recommended.",
+                    |db| {
+                        db.span_suggestion(arg.span, "change this to", replacement);
+                    },
+                );
             }
         }
     }

tests/ui/needless_borrow.rs

@@ -1,4 +1,4 @@
-
+use std::borrow::Cow;
 
 
 fn x(y: &i32) -> i32 {
@@ -51,3 +51,12 @@ fn issue_1432() {
 
     let _ = v.iter().filter(|&a| a.is_empty());
 }
+
+#[allow(dead_code)]
+fn test_cow_with_ref(c: &Cow<[i32]>) {
+}
+
+#[allow(dead_code)]
+fn test_cow(c: Cow<[i32]>) {
+    let _c = c;
+}

tests/ui/needless_borrow.stderr

@@ -38,5 +38,15 @@ error: this pattern creates a reference to a reference
 50 |     let _ = v.iter().filter(|&ref a| a.is_empty());
    |                               ^^^^^ help: change this to: `a`
 
-error: aborting due to 6 previous errors
+a> $DIR/needless_borrow.rs:56:25: 56:36
+b> $DIR/needless_borrow.rs:56:25: 56:36
+error: using a reference to `Cow` is not recommended.
+  --> $DIR/needless_borrow.rs:56:25
+   |
+56 | fn test_cow_with_ref(c: &Cow<[i32]>) {
+   |                         ^^^^^^^^^^^ help: change this to: `Cow<[i32]>`
+   |
+   = note: `-D ptr-arg` implied by `-D warnings`
+
+error: aborting due to 7 previous errors