Rollup merge of rust-lang#40564 - GuillaumeGomez:rustdoc-const, r=frewsxcv

Fix const not displayed in rustdoc

Fixes rust-lang#40331.

r? @rust-lang/docs

Author: Ariel Ben-Yehuda

src/librustdoc/clean/mod.rs

@@ -1387,7 +1387,7 @@ impl<'tcx> Clean<Item> for ty::AssociatedItem {
                         decl: decl,
                         abi: sig.abi(),
 
-                        // trait methods canot (currently, at least) be const
+                        // trait methods cannot (currently, at least) be const
                         constness: hir::Constness::NotConst,
                     })
                 } else {

src/librustdoc/html/render.rs

@@ -60,6 +60,7 @@ use rustc::middle::privacy::AccessLevels;
 use rustc::middle::stability;
 use rustc::hir;
 use rustc::util::nodemap::{FxHashMap, FxHashSet};
+use rustc::session::config::nightly_options::is_nightly_build;
 use rustc_data_structures::flock;
 
 use clean::{self, AttributesExt, GetDefId, SelfTy, Mutability};
@@ -2316,9 +2317,10 @@ fn render_assoc_item(w: &mut fmt::Formatter,
             }
         };
         // FIXME(#24111): remove when `const_fn` is stabilized
-        let vis_constness = match UnstableFeatures::from_environment() {
-            UnstableFeatures::Allow => constness,
-            _ => hir::Constness::NotConst
+        let vis_constness = if is_nightly_build() {
+            constness
+        } else {
+            hir::Constness::NotConst
         };
         let prefix = format!("{}{}{:#}fn {}{:#}",
                              ConstnessSpace(vis_constness),

src/test/rustdoc/const.rs

@@ -0,0 +1,22 @@
+// 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.
+
+#![crate_type="lib"]
+
+#![feature(const_fn)]
+
+pub struct Foo;
+
+impl Foo {
+    // @has const/struct.Foo.html '//*[@id="new.v"]//code' 'const unsafe fn new'
+    pub const unsafe fn new() -> Foo {
+        Foo
+    }
+}