Fix useless_attribute to also whitelist unused_extern_crates.

Author: eddyb

clippy_lints/src/attrs.rs

@@ -180,10 +180,15 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for AttrPass {
                                                                 || is_word(lint, "deprecated") {
                                                 return
                                         },
-                                        ItemKind::ExternCrate(..) => if is_word(lint, "unused_imports")
-                                                                        && skip_unused_imports {
+                                        ItemKind::ExternCrate(..) => {
+                                            if is_word(lint, "unused_imports")
+                                                && skip_unused_imports {
+                                                    return
+                                            }
+                                            if is_word(lint, "unused_extern_crates") {
                                                 return
-                                        },
+                                            }
+                                        }
                                         _ => {},
                                     }
                                 }

tests/ui/useless_attribute.rs

@@ -2,11 +2,12 @@
 
 #![warn(clippy::useless_attribute)]
 
-#[allow(dead_code, unused_extern_crates)]
-#[cfg_attr(feature = "cargo-clippy", allow(dead_code, unused_extern_crates))]
+#[allow(dead_code)]
+#[cfg_attr(feature = "cargo-clippy", allow(dead_code))]
 #[cfg_attr(feature = "cargo-clippy",
-           allow(dead_code, unused_extern_crates))]
+           allow(dead_code))]
 #[allow(unused_imports)]
+#[allow(unused_extern_crates)]
 #[macro_use]
 extern crate clippy_lints;
 

tests/ui/useless_attribute.stderr

@@ -1,16 +1,16 @@
 error: useless lint attribute
  --> $DIR/useless_attribute.rs:5:1
   |
-5 | #[allow(dead_code, unused_extern_crates)]
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![allow(dead_code, unused_extern_crates)]`
+5 | #[allow(dead_code)]
+  | ^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![allow(dead_code)]`
   |
   = note: `-D clippy::useless-attribute` implied by `-D warnings`
 
 error: useless lint attribute
  --> $DIR/useless_attribute.rs:6:1
   |
-6 | #[cfg_attr(feature = "cargo-clippy", allow(dead_code, unused_extern_crates))]
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![cfg_attr(feature = "cargo-clippy", allow(dead_code, unused_extern_crates))`
+6 | #[cfg_attr(feature = "cargo-clippy", allow(dead_code))]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: if you just forgot a `!`, use: `#![cfg_attr(feature = "cargo-clippy", allow(dead_code))`
 
 error: aborting due to 2 previous errors