build aarch64-musl host compiler in CI

[alex]

fixes #75721

I've put this together by cargo culting pieces from different Docker files. It's a bit emberassing, because I frankly have no idea if I did this correctly 😬 The docker image builds, but attempting to run it produces various permission errors that I believe are related to my local docker setup by rootless. In short, I'm very sorry for this mess and please review with a critical eye.

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

r? @pietroalbini

[ehuss]

I assume the PR title "must" is a typo?

Can you update this line in the docs to add a checkmark in the third column to indicate this target is available as a host target?

rust/src/doc/rustc/src/platform-support.md

Line 65 in e15510c

`aarch64-unknown-linux-musl` | ✓ | | ARM64 Linux with MUSL

(FWIW, I ran the docker image, and it failed with Host compiler must support std::atomic! when building LLVM for aarch64-unknown-linux-musl)

[alex]

Sigh. Yes, it's a typo. It's a good thing my job doesn't require a lot of typing, I'm quite bad at it.

[alex]

I've updated the docs.

As for the compilation error. #63623 mentions that same error, but doesn't quite find the underlying cause. My inclination is that it likely means the version of GCC being used to build LLVM is too old -- likely since the base docker image is Ubuntu 16.04 and I'm sure aarch64 has improved a lot since then. Is bumping the base container a reasonable thing?

[ehuss]

I don't really know much about it. One thing I don't understand is the difference between musl-toolchain.sh (which is used for the x86_64-unknown-linux-musl host builder and builds gcc 9.2.0) and musl.sh (which uses the system gcc of 5.4.0). They seem to create the musl root in completely different ways.

[mati865]

They seem to create the musl root in completely different ways.

Thats true.
musl.sh takes system GCC and instructs it to use musl as C library. Obviously musl C++ is missing in this case so it's only able to compile and link C code.
musl-toolchain.sh prepares full GCC toolchain that can build and link C++ code and has kernel headers.

[pietroalbini]

Since this moves AArch64 MUSL from Cross-compilation Tier 2 to Development Platform Tier 2, cc @rust-lang/compiler

@rustbot modify labels: S-waiting-on-team -S-waiting-on-review

[spastorino]

This was discussed today as part of the compiler weekly meeting

[alex]

@ehuss @pietroalbini I think the PR that was closed in favor of this might actually be more complete/better/more functional. I guess I got here first, but I'm very happy to hand it over to @Gelbpunkt unless there's something I missed :-)

[pnkfelix]

This was discussed today as part of the compiler weekly meeting

More specifically, I think the T-compiler team was generally in favor of landing this.

So I'm removing S-waiting-on-team.

(One thing I do not know is whether T-infra has discussed this. Cc @rust-lang/infra )

[pietroalbini]

At @alex's request, closing this in favor of #76420!