Lint passing Cow by reference

clippy_lints/src/ptr.rs

@@ -3,6 +3,7 @@
 use std::borrow::Cow;
 use rustc::hir::*;
 use rustc::hir::map::NodeItem;
+use rustc::hir::QPath;
 use rustc::lint::*;
 use rustc::ty;
 use syntax::ast::NodeId;
@@ -213,6 +214,33 @@ fn check_fn(cx: &LateContext, decl: &FnDecl, fn_id: NodeId, opt_body_id: Option<
                         },
                     );
                 }
+            } else if match_type(cx, ty, &paths::COW) {
+                if_chain! {
+                    if let TyRptr(_, MutTy { ref ty, ..} ) = arg.node;
+                    if let TyPath(ref path) = ty.node;
+                    if let QPath::Resolved(None, ref pp) = *path;
+                    if let [ref bx] = *pp.segments;
+                    if let Some(ref params) = bx.parameters;
+                    if !params.parenthesized;
+                    if let [ref inner] = *params.types;
+                    then {
+                        let replacement = snippet_opt(cx, inner.span);
+                        match replacement {
+                            Some(r) => {
+                                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", "&".to_owned() + &r);
+                                    },
+                                );
+                            },
+                            None => (),
+                        }
+                    }
+                }
             }
         }
     }

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 +38,13 @@ 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
+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: `&[i32]`
+   |
+   = note: `-D ptr-arg` implied by `-D warnings`
+
+error: aborting due to 7 previous errors