Document rustc internal env vars a little bit

[mark-i-m]

cc #436 @pnkfelix @nikomatsakis @Mark-Simulacrum rust-lang/rust#38686

From: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/env.20vars.20and.20x.2Epy.20.2338686

[mark-i-m]

r? @pnkfelix

[Mark-Simulacrum]

Hm, I feel like it's also worth noting that 99% of the time you want to switch from running the bootstrap rustc and instead run stageN/bin/rustc or something along those lines, but that can be left for later.

[mark-i-m]

@Mark-Simulacrum Could you explain what you mean? Do you mean directly run ./stageN/bin/rustc rather than ./x.py run ...?

[Mark-Simulacrum]

Yes, I mean to directly run ./stageN/bin/rustc. I'm not sure what you mean by ./x.py run ......

[mark-i-m]

Got it. At one point this did not work. It gave errors about environment variables... I haven't tried it recently, though.

I'm not sure what you mean by ./x.py run ......

I mean running the compiled compiler via ./x.py (e.g. ./x.py test src/test/ui/blah.rs)

[Mark-Simulacrum]

If ./stageN/bin/rustc gives an error about environment variables, that usually means something is quite wrong -- or you're trying to compile e.g. librustc or libstd or something that depends on environment variables, but I imagine that's rare (and would be the same anywhere if you skip to a rustc invocation from cargo build -vv).

[mark-i-m]

@Mark-Simulacrum added a note (mostly verbatim from you post)

[mark-i-m]

If it looks good to you, please github-approve and merge.

[mark-i-m]

@Mark-Simulacrum ^^^?

[Mark-Simulacrum]

Oh that was referring to me! Sorry. Yes, this looks good to me...