Process cfg_attr right before stripping cfg

Fixes rust-lang#22070.
Fixes rust-lang#19372.

Author: kmcallister

src/libsyntax/config.rs

@@ -12,7 +12,7 @@ use attr::AttrMetaMethods;
 use diagnostic::SpanHandler;
 use fold::Folder;
 use {ast, fold, attr};
-use codemap::Spanned;
+use codemap::{Spanned, respan};
 use ptr::P;
 
 use util::small_vector::SmallVector;
@@ -26,6 +26,7 @@ struct Context<F> where F: FnMut(&[ast::Attribute]) -> bool {
 // Support conditional compilation by transforming the AST, stripping out
 // any items that do not belong in the current configuration
 pub fn strip_unconfigured_items(diagnostic: &SpanHandler, krate: ast::Crate) -> ast::Crate {
+    let krate = process_cfg_attr(diagnostic, krate);
     let config = krate.config.clone();
     strip_items(krate, |attrs| in_cfg(diagnostic, &config, attrs))
 }
@@ -281,3 +282,49 @@ fn in_cfg(diagnostic: &SpanHandler, cfg: &[P<ast::MetaItem>], attrs: &[ast::Attr
         attr::cfg_matches(diagnostic, cfg, &*mis[0])
     })
 }
+
+struct CfgAttrFolder<'a> {
+    diag: &'a SpanHandler,
+    config: ast::CrateConfig,
+}
+
+// Process `#[cfg_attr]`.
+fn process_cfg_attr(diagnostic: &SpanHandler, krate: ast::Crate) -> ast::Crate {
+    let mut fld = CfgAttrFolder {
+        diag: diagnostic,
+        config: krate.config.clone(),
+    };
+    fld.fold_crate(krate)
+}
+
+impl<'a> fold::Folder for CfgAttrFolder<'a> {
+    fn fold_attribute(&mut self, attr: ast::Attribute) -> Option<ast::Attribute> {
+        if !attr.check_name("cfg_attr") {
+            return fold::noop_fold_attribute(attr, self);
+        }
+
+        let (cfg, mi) = match attr.meta_item_list() {
+            Some([ref cfg, ref mi]) => (cfg, mi),
+            _ => {
+                self.diag.span_err(attr.span, "expected `#[cfg_attr(<cfg pattern>, <attr>)]`");
+                return None;
+            }
+        };
+
+        if attr::cfg_matches(self.diag, &self.config[], &cfg) {
+            Some(respan(mi.span, ast::Attribute_ {
+                id: attr::mk_attr_id(),
+                style: attr.node.style,
+                value: mi.clone(),
+                is_sugared_doc: false,
+            }))
+        } else {
+            None
+        }
+    }
+
+    // Need the ability to run pre-expansion.
+    fn fold_mac(&mut self, mac: ast::Mac) -> ast::Mac {
+        fold::noop_fold_mac(mac, self)
+    }
+}

src/libsyntax/ext/base.rs

@@ -528,8 +528,6 @@ fn initial_syntax_expander_table(ecfg: &expand::ExpansionConfig) -> SyntaxEnv {
     syntax_expanders.insert(intern("cfg"),
                             builtin_normal_expander(
                                     ext::cfg::expand_cfg));
-    syntax_expanders.insert(intern("cfg_attr"),
-                            Modifier(box 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

@@ -96,7 +96,6 @@ pub mod ext {
     pub mod base;
     pub mod build;
     pub mod cfg;
-    pub mod cfg_attr;
     pub mod concat;
     pub mod concat_idents;
     pub mod deriving;

src/libsyntax/lib.rs

@@ -0,0 +1,18 @@
+// 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.
+//
+// error-pattern: main function not found
+// compile-flags: --cfg foo
+
+// main is conditionally compiled, but the conditional compilation
+// is conditional too!
+
+#[cfg_attr(foo, cfg(bar))]
+fn main() { }

src/test/compile-fail/cfg-attr-cfg-2.rs

@@ -0,0 +1,17 @@
+// 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.
+//
+// compile-flags: --cfg broken
+
+// https://github.com/rust-lang/rust/issues/21833#issuecomment-72353044
+
+#![cfg_attr(broken, no_std)] //~ ERROR no_std is experimental
+
+fn main() { }

src/test/compile-fail/cfg-attr-crate-2.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.
+
+// main is conditionally compiled, but the conditional compilation
+// is conditional too!
+
+#[cfg_attr(foo, cfg(bar))]
+fn main() { }

src/test/run-pass/cfg-attr-cfg.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.
+
+// https://github.com/rust-lang/rust/issues/21833#issuecomment-72353044
+
+#![cfg_attr(not_used, no_std)]
+
+fn main() { }