Add a cfg_attr syntax extension

This extends cfg-gating to attributes.

```rust
 #[cfg_attr(<cfg pattern>, <attr>)]
```
will expand to
```rust
 #[<attr>]
```
if the `<cfg pattern>` matches the current cfg environment, and nothing
if it does not. The grammar for the cfg pattern has a simple
recursive structure:

 * `value` and `key = "value"` are cfg patterns,
 * `not(<cfg pattern>)` is a cfg pattern and matches if `<cfg pattern>`
    does not.
 * `all(<cfg pattern>, ...)` is a cfg pattern and matches if all of the
    `<cfg pattern>`s do.
 * `any(<cfg pattern>, ...)` is a cfg pattern and matches if any of the
    `<cfg pattern>`s do.

Examples:

```rust
 // only derive Show for assert_eq! in tests
 #[cfg_attr(test, deriving(Show))]
 struct Foo { ... }

 // only derive Show for assert_eq! in tests and debug builds
 #[cfg_attr(any(test, not(ndebug)), deriving(Show))]
 struct Foo { ... }

 // ignore a test in certain cases
 #[test]
 #[cfg_attr(all(not(target_os = "linux"), target_endian = "big"), ignore)]
 fn test_broken_thing() { ... }

 // Avoid duplication when fixing staging issues in rustc
 #[cfg_attr(not(stage0), lang="iter")]
 pub trait Iterator<T> { ... }
```

src/libsyntax/ext/base.rs

@@ -439,6 +439,8 @@ fn initial_syntax_expander_table() -> SyntaxEnv {
     syntax_expanders.insert(intern("cfg"),
                             builtin_normal_expander(
                                     ext::cfg::expand_cfg));
+    syntax_expanders.insert(intern("cfg_attr"),
+                            ItemModifier(ext::cfg_attr::expand));
     syntax_expanders.insert(intern("trace_macros"),
                             builtin_normal_expander(
                                     ext::trace_macros::expand_trace_macros));

src/libsyntax/ext/cfg_attr.rs

@@ -0,0 +1,59 @@
+// Copyright 2014 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.
+
+use std::gc::{Gc, GC};
+
+use ast;
+use attr;
+use codemap::Span;
+use ext::base::ExtCtxt;
+use ext::build::AstBuilder;
+
+pub fn expand(cx: &mut ExtCtxt, sp: Span, mi: Gc<ast::MetaItem>, it: Gc<ast::Item>)
+          -> Gc<ast::Item> {
+    let (cfg, attr) = match mi.node {
+        ast::MetaList(_, ref mis) if mis.len() == 2 => (mis[0], mis[1]),
+        _ => {
+            cx.span_err(sp, "expected `#[cfg_attr(<cfg pattern>, <attr>)]`");
+            return it;
+        }
+    };
+
+    let mut out = (*it).clone();
+    if cfg_matches(cx, cfg) {
+        out.attrs.push(cx.attribute(attr.span, attr));
+    }
+
+    box(GC) out
+}
+
+fn cfg_matches(cx: &mut ExtCtxt, cfg: Gc<ast::MetaItem>) -> bool {
+    match cfg.node {
+        ast::MetaList(ref pred, ref mis) if pred.get() == "any" =>
+            mis.iter().any(|mi| cfg_matches(cx, *mi)),
+        ast::MetaList(ref pred, ref mis) if pred.get() == "all" =>
+            mis.iter().all(|mi| cfg_matches(cx, *mi)),
+        ast::MetaList(ref pred, ref mis) if pred.get() == "not" => {
+            if mis.len() != 1 {
+                cx.span_err(cfg.span, format!("expected 1 value, got {}",
+                                              mis.len()).as_slice());
+                return false;
+            }
+            !cfg_matches(cx, mis[0])
+        }
+        ast::MetaList(ref pred, _) => {
+            cx.span_err(cfg.span,
+                        format!("invalid predicate `{}`", pred).as_slice());
+            false
+        },
+        ast::MetaWord(_) | ast::MetaNameValue(..) =>
+            attr::contains(cx.cfg.as_slice(), cfg),
+    }
+}

src/libsyntax/lib.rs

@@ -83,6 +83,7 @@ pub mod ext {
     pub mod build;
     pub mod bytes;
     pub mod cfg;
+    pub mod cfg_attr;
     pub mod concat;
     pub mod concat_idents;
     pub mod deriving;

src/test/run-pass/cfg_attr.rs

@@ -0,0 +1,55 @@
+// Copyright 2014 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.
+
+// compile-flags:--cfg set1 --cfg set2
+#![allow(dead_code)]
+use std::fmt::Show;
+
+struct NotShowable;
+
+#[cfg_attr(set1, deriving(Show))]
+struct Set1;
+
+#[cfg_attr(notset, deriving(Show))]
+struct Notset(NotShowable);
+
+#[cfg_attr(not(notset), deriving(Show))]
+struct NotNotset;
+
+#[cfg_attr(not(set1), deriving(Show))]
+struct NotSet1(NotShowable);
+
+#[cfg_attr(all(set1, set2), deriving(Show))]
+struct AllSet1Set2;
+
+#[cfg_attr(all(set1, notset), deriving(Show))]
+struct AllSet1Notset(NotShowable);
+
+#[cfg_attr(any(set1, notset), deriving(Show))]
+struct AnySet1Notset;
+
+#[cfg_attr(any(notset, notset2), deriving(Show))]
+struct AnyNotsetNotset2(NotShowable);
+
+#[cfg_attr(all(not(notset), any(set1, notset)), deriving(Show))]
+struct Complex;
+
+#[cfg_attr(any(notset, not(any(set1, notset))), deriving(Show))]
+struct ComplexNot(NotShowable);
+
+fn is_show<T: Show>() {}
+
+fn main() {
+    is_show::<Set1>();
+    is_show::<NotNotset>();
+    is_show::<AllSet1Set2>();
+    is_show::<AnySet1Notset>();
+    is_show::<Complex>();
+}