auto merge of #9991 : LeoTestard/rust/asm-feature-gate, r=huonw

Fixes #9882
Note that the actual checking code is inside a if false in order to compile libstd properly.
libstd uses asm! in rt. If we put ```#[feature(asm)]``` in libstd, it fails to build at stage0 beacause the
asm feature is not known yet by the snapshot compiler.
We must wait that this PR arrives into the snapshot in order to actually activate the checking code.

src/librustc/front/feature_gate.rs

@@ -34,6 +34,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("macro_rules", Active),
     ("struct_variant", Active),
     ("once_fns", Active),
+    ("asm", Active),
 
     // These are used to test this portion of the compiler, they don't actually
     // mean anything
@@ -108,26 +109,29 @@ impl Visitor<()> for Context {
                 }
             }
 
-            ast::item_mac(ref mac) => {
-                match mac.node {
-                    ast::mac_invoc_tt(ref path, _, _) => {
-                        let rules = self.sess.ident_of("macro_rules");
-                        if path.segments.last().identifier == rules {
-                            self.gate_feature("macro_rules", i.span,
-                                              "macro definitions are not \
-                                               stable enough for use and are \
-                                               subject to change");
-                        }
-                    }
-                }
-            }
-
             _ => {}
         }
 
         visit::walk_item(self, i, ());
     }
 
+    fn visit_mac(&mut self, macro: &ast::mac, _: ()) {
+        let ast::mac_invoc_tt(ref path, _, _) = macro.node;
+
+        if path.segments.last().identifier == self.sess.ident_of("macro_rules") {
+            self.gate_feature("macro_rules", path.span, "macro definitions are \
+                not stable enough for use and are subject to change");
+        }
+
+        else if path.segments.last().identifier == self.sess.ident_of("asm") {
+            // NOTE: remove the false once the ASM feature is in the next snapshot
+            if false {
+                self.gate_feature("asm", path.span, "inline assembly is not \
+                    stable enough for use and is subject to change");
+            }
+        }
+    }
+
     fn visit_ty(&mut self, t: &ast::Ty, _: ()) {
         match t.node {
             ast::ty_closure(closure) if closure.onceness == ast::Once => {

src/libsyntax/visit.rs

@@ -90,6 +90,7 @@ pub trait Visitor<E:Clone> {
         walk_struct_def(self, s, i, g, n, e)
     }
     fn visit_struct_field(&mut self, s:@struct_field, e:E) { walk_struct_field(self, s, e) }
+    fn visit_mac(&mut self, m:&mac, e:E) { walk_mac(self, m, e); }
 }
 
 impl<E:Clone> Visitor<E> for @mut Visitor<E> {
@@ -150,6 +151,9 @@ impl<E:Clone> Visitor<E> for @mut Visitor<E> {
     fn visit_struct_field(&mut self, a:@struct_field, e:E) {
         (*self).visit_struct_field(a, e)
     }
+    fn visit_mac(&mut self, macro:&mac, e:E) {
+        (*self).visit_mac(macro, e);
+    }
 }
 
 pub fn walk_crate<E:Clone, V:Visitor<E>>(visitor: &mut V, crate: &Crate, env: E) {
@@ -247,7 +251,7 @@ pub fn walk_item<E:Clone, V:Visitor<E>>(visitor: &mut V, item: &item, env: E) {
                 visitor.visit_trait_method(method, env.clone())
             }
         }
-        item_mac(ref macro) => walk_mac(visitor, macro, env),
+        item_mac(ref macro) => visitor.visit_mac(macro, env),
     }
 }
 
@@ -507,7 +511,7 @@ pub fn walk_stmt<E:Clone, V:Visitor<E>>(visitor: &mut V, statement: &Stmt, env:
         StmtExpr(expression, _) | StmtSemi(expression, _) => {
             visitor.visit_expr(expression, env)
         }
-        StmtMac(ref macro, _) => walk_mac(visitor, macro, env),
+        StmtMac(ref macro, _) => visitor.visit_mac(macro, env),
     }
 }
 
@@ -644,7 +648,7 @@ pub fn walk_expr<E:Clone, V:Visitor<E>>(visitor: &mut V, expression: @Expr, env:
             walk_expr_opt(visitor, optional_expression, env.clone())
         }
         ExprLogLevel => {}
-        ExprMac(ref macro) => walk_mac(visitor, macro, env.clone()),
+        ExprMac(ref macro) => visitor.visit_mac(macro, env.clone()),
         ExprParen(subexpression) => {
             visitor.visit_expr(subexpression, env.clone())
         }

src/test/compile-fail/asm-gated.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 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.
+
+// xfail-test
+
+fn main() {
+    unsafe {
+        asm!(""); //~ ERROR inline assembly is not stable enough
+    }
+}