rustbuild: Fix the --build argument to bootstrap.py

[segevfiner]

This makes the --build argument also apply for the downloading of the stage0 toolchain and building rustbuild.

Fixes #42116

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[shepmaster]

Thanks for the hard work, @segevfiner! We'll get someone top-notch to review this shortly!

/cc @Mark-Simulacrum who is also working in rustbuild world

[Mark-Simulacrum]

It feels like this should be inside build_triple itself, but presumably it might not have access to args today? Not sure.

cc @aidanhs, this affects python bootstrap

[segevfiner]

It doesn't have access. The command line is handled in the bootstrap() function. The parsed args, or stuff derived from them, are set as attributes of the RustBuild object.

[aidanhs]

The fix looks reasonable, but I'm tempted to instead propose that we just remove that piece of documentation and make it configured by the config file. Seems a little odd for --build to be a special case of an argument understood for reconfiguration.

@segevfiner, what do you think about just altering the toml? Or, alternatively, I assume you can pass an argument to configure to set this? (or if you can't, you should be able to!)

Just want to understand the use-case.

[segevfiner]

@aidanhs
I think all the arguments that bootstrap.py parses are like this. They are passed to the underlying bootstrap executable (rustbuild itself) too.

build is configurable via the config file, that's what I did when I wanted to build using a non-default build. Using it from the config is fine, I see there is even a flag so you can use multiple config files, which is useful when building using multiple build compilers in the same source tree. But using --build will be kinda convenient for this too, which I guess is why it exists in the first place. You will than only need one config.toml and there is even a target specific section in config.toml to help with this.

If it's decided this should not be supported, it should probably removed entirely from rustbuild (It appears in --help). But note that we also have --target and --host, and that begs the question if they should be removed than too...

[aidanhs]

Gotcha, so supporting it here is required if we want to support that flag on rustbuild. Thanks for explaining :) I'm suspicious that there are more edge cases here that aren't handled correctly, but clearly this is a step forward so I'm happy.

Any comments from a rustbuild perspective @Mark-Simulacrum?

[Mark-Simulacrum]

I don't think so. It seems good to handle --build in the python script so that it can work for more cases; I don't think there are any other flags that we'd need to handle like this, but I could be wrong. If there are, we probably should do something about it. I've wondered if we should have a "minimal" bootstrap in Rust which takes over some of the more complicated logic in the python script, maybe, but that's likely not possible today (and certainly out of scope for this PR). Overall, I think this is good.

[aidanhs]

Thinking about it briefly, I agree - no flags spring to mind as additional cases that should be considered, but I'll look through all the possible flags tomorrow. In the meantime...

@bors r+

[bors]

📌 Commit c41ac2e has been approved by aidanhs

[bors]

⌛ Testing commit c41ac2e with merge 5c126bd...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: aidanhs
Pushing 5c126bd to master...