Promote powerpc64-unknown-linux-musl to tier 2 with host tools

# Proposal Promote the powerpc64-unknown-linux-musl target to tier 2 with host tools. ## Tier 2 target requirements > * 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.) While big-endian ppc64 is not as common as little-endian ppc64le these days (software side, all little-endian-capable ppc64 hardware is bi-endian), it's still an important target and somewhat widely used and supported. There are several distributions that support ppc64 musl, for example [Gentoo](https://www.gentoo.org/downloads/#ppc), [Chimera Linux](https://chimera-linux.org/about/), [Adélie Linux](https://www.adelielinux.org/download/) and a [community build](https://alpine.ppc64.be/) of Alpine Linux. Notable supported hardware includes e.g. the Power Mac G5, iMac G5 and IBM servers with POWER6 or newer CPUs. > * 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 is actively maintained by myself, @neuschaefer and @famfo, see the [platform support page](https://doc.rust-lang.org/beta/rustc/platform-support/powerpc64-unknown-linux-musl.html). We also already maintain the ppc64le musl target, which is in tier 2 with host tools and, endianness aside, very similar to this target. > * 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. This is not much different from all the other musl targets already at tier 2. It is also a rather modern target, since unlike the existing tier 2 glibc target, it does not use the legacy ELFv1 ABI (which LLD does not support), so it's unlikely to cause issues in the future. > * 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. This is already covered by the platform support documentation linked earlier. > * The target must document its baseline expectations for the features or > versions of CPUs, operating systems, libraries, runtime environments, and > similar. This target use LLVM's ppc64 CPU as the baseline, which is equivalent to a PowerPC 970 (used in the Mac G5) and implies AltiVec (which in turn means that out of IBM server CPUs, POWER6 or newer is required). I'll update the documentation to be more specific on this. The version requirements for Linux and musl are well-documented already. > * If introducing a new tier 2 or higher target that is identical to an existing > Rust target except for the baseline expectations for the features or versions > of CPUs, operating systems, libraries, runtime environments, and similar, > then the proposed target must document to the satisfaction of the approving > teams why the specific difference in baseline expectations provides > sufficient value to justify a separate target. Not applicable. > * 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. This target supports the full standard library functionality. > * 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. The LLVM codegen backend is used. > * 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. The C calling convention is supported. > * The target must build reliably in CI, for all components that Rust's CI > considers mandatory. All components that are typically distributed through rustup build just fine. > * 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. > * 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. > * 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. The target supports cross-compilation and programs could be run in QEMU in CI if desired. The build time is similar to the glibc or little endian targets. > * 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. No license issues to my knowledge. > * 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. Should not cause issues. > * The target maintainers should regularly run the testsuite for the target, and > should fix any test failures in a reasonably timely fashion. We regularly run the testsuite for the powerpc64(le) targets. > * All requirements for tier 3 apply. Ack. # Process The main points of the [Major Change Process][MCP] are as follows: * [x] File an issue describing the proposal. * [ ] A compiler team member or contributor who is knowledgeable in the area can **second** by writing `@rustbot second`. * Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a `-C flag`, then full team check-off is required. * Compiler team members can initiate a check-off via `@rfcbot fcp merge` on either the MCP or the PR. * [ ] Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered **approved**. You can read [more about Major Change Proposals on forge][MCP]. [MCP]: https://forge.rust-lang.org/compiler/mcp.html # Comments **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.**