Promote tier 3 *-pc-windows-gnullvm targets to tier 2

[mati865]

Proposal

Promote aarch64-pc-windows-gnullvm, i686-pc-windows-gnullvm and x86_64-pc-windows-gnullvm targets to tier 2.

AArch64 and x86_64 targets were initially added in rust-lang/rust#94872 and gradually improved in subsequent pull requests. Recently i686 target has been also added in rust-lang/rust#115687, it has been skipped initially because it used to require patched LLVM.

A tier 2 target must have value to people other than its maintainers. (It may still be a niche target, but it must not be exclusively useful for an inherently closed group.)

There are users of MSYS2 and and llvm-mingw projects interested in having Rust targets that work well with LLVM toolchains. I assume more folks would also be interested in open source Windows targets with following features:

• native TLS
• Control Flow Guard
• doesn't suffer from some of windows-gnu issues (like https://github.com/rust-lang/rust/blob/3223b0b5e8dadda3f76c3fd1a8d6c5addc09599e/compiler/rustc_target/src/spec/windows_gnu_base.rs#L78 and https://github.com/rust-lang/rust/blob/3223b0b5e8dadda3f76c3fd1a8d6c5addc09599e/compiler/rustc_codegen_llvm/src/back/archive.rs#L152)
• has working ASan, UBSan and profilers (including PGO and coverage) but not currently provided by Rust

A tier 2 target must have a designated team of developers (the "target maintainers") available to consult on target-specific build-breaking issues, or if necessary to develop target-specific language or library implementation details. This team must have at least 2 developers.

The target maintainers should not only fix target-specific issues, but should use any such issue as an opportunity to educate the Rust community about portability to their target, and enhance documentation of the target.

I'm the only maintainer currently but @thomcc have expressed their willingness to step up as the second maintainer if necessary.

The target must not place undue burden on Rust developers not specifically concerned with that target. Rust developers are expected to not gratuitously break a tier 2 target, but are not expected to become experts in every tier 2 target, and are not expected to provide target-specific implementations for every tier 2 target.

These targets are very similar to tier 1 -pc-windows-gnu targets but use slightly different ABI and boast more features. Their maintenance should be very similar, except it's actually easier to obtain a working LLVM based C/C++ toolchain.

The target must provide documentation for the Rust community explaining how to build for the target using cross-compilation, and explaining how to run tests for the target. If at all possible, this documentation should show how to run Rust programs and tests for the target using emulation, to allow anyone to do so. If the target cannot be feasibly emulated, the documentation should explain how to obtain and work with physical hardware, cloud systems, or equivalent.

Cross compilation and testing are explained on the targets page.

The target must document its baseline expectations for the features or versions of CPUs, operating systems, libraries, runtime environments, and similar.

These targets follow the same requirements as other Windows targets with corresponding CPU.

Tier 2 targets must not leave any significant portions of core or the standard library unimplemented or stubbed out, unless they cannot possibly be supported on the target.

It does not differ from tier 1 Windows targets in this regard.

The code generation backend for the target should not have deficiencies that invalidate Rust safety properties, as evaluated by the Rust compiler team. (This requirement does not apply to arbitrary security enhancements or mitigations provided by code generation backends, only to those properties needed to ensure safe Rust code cannot cause undefined behavior or other unsoundness.) If this requirement does not hold, the target must clearly and prominently document any such limitations as part of the target's entry in the target tier list, and ideally also via a failing test in the testsuite. The Rust compiler team must be satisfied with the balance between these limitations and the difficulty of implementing the necessary features.

I'm not aware of such issues, in fact these targets are free from thread local issues that affect tier 1 *-windows-gnu targets: rust-lang/rust#100917

If the target supports C code, and the target has an interoperable calling convention for C code, the Rust target must support that C calling convention for the platform via extern "C". The C calling convention does not need to be the default Rust calling convention for the target, however.

It does not differ from tier 1 Windows targets in this regard.

The target must build reliably in CI, for all components that Rust's CI considers mandatory.

It builds reliably on my PC, it has to be accepted as tier 2 before it can be tested on the CI.

The approving teams may additionally require that a subset of tests pass in CI, such as enough to build a functional "hello world" program, ./x.py test --no-run, or equivalent "smoke tests". In particular, this requirement may apply if the target builds host tools, or if the tests in question provide substantial value via early detection of critical problems.

I don't think Rust's build system allows to run such tests when cross-compiling but it can be tested using nightly with -Z build-std.

Building the target in CI must not take substantially longer than the current slowest target in CI, and should not substantially raise the maintenance burden of the CI infrastructure. This requirement is subjective, to be evaluated by the infrastructure team, and will take the community importance of the target into account.

Building only std should be as fast as for the other targets using dist-various builder. Providing host tools would be a bit faster than dist-mingw builders but I doubt it's justified right now.

Tier 2 targets should, if at all possible, support cross-compiling. Tier 2 targets should not require using the target as the host for builds, even if the target supports host tools.

This is fully supported.

In addition to the legal requirements for all targets (specified in the tier 3 requirements), because a tier 2 target typically involves the Rust project building and supplying various compiled binaries, incorporating the target and redistributing any resulting compiled binaries (e.g. built libraries, host tools if any) must not impose any onerous license requirements on any members of the Rust project, including infrastructure team members and those operating CI systems. This is a subjective requirement, to be evaluated by the approving teams.

They share C library with tier 1 Windows targets and pair it with more liberal LLVM tools.

Tier 2 targets must not impose burden on the authors of pull requests, or other developers in the community, to ensure that tests pass for the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on tests failing for the target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding the PR breaking tests on a tier 2 target, unless they have opted into such messages.

Agreed.

The target maintainers should regularly run the testsuite for the target, and should fix any test failures in a reasonably timely fashion.

Agreed once its accepted and becomes testable without obscure hacks.

All requirements for tier 3 apply.

They are tier 3 targets already.

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Process

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Comments

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[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

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cc @rust-lang/compiler @rust-lang/compiler-contributors

[davidtwco]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted