enable feature `impl_header_lifetime_elision`

src/librustc_data_structures/lib.rs

@@ -21,6 +21,7 @@
       html_root_url = "https://doc.rust-lang.org/nightly/")]
 
 #![feature(in_band_lifetimes)]
+#![feature(impl_header_lifetime_elision)]
 #![feature(unboxed_closures)]
 #![feature(fn_traits)]
 #![feature(unsize)]

src/librustc_mir/lib.rs

@@ -17,6 +17,7 @@
 #![cfg_attr(not(stage0), feature(nll))]
 #![feature(infer_outlives_requirements)]
 #![feature(in_band_lifetimes)]
+#![feature(impl_header_lifetime_elision)]
 #![feature(slice_patterns)]
 #![feature(slice_sort_by_cached_key)]
 #![feature(box_patterns)]