rustc_target: Add OpenEmbedded/Yocto Linux base targets

[DeepeshWR]

Add built-in OpenEmbedded/Yocto Linux targets for x86_64, i686, aarch64, armv7, and riscv64.

These targets inherit from the corresponding upstream Linux GNU targets and provide OpenEmbedded-specific target triples and default linker settings. They are intended to serve as base targets for downstream Yocto/OpenEmbedded-generated targets, allowing vendor-specific targets to override metadata (for example, TARGET_VENDOR) while reusing a common target configuration.

This reduces duplication in Yocto-based Rust integrations and avoids maintaining downstream copies of largely identical target specifications.

Tier 3 Policy Notes

To cover the Tier 3 requirements:

A tier 3 target must have a designated developer or developers

These targets have a designated maintainer: @DeepeshWR, as documented in the platform support documentation added by this PR.

Targets must use naming consistent with any existing targets

The targets follow the existing target naming conventions and are derived from the corresponding upstream Linux GNU targets:

x86_64-oe-linux-gnu
aarch64-oe-linux-gnu
i686-oe-linux-gnu
armv7-oe-linux-gnueabihf
riscv64-oe-linux-gnu

The oe vendor component identifies OpenEmbedded/Yocto-based systems while preserving the established architecture/OS/ABI naming scheme.

Tier 3 targets may have unusual requirements to build or use, but must not create legal issues or impose onerous legal terms for the Rust project or for Rust developers or users

These targets do not introduce any additional legal requirements. They use the standard OpenEmbedded/Yocto cross-compilation toolchains and are intended to behave similarly to the corresponding Linux GNU targets.

Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate

These targets inherit from the corresponding Linux GNU targets and support the standard library in the same manner as their upstream counterparts.

The target must provide documentation for the Rust community explaining how to build for the target

This PR adds platform-support documentation describing the targets, maintainership, requirements, and intended usage.

Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target

The targets are thin wrappers around existing Linux GNU targets and primarily provide canonical OpenEmbedded/Yocto target triples and default linker configuration. They do not require additional CI infrastructure or special maintenance beyond keeping them aligned with their upstream Linux GNU base targets.

Patches adding or updating tier 3 targets must not break any existing tier 2 or tier 1 target, and must not knowingly break another tier 3 target without approval of either the compiler team or the maintainers of the other tier 3 target

Noted.

Tier 3 targets must be able to produce assembly using at least one of rustc's supported backends from any host target

These targets use LLVM through the existing Rust code generation pipeline, just like their corresponding Linux GNU targets.

r? @davidtwco

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

[Noratrieb]

can you elaborate a bit more on what this is useful for?

avoids maintaining downstream copies of largely identical target specifications.

I don't understand this part, basically nothing about those targets here is copied as they seem to basically be identical.

[davidtwco]

can you elaborate a bit more on what this is useful for?

avoids maintaining downstream copies of largely identical target specifications.

I don't understand this part, basically nothing about those targets here is copied as they seem to basically be identical.

There's some context here - #yocto > custom targets @ 💬

[davidtwco]

Could you reply just asserting that these targets comply with our target tier policy? (e.g. like #150863)

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[DeepeshWR]

Could you reply just asserting that these targets comply with our target tier policy? (e.g. like #150863)

Thanks for pointing that out. I've updated the PR description with a "Tier 3 Policy Notes" section covering the target tier requirements.