Use a crate attribute to load plugins

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

becomes a crate attribute

    #![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]

src/doc/reference.md

@@ -2014,6 +2014,11 @@ type int8_t = i8;
 - `no_start` - disable linking to the `native` crate, which specifies the
   "start" language item.
 - `no_std` - disable linking to the `std` crate.
+- `plugin` — load a list of named crates as compiler plugins, e.g.
+             `#![plugin(foo, bar)]`. Optional arguments for each plugin,
+             i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's
+             registrar function.  The `plugin` feature gate is required to use
+             this attribute.
 
 ### Module-only attributes
 
@@ -2082,7 +2087,7 @@ On `struct`s:
   remove any padding between fields (note that this is very fragile and may
   break platforms which require aligned access).
 
-### Macro- and plugin-related attributes
+### Macro-related attributes
 
 - `macro_use` on a `mod` — macros defined in this module will be visible in the
   module's parent, after this module has been included.
@@ -2097,13 +2102,8 @@ On `struct`s:
 
 - `macro_export` - export a macro for cross-crate usage.
 
-- `plugin` on an `extern crate` — load this crate as a [compiler
-  plugin][plugin].  The `plugin` feature gate is required.  Any arguments to
-  the attribute, e.g. `#[plugin=...]` or `#[plugin(...)]`, are provided to the
-  plugin.
-
-- `no_link` on an `extern crate` — even if we load this crate for macros or
-  compiler plugins, don't link it into the output.
+- `no_link` on an `extern crate` — even if we load this crate for macros, don't
+  link it into the output.
 
 See the [macros section of the
 book](book/macros.html#scoping-and-macro-import/export) for more information on

src/doc/trpl/plugins.md

@@ -30,14 +30,14 @@ information.
 extend the compiler's behavior with new syntax extensions, lint checks, etc.
 
 A plugin is a dynamic library crate with a designated *registrar* function that
-registers extensions with `rustc`. Other crates can use these extensions by
-loading the plugin crate with `#[plugin] extern crate`. See the
+registers extensions with `rustc`. Other crates can load these extensions using
+the crate attribute `#![plugin(...)]`.  See the
 [`rustc::plugin`](../rustc/plugin/index.html) documentation for more about the
 mechanics of defining and loading a plugin.
 
-Arguments passed as `#[plugin=...]` or `#[plugin(...)]` are not interpreted by
-rustc itself.  They are provided to the plugin through the `Registry`'s [`args`
-method](../rustc/plugin/registry/struct.Registry.html#method.args).
+If present, arguments passed as `#![plugin(foo(... args ...))]` are not
+interpreted by rustc itself.  They are provided to the plugin through the
+`Registry`'s [`args` method](../rustc/plugin/registry/struct.Registry.html#method.args).
 
 # Syntax extensions
 
@@ -110,8 +110,7 @@ Then we can use `rn!()` like any other macro:
 
 ```ignore
 #![feature(plugin)]
-
-#[plugin] extern crate roman_numerals;
+#![plugin(roman_numerals)]
 
 fn main() {
     assert_eq!(rn!(MMXV), 2015);
@@ -219,7 +218,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
 Then code like
 
 ```ignore
-#[plugin] extern crate lint_plugin_test;
+#![plugin(lint_plugin_test)]
 
 fn lintme() { }
 ```