Add Trusty OS as tier 3 target

[randomPoison]

This PR adds support for the Trusty secure operating system as a Tier 3 supported target. This upstreams the patch that we have been using internally. This also revives #103895 which was closed due to inactivity, and is being resumed now that time allows.

And MCP has already been done for adding this platform: rust-lang/compiler-team/issues/568

Target Tier Policy Acknowledgements

A tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)

• Nicole LeGare (@randomPoison)
• Stephen Crane (@rinon)
• As a fallback trusty-dev-team@google.com can be contacted

Targets must use naming consistent with any existing targets; for instance, a target for the same CPU or OS as an existing Rust target should use the same name for that CPU or OS. Targets should normally use the same names and naming conventions as used elsewhere in the broader ecosystem beyond Rust (such as in other toolchains), unless they have a very good reason to diverge. Changing the name of a target can be highly disruptive, especially once the target reaches a higher tier, so getting the name right is important even for a tier 3 target.

The two new Trusty targets, aarch64-unknown-trusty and armv7-unknown-trusty both follow the existing naming convention for similar targets.

Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.

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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.

There are no known legal issues or license incompatibilities.

Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.

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Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate (core for most targets, alloc for targets that can support dynamic memory allocation, std for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions.

This PR only adds the targets for the platform. std support will be added once platform support is added to the libc crate, which depends on the language targets being added to rustc.

The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running binaries, or running tests (even if they do not pass), the documentation must explain how to run such binaries or tests for the target, using emulation if possible or dedicated hardware if necessary.

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Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain 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 a tier 3 target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.

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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.

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Tier 3 targets must be able to produce assembly using at least one of rustc's supported backends from any host target. (Having support in a fork of the backend is not sufficient, it must be upstream.)

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[rustbot]

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[rustbot]

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

Some changes occurred in tests/ui/check-cfg

cc @Urgau

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

cc @Noratrieb

[randomPoison]

r? compiler-team

[Urgau]

@workingjubilee wrote in this comment:

Slightly surprised to see this target enables Thumb instructions but is not named thumbv7.

[Urgau]

Looking at our targets many of them have thumb2 activated without being named thumbv7.

It seems like our thumbv7 targets are either based on thumbv7 llvm targets or +thumb-mode.

I therefore thinks the naming of this target is consistent with our existing targets.

[workingjubilee]

Ah, sure then. I was mostly curious I guess.

[Urgau]

Looks good to me. Thanks.

r? @Urgau
@bors r+ rollup

[bors]

📌 Commit 5d0ce4c has been approved by Urgau

It is now in the queue for this repository.

[kadiwa4]

This seems like a mistake. It's supposed to be tier 3, isn't it?

[Urgau]

Indeed my bad, missed it during my review.

@randomPoison may I ask you to send a PR fixing the target tier here and in the other target.

[randomPoison]

Ah yup this was a copy-paste error. I've put up #129712 to fix it.