explain in more depth about `keep-stage`

[nikomatsakis]

#191 added some commentary about --keep-stage 1 but there is probably more to say.

@Mark-Simulacrum wrote a comment explaining some more workflows but I confess I haven't fully processed it.

It seems like we should dedicate an entire section (I'd make a new file, perosnally) to keep-stage and explain:

• What exactly it does (e.g., what is the effect of --keep-stage 0)
• Common workflows where it can be useful

[dwijnand]

I've so many questions for @Mark-Simulacrum about his workflows in #191 (comment):

How does x.py build --stage 1 --keep-stage 0 src/libstd "build the compiler once and only once"? Doesn't it skip compiling the stage 0 compiler and never get to compiling the stage 1 compiler?

Instead of x.py build --stage 1 src/libstd --keep-stage 1 wouldn't it be faster to do x.py build --stage 0 src/rustc? I think it is but it's missing the uplifting of rustc and sysroot into build/HOST/stage1.

Is there a difference between x.py build --stage 2 --keep-stage 0 and x.py build --stage 1? I think it's not because each stage isn't the same so it's not simply "do two dogfood iterations".

For cross-linking, my general staging discussions are happening in rust-lang/rust#57963.

[dwijnand]

Perhaps a good starting point would be off of @nikomatsakis' comment: #191 (comment)

What's does --keep-stage 1 do?

[mark-i-m]

Discussed a bit in rust-lang/rust#57963

[mark-i-m]

There is more discussion in https://rust-lang.github.io/rustc-guide/building/bootstrapping.html

So I will close this issue, but feel free to reopen...