This commit enables ThinLTO for the compiler as well as multiple codegen units.
This is intended to get the benefits of parallel codegen while also avoiding
any major loss of perf. Finally this commit is also intended as further testing
for #45320 and shaking out bugs.

…ker.

At stage 1 rustdoc is built at stage 0 so the error index generator should be as well.

This fixes `x.py --stage 1 doc` as rustdoc doesn't even build at stage 1.

This commit moves compression of the bytecode from the `link` module to the
`write` module, namely allowing it to be (a) cached by incremental compilation
and (b) produced in parallel. The parallelization may show up as some nice wins
during normal compilation and the caching in incremental mode should be
beneficial for incremental compiles! (no more need to recompress the entire
crate's bitcode on all builds)

…ster

rustc: Move bytecode compression into codegen

This commit moves compression of the bytecode from the `link` module to the
`write` module, namely allowing it to be (a) cached by incremental compilation
and (b) produced in parallel. The parallelization may show up as some nice wins
during normal compilation and the caching in incremental mode should be
beneficial for incremental compiles! (no more need to recompress the entire
crate's bitcode on all builds)

rustbuild: Compile rustc with ThinLTO

This commit enables ThinLTO for the compiler as well as multiple codegen units.
This is intended to get the benefits of parallel codegen while also avoiding
any major loss of perf. Finally this commit is also intended as further testing
for #45320 and shaking out bugs.

update jobserver version to work around macos bug

Update `jobserver` crate to fix rust-lang/cargo#4643, a panic which can't happen according to `libc::poll`'s man page but was nevertheless reported on macOS 10.9 and 10.10.

r? @alexcrichton

rustbuild: Fix path for the nomicon

…lacrum

rustbuild: Build stage 1 error index generator at stage 0

At stage 1 rustdoc is built at stage 0 so the error index generator should be as well.

This fixes `x.py --stage 1 doc` as rustdoc doesn't even build at stage 1.

It was broken by #44605.

r? @alexcrichton

…etrochenkov

Cleanly error for non-const variable in associated const

Not sure if wrapping the whole `visit::walk_impl_item` call is correct.
Closes #44239

Fix rustbuild --color conflict when building on Travis outside of Docker

When trying to build rust on Travis without using `stamp` or `docker`, both `RUSTC_COLOR=1` and `TRAVIS=true` will separately pass `--color always` to the command line. This causes the build to fail due to "*Option 'color' given more than once*".

In this PR, the `RUSTC_COLOR=1` will not be passed in the CI environment.

Improve docs around `Once::call_once_force` and `OnceState`.

Added some examples, clarify behavior, etc etc.

Fixes #43472.