riscv32im-risc0-zkvm-elf: add target

[SchmErik]

This pull request adds RISC Zero's Zero Knowledge Virtual Machine (zkVM) as a target for rust. The zkVM used to produce proofs of execution of RISC-V ELF binaries. In order to do this, the target will execute the ELF to generate a receipt containing the output of the computation along with a cryptographic seal. This receipt can be verified to ensure the integrity of the computation and its result. This target is implemented as software only; it has no hardware implementation.

Tier 3 target policy:

Here is a copy of the tier 3 target policy:

Tier 3 target policy:

At this tier, the Rust project provides no official support for a target, so we
place minimal requirements on the introduction of targets.

A proposed new tier 3 target must be reviewed and approved by a member of the
compiler team based on these requirements. The reviewer may choose to gauge
broader compiler team consensus via a [Major Change Proposal (MCP)](https://forge.rust-lang.org/compiler/mcp.html).

A proposed target or target-specific patch that substantially changes code
shared with other targets (not just target-specific code) must be reviewed and
approved by the appropriate team for that shared code before acceptance.

• 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.)

The maintainers are named in the target description file

• 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.
• If possible, use only letters, numbers, dashes and underscores for the name.
  Periods (.) are known to cause issues in Cargo.

We understand.

• 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.

We understand and will not introduce incompatibilities. All of our code that we publish is licensed under Apache-2.0.

• Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).

We understand. We are open to either license for the Rust repository.

• 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.

We understand. The runtime libraries and the execution environment and software associated with this environment uses Apache-2.0 so this should not be an issue.

• 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.

We understand. We only depend on FOSS libraries. Dependencies such as runtime libraries for this target are licensed as Apache-2.0.

• "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.

There are no such terms present

• 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.

I am not the reviewer of this pull request

• 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.

We understand.

• 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 target implements core and alloc. And std support is currently experimental as some functionalities in std are either a) not applicable to our target or b) more work in research and experimentation needs to be done. For more information about the characteristics of this target, please refer to the target description file.

• 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.

See file target description file

• 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.

We understand.

• 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.

We understand.

• 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 understand.

If a tier 3 target stops meeting these requirements, or the target maintainers
no longer have interest or time, or the target shows no signs of activity and
has not built for some time, or removing the target would improve the quality
of the Rust codebase, we may post a PR to remove it; any such PR will be CCed
to the target maintainers (and potentially other people who have previously
worked on the target), to check potential interest in improving the situation.

We understand.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @thomcc (or someone else) soon.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[rustbot]

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

[SchmErik]

r? compiler-team

[davidtwco]

t-compiler changes and target policy compliance looks good to me - you'll need to rebase instead of merge to update though. I'll re-assign someone from t-libs to check those parts.

[davidtwco]

r? t-libs

[davidtwco]

@bors r=davidtwco,Mark-Simulacrum

[bors]

📌 Commit 42556b4 has been approved by davidtwco,Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 42556b4 with merge 8b94152...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco,Mark-Simulacrum
Pushing 8b94152 to master...

[rust-timer]

Finished benchmarking commit (8b94152): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.9%	[-1.9%, -1.9%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[1.8%, 3.4%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-4.3%	[-4.3%, -4.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-4.3%	[-4.3%, -4.3%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 662.94s -> 663.309s (0.06%)
Artifact size: 308.35 MiB -> 308.35 MiB (-0.00%)