Install rust toolchain on building source distribution

[padix-key]

Currently, maturin requires the rust toolchain (cargo, rustc) already to be installed. This is no problem in a development scenario as Rust developers presumably have Rust installed.

However, when a user installs the source distribution of a package built with maturin, the user also requires the Rust toolchain, although they may be only interested in the package, but not in Rust. Rust is also not a part (yet) of the major OS distributions, so that user would need to go the extra mile of running rustup first.

Therefore, I propose installing the Rust toolchain in an isolated environment (e.g. similar to https://github.com/chriskuehl/rustenv) when maturin is used as [build-system], at least in case the rust toolchain is not installed, yet.

[messense]

I don't think this should be done in maturin, when using source distribution, users should be aware of the build system requirement.

Even though gcc/clang is part of the major OS distributions, when using sdist, it's still possible that they are missing from the OS environment of the end user, in such case should setuptools or other build backends install gcc/clang for the users? Absolutely not, IMO.

[konstin]

Personally, i think a source distribution should install all build dependencies it needs; it should not be the responsibility of a python user to understand the native build system of their transitive dependencies only because they are let's say on an arm platform without wheels. We'd need a way to bootstrap rust and cargo for this though that doesn't change the users environment (like rustup does). It would be ideal to have wheels for cargo and rustc on pypi like we have with zig. Relying on (linux) distros is often insufficient, they require root access to install new packages that is not available in many environments and only ship specific versions of tools. It would honestly be amazing if we could get gcc and clang from pypi too.

[messense]

It sounds amazing but surely there will be people start complain that using sdist download lots of huge dependencies.

IMO if it's really desirable, perhaps it should just be another dependency in [build-system] table that gets installed by a build frontend, instead of installing by maturin?

[konstin]

We can check for rust and cargo in get_requires_for_build_wheel and only if it is not installed add a dependency on the pypi package(s). For me, maturin should handle as much as possible end-to-end without the developer specifying something additionally.

[messense]

We can check for rust and cargo in get_requires_for_build_wheel and only if it is not installed add a dependency on the pypi package(s).

I'm fine with this solution if it works, but consider when a sdist requires a specific
version of nightly Rust toolchain that wasn't not installed, it seems impractical to have every single nightly Rust toolchain on pypi.

[konstin]

Totally, this would be limited to stable rust

[padix-key]

Thanks for the discussion. For me both options, an explicit extra dependency in build-system or an implicit installation in an environment by maturin would work.

[Owen-CH-Leung]

I'll try to file a PR to address this in the coming few days