Auto merge of #21257 - alexcrichton:issue-20064, r=pnkfelix

These two attributes are used to change the entry point into a Rust program, but
for now they're being put behind feature gates until we have a chance to think
about them a little more. The #[start] attribute specifically may have its
signature changed.

This is a breaking change to due the usage of these attributes generating errors
by default now. If your crate is using these attributes, add this to your crate
root:

    #![feature(start)] // if you're using the #[start] attribute
    #![feature(main)]  // if you're using the #[main] attribute

cc #20064

Author: bors

src/doc/trpl/unsafe.md

@@ -447,7 +447,7 @@ in the same format as C:
 
 ```
 #![no_std]
-#![feature(lang_items)]
+#![feature(lang_items, start)]
 
 // Pull in the system libc library for what crt0.o likely requires
 extern crate libc;
@@ -475,7 +475,7 @@ compiler's name mangling too:
 ```ignore
 #![no_std]
 #![no_main]
-#![feature(lang_items)]
+#![feature(lang_items, start)]
 
 extern crate libc;
 
@@ -529,7 +529,7 @@ vectors provided from C, using idiomatic Rust practices.
 
 ```
 #![no_std]
-#![feature(lang_items)]
+#![feature(lang_items, start)]
 
 # extern crate libc;
 extern crate core;
@@ -653,7 +653,7 @@ sugar for dynamic allocations via `malloc` and `free`:
 
 ```
 #![no_std]
-#![feature(lang_items, box_syntax)]
+#![feature(lang_items, box_syntax, start)]
 
 extern crate libc;
 

src/libsyntax/feature_gate.rs

@@ -78,6 +78,8 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("while_let", Accepted),
 
     ("plugin", Active),
+    ("start", Active),
+    ("main", Active),
 
     // A temporary feature gate used to enable parser extensions needed
     // to bootstrap fix for #5723.
@@ -279,6 +281,18 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                     self.gate_feature("plugin_registrar", i.span,
                                       "compiler plugins are experimental and possibly buggy");
                 }
+                if attr::contains_name(&i.attrs[], "start") {
+                    self.gate_feature("start", i.span,
+                                      "a #[start] function is an experimental \
+                                       feature whose signature may change \
+                                       over time");
+                }
+                if attr::contains_name(&i.attrs[], "main") {
+                    self.gate_feature("main", i.span,
+                                      "declaration of a nonstandard #[main] \
+                                       function may change over time, for now \
+                                       a top-level `fn main()` is required");
+                }
             }
 
             ast::ItemStruct(..) => {

src/test/compile-fail/feature-gate-main.rs

@@ -0,0 +1,12 @@
+// 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]
+fn foo() {} //~ ERROR: declaration of a nonstandard #[main] function may change over time

src/test/compile-fail/feature-gate-start.rs

@@ -0,0 +1,13 @@
+// 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.
+
+#[start]
+fn foo() {} //~ ERROR: a #[start] function is an experimental feature
+

src/test/compile-fail/issue-9575.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(start)]
+
 #[start]
 fn start(argc: isize, argv: *const *const u8, crate_map: *const u8) -> isize {
     //~^ ERROR incorrect number of function parameters

src/test/compile-fail/lang-item-missing.rs

@@ -14,6 +14,7 @@
 // error-pattern: requires `sized` lang_item
 
 #![no_std]
+#![feature(start)]
 
 #[start]
 fn start(argc: isize, argv: *const *const u8) -> isize {

src/test/compile-fail/lint-dead-code-2.rs

@@ -10,6 +10,7 @@
 
 #![allow(unused_variables)]
 #![deny(dead_code)]
+#![feature(main, start)]
 
 struct Foo;
 

src/test/compile-fail/multiple-main-2.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(main)]
+
 #[main]
 fn bar() {
 }

src/test/compile-fail/multiple-main-3.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(main)]
+
 #[main]
 fn main1() {
 }

src/test/compile-fail/privacy1.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(lang_items)]
+#![feature(lang_items, start)]
 #![no_std] // makes debugging this test *a lot* easier (during resolve)
 
 #[lang="sized"]