[nll] bootstrap rustc with NLL enabled

[nikomatsakis]

As discussed in today's meeting, we need to get rustc bootstrapping with NLL enabled -- if there are problems, we should isolate them.

@lqd is going to try and push this through and report back.

[lqd]

Here's a summary of this adventure :)

I'll ignore the cases where MIR borrowck correctly adds new "unused mut" lint warnings which were previously not found by the AST borrowck: while they are new warnings, the unused muts were easily removed from the rustc code in order to keep building, so I marked them as ok (but can produce the list of cases if need be).

I tested by manually enabling the NLL feature in crates, #![cfg_attr(not(stage0), feature(nll))], and building. This doesn't test the submodules, stdsimd, and tools.

Interesting cases

• liballoc: doesn't build, already tracked as NLL / MIR-borrowck regression in liballoc #48224 (NLL / MIR-borrowck regression in liballoc #48224 (comment) mentions Niko might have a fix already)

• librustc_passes: doesn't build right now and seems correct (apparently this pattern comes up often, so maybe we can accept this in the future, with additions to 2-phase-borrows), minimized repro here for convenience.

• librustc_typeck: doesn't build because of incorrect "unused mut" lint warnings [NLL] variable does not need to be mutable #51830. and because of ICE cannot convert ... to a region vid which is librustc_mir/borrow_check/nll/universal_regions.rs:824: cannot convert ReFree ... #51351, and has a minimized repro.

• libsyntax_ext: doesn't build because of "broken MIR" ICE ICE bootstrapping rustc with NLL enabled #48071, which has a minimal repro here.

Cases of incorrect "unused mut" lint warnings

• libcore: used to ICE with region_obligations not empty (known issue region_obligations not empty #51649).
  This error has, after I started, become less prominent on master, and now 4 denied warnings prevent building. These are incorrect "unused mut" lint warnings when using move closures, and occurrences of known issue nll: incorrect unused_mut with move closure #51801 (which might be a more minimal reproduction of [NLL] mutability difference #50897)

• librustc doesn't build because of incorrect "unused mut" lint warnings nll: incorrect unused_mut with move closure #51801 and [NLL] variable does not need to be mutable #51830, the latter mentions removing a "mut" that doesn't exist in the surface code, and minimized in the issues and for convenience here

• librustc_data_structures: used to ICE with region_obligations not empty (known issue region_obligations not empty #51649). Now doesn't build because of incorrect "unused mut" lint warnings nll: incorrect unused_mut with move closure #51801.

• librustc_driver: doesn't build because of incorrect "unused mut" lint warnings nll: incorrect unused_mut with move closure #51801.

• librustc_lint: doesn't build because of incorrect "unused mut" lint warnings [NLL] variable does not need to be mutable #51830.

• librustc_traits: doesn't build because of incorrect "unused mut" lint warnings [NLL] variable does not need to be mutable #51830.

• libserialize: doesn't build because of incorrect "unused mut" lint warnings nll: incorrect unused_mut with move closure #51801.

• libsyntax: doesn't build because of incorrect "unused mut" lint warnings which I'm not sure we tracked so I filed it as NLL: incorrect "unused mut" lint warning, with multiple closures #51904. A reduced repro is available here. The fact that there are 2 closures seem important here.

Crates without issues

• liballoc_jemalloc: seems ok (unsure whether it's built when jemalloc is disabled in the config.toml)
• liballoc_system: seems ok
• libarena: ok
• libfmt_macros: ok
• libgraphviz: ok
• libpanic_abort: ok
• libpanic_unwind: ok
• libproc_macro: ok
• libprofiler_builtins: ok
• librustc_allocator: ok
• librustc_apfloat: ok
• librustc_asan: ok
• librustc_borrowck: ok
• librustc_codegen_llvm: ok
• librustc_codegen_utils: ok
• librustc_cratesio_shim: ok, but it's just 2 extern crates
• librustc_errors: ok
• librustc_incremental: ok
• librustc_llvm: ok
• librustc_lsan: ok
• librustc_metadata: ok
• librustc_mir: used to ICE with region_obligations not empty (known issue region_obligations not empty #51649). Now ok.
• librustc_msan: ok
• librustc_platform_intrinsics: ok
• librustc_plugin: ok
• librustc_privacy: ok
• librustc_resolve: ok
• librustc_save_analysis: ok
• librustc_target: ok
• librustc_tsan: ok
• librustdoc: ok
• libstd: ok
• libstd_unicode: the little of what is there is ok :)
• libsyntax_pos: ok
• libterm: ok
• libtest: ok
• libunwind: ok
• rustc: if it is built by x.py build, then it is ok.

[nikomatsakis]

Tagging this meta bug as NLL-deferred until the following issues are fixed:

• nll: incorrect unused-mut warnings #51918
• librustc_mir/borrow_check/nll/universal_regions.rs:824: cannot convert ReFree ... #51351
• ICE bootstrapping rustc with NLL enabled #48071
• NLL / MIR-borrowck regression in liballoc #48224
• ICE: mir_borrowck encountered mutable promoted #52671

[matthewjasper]

Now that all of those issues are fixed, I've added #52671 to the list since it is also blocking bootstrap.

With that fix bootstrapping still breaks due to the unused mut lint and a case where two phase borrows don't track activation through unsized coercions. I've created PRs for those.