Link a rustup toolchain automatically in `x.py setup`

[Sl1mb0]

Issue tracker for linking a rustup toolchain automatically in x.py setup.

[Mark-Simulacrum]

Can you say more about the motivation? Initially, I would say that it is pretty likely that we do not want to support this kind of "aggregation" in the x.py interface; it complicates the information and I'm not sure it improves the user interface.

[Sl1mb0]

I agree that I don't think it improves the user interface much, but I think that the interface for this specific feature could be made better. My only argument for why I think this capability should be added is that it's natural to create a toolchain for the compiler immediately after having built it, so why not give ./x.py the capability to do so?

However, as I understand it you only need to run rustup once to create a toolchain for the build, and subsequent builds will always be used by the toolchain. So I can see why this is kind of pointless.

[Mark-Simulacrum]

I think streamlining the toolchain creation process is worthwhile to invest into; e.g., we could give a suggestion at the end of the first x.py build in a new build directory (with a stamp file just saying 'did that') to run rustup link .... with the appropriate path.

[Sl1mb0]

I like that idea. I'll wait for more feedback before I start looking into it though.

[jyn514]

Hmm, I would rather set up the toolchain automatically when running x.py setup, and give a warning if a toolchain named stage1 already exists, along with the command to set it up yourself.

If we gave a warning the first time you run x.py build, then a) people who aren't developing locally get confused (e.g. distros building from source) because they don't actually care about this, and b) x.py has no way of knowing whether you actually add the toolchain or not; I can definitely see people missing it the first time and then never getting the warning again just because x.py is using a stamp file.

[Mark-Simulacrum]

Yeah, x.py setup seems better. I think rustup won't let you do it without the sysroot somewhat existing, though:

$ rustup toolchain link foo foo
error: not a directory: 'foo/lib'

But I think we have enough in setup that we can probably create the directory temporarily...

[jyn514]

Mentoring instructions: just after

rust/src/bootstrap/setup.rs

Line 108 in bf64232

println!("`x.py` will now use the configuration at {}", include_path.display());

, call rustup toolchain link stage1 build/TARGET/stage1, where TARGET is builder.config.build. If that errors, give a warning and suggest that the user do it themselves.

[Sl1mb0]

where TARGET is builder.config.build

Could you clarify this? I assumed you were referring to build::Builder, but there is no config field there..