Auto merge of #54778 - scottmcm:stabilize-ihle, r=pnkfelix

Stabilize impl_header_lifetime_elision in 2015

~~This is currently blocked on #54902; it should be good after that~~

It's already stable in 2018; this finishes the stabilization.

FCP completed (#15872 (comment)), proposal (#15872 (comment)).

Tracking issue: #15872
Usage examples (from libcore): #54687

Author: bors

src/libcore/lib.rs

@@ -87,7 +87,7 @@
 #![feature(doc_spotlight)]
 #![feature(extern_types)]
 #![feature(fundamental)]
-#![feature(impl_header_lifetime_elision)]
+#![cfg_attr(stage0, feature(impl_header_lifetime_elision))]
 #![feature(intrinsics)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]

src/libcore/tests/lib.rs

@@ -19,7 +19,7 @@
 #![feature(flt2dec)]
 #![feature(fmt_internals)]
 #![feature(hashmap_internals)]
-#![feature(impl_header_lifetime_elision)]
+#![cfg_attr(stage0, feature(impl_header_lifetime_elision))]
 #![feature(pattern)]
 #![feature(range_is_empty)]
 #![feature(raw)]