[nll] use NLL during bootstrapping

[nikomatsakis]

An important correctness milestone is to have the compiler itself use NLL when bootstrapping. We've made some stabs at this but we need to start doing it all the time.

Instructions

• Log into Zulip and head to correct thread to say hi =)
• Select a crate from the list below
• Leave a comment that you are trying it
• Verify that ./x.py build --stage 1 works for you
• Add #![cfg_attr(not(stage0), feature(nll))] to the lib.rs file
  • This incantation adds #![feature(nll)], but only once we get to stage1 -- otherwise we'd be using the beta version of NLL, which is old and buggy.)
• Try ./x.py build --stage 1 again and see if it works
• If so, open a PR

Crate list

• liballoc -- #53211
• liballoc_jemalloc -- #53211
• liballoc_system -- #53211
• libarena -- #53214
• libbacktrace
• libcompiler_builtins
• libcore -- #53230
• libfmt_macros -- #53219
• libgraphviz -- #53211
• liblibc
• libpanic_abort -- #53219
• libpanic_unwind -- #53219
• libproc_macro -- #53230
• libprofiler_builtins -- #53230
• librustc -- #53230
• librustc_allocator -- #53230
• librustc_apfloat -- #53219
• librustc_asan -- #53214
• librustc_borrowck -- #53211
• librustc_codegen_llvm -- #53230
• librustc_codegen_utils -- #53230
• librustc_cratesio_shim -- #53219
• librustc_data_structures -- #53230
• librustc_driver -- #53219
• librustc_errors -- #53214
• librustc_incremental -- #53219
• librustc_lint -- #53211
• librustc_llvm -- #53230
• librustc_lsan -- #53230
• librustc_metadata -- #53214
• librustc_mir -- #53230
• librustc_msan -- #53230
• librustc_passes -- #53214
• librustc_platform_intrinsics -- #53230
• librustc_plugin -- #53214
• librustc_privacy -- #53214
• librustc_resolve -- #53214
• librustc_save_analysis -- #53214
• librustc_target -- #53219
• librustc_traits -- #53214
• librustc_tsan -- #53214
• librustc_typeck -- #53230
• librustdoc -- #53230
• libserialize -- #53230
• libstd -- #53230
• libstd_unicode -- rolled into libcore
• libsyntax -- #53230
• libsyntax_ext -- #53230
• libsyntax_pos -- #53219
• libterm -- #53219
• libtest -- #53230
• libunwind -- #53219
• stdsimd

[lovesegfault]

I'm going to be attempting this for librustc_mir just to start.

[pnkfelix]

The remaining four crates:

• liblibc
• libbacktrace
• libcompiler_builtins
• stdsimd

are all "external dependencies", in the sense that they are crates that have been factored out and are hosted on separate repositories (still within the rust-lang or rust-lang-nursery ecosystems).

So, it seems like the vast majority of work here is done, and the important thing is: It seems clear that we can bootstrap ourselves using NLL.

Therefore, I am closing this task as "effectively finished"

[memoryruins]

Note: locally, libcompiler_builtins successfully built with NLL. I expect stdsimd and any others would too.

Well done, everyone :)