Add feature gate

src/doc/reference.md

@@ -1178,10 +1178,11 @@ let px: i32 = match p { Point(x, _) => x };
 ```
 
 A _unit-like struct_ is a structure without any fields, defined by leaving off
-the list of fields entirely. Such structure implicitly defines a constant of
+the list of fields entirely. Such a structure implicitly defines a constant of
 its type with the same name. For example:
 
 ```
+# #![feature(braced_empty_structs)]
 struct Cookie;
 let c = [Cookie, Cookie {}, Cookie, Cookie {}];
 ```
@@ -1189,6 +1190,7 @@ let c = [Cookie, Cookie {}, Cookie, Cookie {}];
 is equivalent to
 
 ```
+# #![feature(braced_empty_structs)]
 struct Cookie {}
 const Cookie: Cookie = Cookie {};
 let c = [Cookie, Cookie {}, Cookie, Cookie {}];
@@ -2420,6 +2422,7 @@ The currently implemented features of the reference compiler are:
                               terms of encapsulation).
 * - `default_type_parameter_fallback` - Allows type parameter defaults to
                                         influence type inference.
+* - `braced_empty_structs` - Allows use of empty structs with braces.
 
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about `#![feature]` directives which enabled

src/libsyntax/feature_gate.rs

@@ -191,6 +191,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
 
     // allow `#[unwind]`
     ("unwind_attributes", "1.4.0", None, Active),
+
+    // allow empty structs/enum variants with braces
+    ("braced_empty_structs", "1.5.0", None, Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -775,7 +778,7 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                 }
             }
 
-            ast::ItemStruct(..) => {
+            ast::ItemStruct(ref def, _) => {
                 if attr::contains_name(&i.attrs[..], "simd") {
                     self.gate_feature("simd", i.span,
                                       "SIMD types are experimental and possibly buggy");
@@ -794,6 +797,10 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                         }
                     }
                 }
+                if def.fields.is_empty() && def.ctor_id.is_none() {
+                    self.gate_feature("braced_empty_structs", i.span,
+                                      "empty structs with braces are unstable");
+                }
             }
 
             ast::ItemDefaultImpl(..) => {
@@ -843,6 +850,12 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                   "box expression syntax is experimental; \
                                    you can call `Box::new` instead.");
             }
+            ast::ExprStruct(_, ref fields, ref expr) => {
+                if fields.is_empty() && expr.is_none() {
+                    self.gate_feature("braced_empty_structs", e.span,
+                                      "empty structs with braces are unstable");
+                }
+            }
             _ => {}
         }
         visit::walk_expr(self, e);
@@ -867,6 +880,12 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                   pattern.span,
                                   "box pattern syntax is experimental");
             }
+            ast::PatStruct(_, ref fields, dotdot) => {
+                if fields.is_empty() && !dotdot {
+                    self.gate_feature("braced_empty_structs", pattern.span,
+                                      "empty structs with braces are unstable");
+                }
+            }
             _ => {}
         }
         visit::walk_pat(self, pattern)

src/test/compile-fail/empty-struct-with-braces-1.rs

@@ -10,6 +10,8 @@
 
 // Empty struct defined with braces shouldn't add names into value namespace
 
+#![feature(braced_empty_structs)]
+
 struct Empty {}
 
 fn main() {

src/test/compile-fail/empty-struct-with-braces-2.rs

@@ -10,6 +10,7 @@
 
 // Empty struct defined with braces shouldn't add names into value namespace
 
+#![feature(braced_empty_structs)]
 #![deny(warnings)]
 
 struct Empty {}

src/test/compile-fail/empty-struct-with-braces-3.rs

@@ -0,0 +1,21 @@
+// 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.
+
+// Feature gate test for empty struct with braces
+
+struct Empty {} //~ ERROR empty structs with braces are unstable
+
+fn main() {
+    let e = Empty {}; //~ ERROR empty structs with braces are unstable
+
+    match e {
+        Empty {} => {} //~ ERROR empty structs with braces are unstable
+    }
+}

src/test/run-pass/empty-struct-with-braces.rs

@@ -11,6 +11,8 @@
 // Empty struct defined with braces add names into type namespace
 // Empty struct defined without braces add names into both type and value namespaces
 
+#![feature(braced_empty_structs)]
+
 struct Empty1 {}
 struct Empty2;
 struct Empty3 {}

src/test/run-pass/issue-16819.rs

@@ -10,6 +10,8 @@
 
 //`#[cfg]` on struct field permits empty unusable struct
 
+#![feature(braced_empty_structs)]
+
 struct S {
     #[cfg(untrue)]
     a: int,