Promote riscv64gc-unknown-linux-musl to tier 2

[Amanieu]

Proposal

Promote the riscv64gc-unknown-linux-musl target to tier 2.

Musl targets are hard to use with -Zbuild-std because they require a full musl sysroot to be available, including an implementation of libunwind. Shipping these in rustup makes them much easier to use.

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.)

• 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.

Myself and @kraj will be maintaining 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.

• 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 will be included in the PR which adds the target.

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

This target is for baseline RV64GC.

• 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 standard codegen backends are 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.

Yes.

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

Yes.

• 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.

Yes.

• 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.

• 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.

Yes.

• All requirements for tier 3 apply.

Yes.

Mentors or Reviewers

If you have a reviewer or mentor in mind for this work, mention them
here. You can put your own name here if you are planning to mentor the
work.

Process

The main points of the Major Change Process are as follows:

• 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.

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.

[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.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rustbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rustbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

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

[lcnr]

@rustbot second

[apiraino]

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