use zeroed allocation instead of eagerly initializing the memory

compiler/rustc_middle/src/lib.rs

@@ -23,6 +23,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(allocator_api)]
 #![feature(array_windows)]
 #![feature(assert_matches)]
 #![feature(backtrace)]
@@ -33,6 +34,7 @@
 #![feature(discriminant_kind)]
 #![feature(never_type)]
 #![feature(extern_types)]
+#![feature(new_uninit)]
 #![feature(nll)]
 #![feature(once_cell)]
 #![feature(min_specialization)]