We should document the exact minimum versions of build tools required to build a given Rust toolchain

[jieyouxu]

As noticed in #128722 (comment) when trying to bump cc to a version which drops support for VS 2013.

Also cc:

We also don't document minium versions of binutils etc on the platform support page. Maybe we should! But that's a pre-existing issue and bumping cc or not doesn't change that.

Originally posted by @ChrisDenton in #129290 (comment)

INSTALL.md exists, but that seems more like some general advice for getting started, but not actual documented guarantees in terms of what kind of build environment is supported.

[Mark-Simulacrum]

I think this makes sense, but it should be noted that just documenting is very likely to get accidentally stale. Unfortunately, it's not clear that we can easily test those in CI -- often the minimum requires a good deal of work to actually obtain, especially on e.g. macOS or Windows.

[dpaoliello]

We'd probably have to have a tier system similar to platform support:

• Recommended/tested tools: these are what the Rust CI uses.
• Supported tools: these aren't always tested but should work and the Rust project will investigate any reported issues.

[dpaoliello]

likely to get accidentally stale

This is already happening with the min VS version: INSTALL.md and the error message for not finding MSVC were both updated to VS 2017, but rustup's docs were still saying VS 2013 (the PR to bump to 2017 has been merged in, but the new docs haven't been deployed yet).

If we're going to document tooling versions, then it really needs to be in a single place that everything else references, and we need to have pointers between the code and those docs to ensure that things are updated together.

[dpaoliello]

I've put together some thoughts about this into a Pre-RFC: https://internals.rust-lang.org/t/pre-rfc-build-tool-support-policy-aka-tool-tier-policy/21596

[tgross35]

We probably need to bump cc in the next couple of days to try to help a bug, which means dropping support for vs2013. @dpaoliello would you be up for putting up a PR documenting this change? I don't think we can wait on official policy for supported build tool tiers (though I like that idea), but just making sure it is noted in the target docs and/or INSTALL.md seems reasonable until we have something better.

[dpaoliello]

@tgross35 I'm not sure there needs to be anything changed in Rust's docs: INSTALL.md and Rust's error messages both say you need VS 2017+, and I already updated rustup's docs to match (that was back in May, I don't know why that hasn't been deployed yet).

Once rustup deploys their new docs there will be nothing left that indicates that VS 2013 (or 2015) is supported.