Add SOLID targets

[kawadakk]

This PR introduces new tier 3 targets for SOLID embedded development platform by Kyoto Microcomputer Co., Ltd.

Target name	target_arch	target_vendor	target_os
aarch64-kmc-solid_asp3	aarch64	kmc	solid_asp3
armv7a-kmc-solid_asp3-eabi	arm	kmc	solid_asp3
armv7a-kmc-solid_asp3-eabihf	arm	kmc	solid_asp3

Related PRs

• libc: Add SOLID target support libc#2227
• cc: https://github.com/alexcrichton/cc-rs/pull/609

Non-blocking Issues

• The target kernel can support Thread::unpark directly, but this property is not utilized because the underlying kernel feature is used to implement Condvar and it's unclear whether std should guarantee that parking tokens are not clobbered by other synchronization primitives.
• The rustc book: The page title "*-kmc-solid-*" shows up as "-kmc-solid-" in TOC

Tier 3 Target Policy

As tier 3 targets, the new targets are required to adhere to the tier 3 target policy requirements. This section quotes each requirement in entirety and describes how they are met.

• 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.)

See src/doc/rustc/src/platform-support/kmc-solid.md.

• 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.
  • 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.

The new target names follow this format: $ARCH-$VENDOR-$OS-$ABI, which is already adopted by most existing targets. $ARCH and $ABI follow the convention: aarch64-* for AArch64, armv7a-*-eabi for Armv7-A with EABI. $OS is used to distinguish multiple variations of the platform in a somewhat similar way to the Apple targets, though we are only adding one variation in this PR. $VENDOR denotes the platform vendor name similarly to the Apple, Solaris, SGX, and VxWorks targets.

$OS corresponds to the value of target_os and takes the format solid-$KERNEL. The inclusion of a hyphen prevents unique decomposition of target names, though the mapping between target names and target attributes isn't trivial in the first place, e.g., because of the Android targets.

More targets may be added later, as we support other base kernels (there are at least three at the point of writing) and are interested in supporting other processor architectures in the future.

• 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.
  • The target must not introduce license incompatibilities.
  • Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).
  • The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements.
  • If the target supports building host tools (such as rustc or cargo), those host tools must not depend on proprietary (non-FOSS) libraries, other than ordinary runtime libraries supplied by the platform and commonly used by other binaries built for the target. For instance, rustc built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3.
  • Targets should not require proprietary (non-FOSS) components to link a functional binary or library.
  • "onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.

We intend to make the contribution fully available under the standard Rust license with no additional legal restrictions whatsoever. This PR does not introduce any new dependency less permissive than the Rust license policy, and we are willing to ensure this doesn't happen for future contributions regarding the new targets.

The new targets don't support building host tools.

Although the new targets use a platform-provided C compiler toolchain, it can be substituted by GNU Arm Embedded Toolchain for testing purposes.

• 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.

Most features are implemented. The following features are not implemented due to the lack of native support:

• fs::File::{file_attr, truncate, duplicate, set_permissions}
• fs::{symlink, link, canonicalize}
• Process creation
• Command-line arguments

Networking is not implemented yet, and we intend to add it as soon as it's ready.
Edit (2021-07-07): Networking is now implemented.

Backtrace generation is not really a good fit for embedded targets, so it's intentionally left unimplemented. Unwinding is functional, however.

• 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 tests (even if they do not pass), the documentation must explain how to run tests for the target, using emulation if possible or dedicated hardware if necessary.

See src/doc/rustc/src/platform-support/kmc-solid.md. Running tests is not supported.

• 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.
  • This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.
• 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.
  • Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.
• 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.
  • In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.

We acknowledge these requirements and intend to ensure they are met.

There are no closely related targets at the moment.