Feature-gate associated constants.

Author: quantheory

src/doc/reference.md

@@ -2355,7 +2355,10 @@ The currently implemented features of the reference compiler are:
           semantics are likely to change, so this macro usage must be opted
           into.
 
-* `associated_types` - Allows type aliases in traits. Experimental.
+* `associated_consts` - Allows constants to be defined in `impl` and `trait`
+                        blocks, so that they can be associated with a type or
+                        trait in a similar manner to methods and associated
+                        types.
 
 * `box_patterns` - Allows `box` patterns, the exact semantics of which
                    is subject to change.

src/libsyntax/feature_gate.rs

@@ -155,6 +155,10 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     // Allows use of unary negate on unsigned integers, e.g. -e for e: u8
     ("negate_unsigned", "1.0.0", Active),
+
+    // Allows the definition of associated constants in `trait` or `impl`
+    // blocks.
+    ("associated_consts", "1.0.0", Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -659,6 +663,30 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
         }
         visit::walk_fn(self, fn_kind, fn_decl, block, span);
     }
+
+    fn visit_trait_item(&mut self, ti: &'v ast::TraitItem) {
+        match ti.node {
+            ast::ConstTraitItem(..) => {
+                self.gate_feature("associated_consts",
+                                  ti.span,
+                                  "associated constants are experimental")
+            }
+            _ => {}
+        }
+        visit::walk_trait_item(self, ti);
+    }
+
+    fn visit_impl_item(&mut self, ii: &'v ast::ImplItem) {
+        match ii.node {
+            ast::ConstImplItem(..) => {
+                self.gate_feature("associated_consts",
+                                  ii.span,
+                                  "associated constants are experimental")
+            }
+            _ => {}
+        }
+        visit::walk_impl_item(self, ii);
+    }
 }
 
 fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,

src/test/auxiliary/associated-const-cc-lib.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_consts)]
+
 #![crate_type="lib"]
 
 use std::marker::MarkerTrait;

src/test/compile-fail/associated-const-dead-code.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_consts)]
 #![deny(dead_code)]
 
 struct MyFoo;

src/test/compile-fail/associated-const-impl-wrong-type.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_consts)]
+
 use std::marker::MarkerTrait;
 
 trait Foo: MarkerTrait {

src/test/compile-fail/associated-const-private-impl.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_consts)]
+
 use std::marker::MarkerTrait;
 
 mod bar1 {

src/test/compile-fail/associated-const-upper-case-lint.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_consts)]
 #![deny(non_upper_case_globals)]
 #![allow(dead_code)]
 

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

@@ -0,0 +1,25 @@
+// 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.
+
+use std::marker::MarkerTrait;
+
+trait MyTrait: MarkerTrait {
+    const C: bool;
+    //~^ associated constants are experimental
+    //~| add #![feature(associated_consts)] to the crate attributes to enable
+}
+
+struct Foo;
+
+impl Foo {
+    const C: bool = true;
+    //~^ associated constants are experimental
+    //~| add #![feature(associated_consts)] to the crate attributes to enable
+}

src/test/compile-fail/impl-wrong-item-for-trait.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_consts)]
+
 trait Foo {
     fn bar(&self);
     const MY_CONST: u32;

src/test/run-pass/associated-const-cross-crate-defaults.rs

@@ -10,6 +10,8 @@
 
 // aux-build:associated-const-cc-lib.rs
 
+#![feature(associated_consts)]
+
 extern crate associated_const_cc_lib as foolib;
 
 pub struct LocalFooUseDefault;