Auto merge of #22026 - kmcallister:plugin, r=sfackler

```rust
#[plugin] #[no_link] extern crate bleh;
```

becomes a crate attribute

```rust
#![plugin(bleh)]
```

The feature gate is still required.

It's almost never correct to link a plugin into the resulting library / executable, because it will bring all of libsyntax and librustc with it. However if you really want this behavior, you can get it with a separate `extern crate` item in addition to the `plugin` attribute.

Fixes #21043.
Fixes #20769.

[breaking-change]

4 files changed, 42 insertions, 3 deletions

diff --git a/src/test/compile-fail/multi-plugin-attr.rs b/src/test/compile-fail/malformed-plugin-1.rs
index 1d98cd26a38..254a797ef1c 100644
--- a/src/test/compile-fail/multi-plugin-attr.rs
+++ b/src/test/compile-fail/malformed-plugin-1.rs
@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[plugin]
-#[plugin]  //~ ERROR #[plugin] specified multiple times
-extern crate std;
+#![plugin] //~ ERROR malformed plugin attribute
 
 fn main() {}
diff --git a/src/test/compile-fail/malformed-plugin-2.rs b/src/test/compile-fail/malformed-plugin-2.rs
new file mode 100644
index 00000000000..884087b7bc5
--- /dev/null
+++ b/src/test/compile-fail/malformed-plugin-2.rs
@@ -0,0 +1,13 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![plugin="bleh"] //~ ERROR malformed plugin attribute
+
+fn main() {}
diff --git a/src/test/compile-fail/malformed-plugin-3.rs b/src/test/compile-fail/malformed-plugin-3.rs
new file mode 100644
index 00000000000..4885bb901df
--- /dev/null
+++ b/src/test/compile-fail/malformed-plugin-3.rs
@@ -0,0 +1,13 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![plugin(foo="bleh")] //~ ERROR malformed plugin attribute
+
+fn main() {}
diff --git a/src/test/compile-fail/plugin-extern-crate-attr-deprecated.rs b/src/test/compile-fail/plugin-extern-crate-attr-deprecated.rs
new file mode 100644
index 00000000000..ccda5cbdceb
--- /dev/null
+++ b/src/test/compile-fail/plugin-extern-crate-attr-deprecated.rs
@@ -0,0 +1,15 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[plugin]  //~ ERROR #[plugin] on `extern crate` is deprecated
+//~^ HELP use a crate attribute instead, i.e. #![plugin(std)]
+extern crate std;
+
+fn main() {}