extend x.py so it is easy to pass custom -Z flags when building after stage 0

[nikomatsakis]

We'd like to start trying to bootstrap with -Znll enabled. From what I can tell, there is no easy way to do this at present: using RUSTFLAGS=-Znll will cause the flag to be passed to all stages, but the support for NLL in stage0 is not up to the task.

We could extend x.py to support RUSTFLAGS_STAGE{0,1,2} (which would get "copied into" the RUSTFLAGS environment when building stage 0, 1, or 2.

Or we could make some kind of flag.

It might also be nice to just support RUSTFLAGS_NOT_STAGE0 since usually that's what you want.

Or maybe there's an existing solution here?

[Mark-Simulacrum]

So we can try to solve this generally (I know @eddyb has requested RUSTFLAGS_STAGE_N support before), which wouldn't be too hard. Alternatively, we can also make nll an option in config.toml. Which would we prefer? I'm open to either; once I hear back we can probably have an implementation up in a day at most I'd imagine.

[nikomatsakis]

@Mark-Simulacrum I think I prefer RUSTFLAGS_STAGE_N. I agree it's quite easy to do. I would probably prefer to have:

• RUSTFLAGS_STAGE_0
• RUSTFLAGS_STAGE_1
• RUSTFLAGS_STAGE_2
• RUSTFLAGS_STAGE_1_OR_2 // i.e., not(stage0)

But what to call that final one I don't know. Maybe RUSTFLAGS_BOOTSTRAP? It seems like it is what you are going to want most of the time.

[eddyb]

RUSTFLAGS_STAGE_NOT_0 (alternatively, RUSTFLAGS_STAGE_GT_0)

[Mark-Simulacrum]

Okay, I'll try and get an implementation of that up by the end of the week. I've assigned myself, but of course, if anyone wants to do this they are welcome to..