Bump to 1.31.0 and bootstrap from 1.30 beta

src/libstd/lib.rs

@@ -250,8 +250,7 @@
 #![feature(cfg_target_vendor)]
 #![feature(char_error_internals)]
 #![feature(compiler_builtins_lib)]
-#![cfg_attr(stage0, feature(const_fn))]
-#![cfg_attr(not(stage0), feature(min_const_fn))]
+#![feature(min_const_fn)]
 #![feature(const_int_ops)]
 #![feature(const_ip)]
 #![feature(core_intrinsics)]
@@ -270,10 +269,9 @@
 #![feature(libc)]
 #![feature(link_args)]
 #![feature(linkage)]
-#![cfg_attr(stage0, feature(macro_vis_matcher))]
 #![feature(needs_panic_runtime)]
 #![feature(never_type)]
-#![cfg_attr(not(stage0), feature(nll))]
+#![feature(nll)]
 #![feature(exhaustive_patterns)]
 #![feature(on_unimplemented)]
 #![feature(optin_builtin_traits)]
@@ -286,7 +284,7 @@
 #![feature(rustc_attrs)]
 #![feature(rustc_const_unstable)]
 #![feature(std_internals)]
-#![feature(stdsimd)]
+#![cfg_attr(not(stage0), feature(stdsimd))]
 #![feature(shrink_to)]
 #![feature(slice_concat_ext)]
 #![feature(slice_internals)]
@@ -302,15 +300,12 @@
 #![feature(unboxed_closures)]
 #![feature(untagged_unions)]
 #![feature(unwind_attributes)]
-#![cfg_attr(stage0, feature(use_extern_macros))]
 #![feature(doc_cfg)]
 #![feature(doc_masked)]
 #![feature(doc_spotlight)]
-#![cfg_attr(all(windows, stage0), feature(used))]
 #![feature(doc_alias)]
 #![feature(doc_keyword)]
 #![feature(panic_info_message)]
-#![cfg_attr(stage0, feature(panic_implementation))]
 #![feature(non_exhaustive)]
 
 #![default_lib_allocator]