Lint empty #[derive()] as unused attribute.

src/libsyntax/ext/derive.rs

@@ -30,10 +30,6 @@ pub fn collect_derives(cx: &mut ExtCtxt<'_>, attrs: &mut Vec<ast::Attribute>) ->
 
         match attr.parse_list(cx.parse_sess,
                               |parser| parser.parse_path_allowing_meta(PathStyle::Mod)) {
-            Ok(ref traits) if traits.is_empty() => {
-                cx.span_warn(attr.span, "empty trait list in `derive`");
-                false
-            }
             Ok(traits) => {
                 result.extend(traits);
                 true

src/test/ui/derives/deriving-meta-empty-trait-list.rs

@@ -1,6 +1,6 @@
-// compile-pass
+#![deny(unused)]
 
-#[derive()] //~ WARNING empty trait list in `derive`
-struct Bar;
+#[derive()] //~ ERROR unused attribute
+struct _Bar;
 
 pub fn main() {}

src/test/ui/derives/deriving-meta-empty-trait-list.stderr

@@ -1,6 +1,15 @@
-warning: empty trait list in `derive`
+error: unused attribute
   --> $DIR/deriving-meta-empty-trait-list.rs:3:1
    |
 LL | #[derive()]
    | ^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/deriving-meta-empty-trait-list.rs:1:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: #[deny(unused_attributes)] implied by #[deny(unused)]
+
+error: aborting due to previous error
 

src/test/ui/malformed/malformed-derive-entry.rs

@@ -4,9 +4,6 @@ struct Test1;
 #[derive(Copy="bad")] //~ ERROR expected one of `)`, `,`, or `::`, found `=`
 struct Test2;
 
-#[derive()] //~ WARNING empty trait list
-struct Test3;
-
 #[derive] //~ ERROR malformed `derive` attribute input
 struct Test4;
 

src/test/ui/malformed/malformed-derive-entry.stderr

@@ -10,14 +10,8 @@ error: expected one of `)`, `,`, or `::`, found `=`
 LL | #[derive(Copy="bad")]
    |              ^ expected one of `)`, `,`, or `::` here
 
-warning: empty trait list in `derive`
-  --> $DIR/malformed-derive-entry.rs:7:1
-   |
-LL | #[derive()]
-   | ^^^^^^^^^^^
-
 error: malformed `derive` attribute input
-  --> $DIR/malformed-derive-entry.rs:10:1
+  --> $DIR/malformed-derive-entry.rs:7:1
    |
 LL | #[derive]
    | ^^^^^^^^^ help: missing traits to be derived: `#[derive(Trait1, Trait2, ...)]`