RFC: Requiring Swift to build the Swift toolchain

[itaiferber]

This topic has been on my mind lately given my recent^[1] experiences^[2] getting Swift going on Gentoo, primarily because the idiomatic Gentoo approach to packages presents effectively a worst-case scenario for bootstrapping Swift:

1. It is strongly preferred that packages build from source (ideally on the machine they're going to run on), and cross-compilation wouldn't be a preferred solution;
2. It is atypical (though possible!) for multiple versions of a package to be installable simultaneously, so
3. It's quite uncommon of for a version of a package to depend on an earlier version of the same unless absolutely necessary

(Of course, Gentoo is... niche... and I wouldn't expect (nor suggest) that the Swift project at large consider it a meaningful use-case, but it is interesting to design with the potential of this in mind.)

I was initially concerned, but with a bit of thought, this seems pretty reasonably surmountable:

@Douglas_Gregor

@etcwilde

That said, I'd like to ask that we amend the requirement of what the version of Swift the Swift source files in the Swift compiler must build with to 5.10 so that we can avoid the multi-hour multi-stage toolchain-build-hopping dance, and that anything that doesn't compile with the 5.10 compiler can be reverted aggressively.

Sure, that's fine. We've already been doing this in practice.

So long as there's a guarantee that Swift 5.10 can be relied upon to bootstrap future compilers, one possible path forward on the platform:

1. Split the current swift-5.10.1 package (which builds Swift 5.10.1, installs it into a versioned dir, and symlinks some binaries into /usr/bin) into swift-toolchain-5.10.1 (which just builds and installs) and swift-5.10.1 (which depends on swift-toolchain-5.10.1, and just symlinks)
   • Since swift-toolchain-<vers> is standalone, I can slot it to allow multiple major.minor versions to be installed simultaneously
   • Since swift-<vers> is not, it will continue to only allow one version at a time
2. Future versions of swift-<vers> can then depend on swift-toolchain-5.10.1 and swift-toolchain-<vers>

Of course, these details are pretty Gentoo-specific so no need to pay too close attention, but this is at least a vote of confidence that any platform which the Swift 5.10 line can be updated to support can bootstrap itself without needing cross-compilation. (Which, again, is a sort of worst-case scenario for support.)

So, +1 from me. (Especially if we're comfortable marking Swift 5.10.x as a sort of LTS release, given the special nature of this change.)

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1. Swift 5.10.1 for Gentoo on GURU ↩︎

2. Building the Swift Project on Linux with lld instead of gold ↩︎