Add std Xtensa targets support

[SergioGasquez]

Adds std Xtensa targets. This enables using Rust on ESP32, ESP32-S2 and ESP32-S3 chips.

Tier 3 policy:

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.)

@MabezDev, @ivmarkov and I (@SergioGasquez) will maintain the targets.

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 target triple is consistent with other 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.
If possible, use only letters, numbers, dashes and underscores for the name. Periods (.) are known
to cause issues in Cargo.

We follow the same naming convention as other targets.

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 does not introduce any legal issues.

The target must not introduce license incompatibilities.

There are no license incompatibilities

Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).
Everything added is under that licenses

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.

Requirements are not changed for any other target.

Compiling, linking, and emitting functional binaries, libraries, or other code for the target
(whether hosted on the target itself or cross-compiling from another target) must not depend on
proprietary (non-FOSS) libraries. Host tools built for the target itself may depend on the ordinary
runtime libraries supplied by the platform and commonly used by other applications built for the
target, but those libraries must not be required for code generation for the target;
cross-compilation to the target must not require such libraries at all. 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.

The linker used by the targets is the GCC linker from the GCC toolchain cross-compiled for Xtensa.
GNU GPL.

"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.

No such terms exist for this target

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.

Understood

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.

The targets implement libStd almost in its entirety, except for the missing support for process, as
this is a bare metal platform. The process sys\unix module is currently stubbed to return "not
implemented" errors.

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.

Here is how to build for the target https://docs.esp-rs.org/book/installation/riscv-and-xtensa.html
and it also covers how to run binaries on the target.

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.

Understood

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.

No other targets should be affected

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

It can produce assembly, but it requires a custom LLVM with Xtensa support
(https://github.com/espressif/llvm-project/). The patches are trying to be upstreamed
(espressif/llvm-project#4)

[rustbot]

r? @oli-obk

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[rustbot]

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

cc @Nilstrieb

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

[SergioGasquez]

This PR is a "follow-up" of #125141.

cc: @davidtwco, @Kobzol, @ivmarkov

[oli-obk]

r? @davidtwco

[Kobzol]

Are there any more Xtensa targets that you want to add, or it's these 6 targets and that's it, btw?

[SergioGasquez]

Are there any more Xtensa targets that you want to add, or it's these 6 targets and that's it, btw?

Only these 6! We don't plan any other target.

[davidtwco]

@bors r+

[bors]

📌 Commit b092798 has been approved by davidtwco

It is now in the queue for this repository.

[workingjubilee]

It seems that the assembly test for the xtensa targets https://github.com/rust-lang/rust/blob/master/tests/assembly/targets/targets-elf.rs have not been enabled?

[workingjubilee]

Ah, that is expected to take some time. Hm, we have blocked other targets on LLVM support, but I suppose that is what we are doing today.