Auto merge of rust-lang#42809 - seanmonstar:stable-associated-consts, r=nikomatsakis

remove associated_consts feature gate

Currently struggling to run tests locally (something about jemalloc target missing).

cc rust-lang#29646

Author: bors

src/doc/unstable-book/src/language-features/associated-consts.md

@@ -70,7 +70,6 @@
 #![feature(allow_internal_unstable)]
 #![feature(asm)]
 #![feature(associated_type_defaults)]
-#![feature(associated_consts)]
 #![feature(cfg_target_feature)]
 #![feature(cfg_target_has_atomic)]
 #![feature(concat_idents)]
@@ -94,6 +93,8 @@
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
 
+#![cfg_attr(stage0, feature(associated_consts))]
+
 #[prelude_import]
 #[allow(unused)]
 use prelude::v1::*;