Building the standard library from source

[lylythechosenone]

Goals

Build libcore, liballoc, and their dependencies from source using rules_rust, preferably with bzlmod.

Reasoning

When compiling to bare-metal targets, as well as custom targets, one is required to recompile the standard library from source. Currently, this is not well-supported by rules_rust.

Prior art

In bazel-rust-cross, @colatkinson implemented this using rust_repositories, although that rule is now deprecated.
In #2068, @daivinhtran made a first attempt at this, although it was quite bulky. It also manually downloaded the rust source files.

[lylythechosenone]

To make this more concrete, the only thing missing is an ability to fetch the rust-src component (as it is in rustup) along with a given toolchain. This was possible with rust_repositories, but does not seem to be possible with newer versions of rules_rust. Once those sources are available, it is as simple as making rust_library targets, making a rust_stdlib_filegroup, and creating a custom toolchain.

[lylythechosenone]

I have created a proof-of-concept exposing this component in lylythechosenone/rules_rust. It seems to work correctly. However, downloading rust_src is useless for most users, so it would be nice to make it optional. I'll await a contributor's response on how exactly to do that.

[lylythechosenone]

After playing with this a bit, it seems like more components would be needed. rust_toolchain_repository and rust_toolchain both take an exec_triple field, and it seems like it would be quite difficult to determine the exec triple in a MODULE file. It would be nice to contain this all inside of rules_rust in a simple build_std flag on rust_toolchain_repository or register_rust_toolchains. I may make another proof-of-concept implementing this soon, but I very much want contributor input on this issue.

[illicitonion]

@krasimirgg I think you have experience of bootstrapping a libstd with rules_rust, right?