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COMPATIBILITY.md

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rules_rust backwards compatibility policy

This document defines the backwards compatibility policy for rules_rust and defines the process for making compatibility breaking changes. Any exception to this process will have to be thoroughly discussed and there will have to be a consensus between maintainers that the exception should be granted.

What is a compatibility breaking change?

A backwards compatible change has the property that a build that is green, correct, is using stable APIs of rules_rust, and is using a supported version of Bazel before the change is still green and correct after the change. In practice it means that users should not have to modify their project source files, their BUILD files, their bzl files, their rust_toolchain definitions, their platform definitions, or their build settings.

A backwards compatible release has the property that it only contains backward compatible changes.

rules_rust follow SemVer 2.0.0. rules_rust promise that all minor version number and patch version number releases only contain backwards compatible changes.

rules_rust don't make any guarantees about compatibility between a released version and the state of the rules_rust at HEAD.

Backwards incompatible changes will have to follow a process in order to be released. Users will be given at least one release where the old behavior is still present but deprecated to allow smooth migration of their project to the new behavior.

Compatibility before 1.0

All minor version number releases before version 1.0 can be backwards incompatible. Backwards incompatible changes still have to follow the the process, but minimum time for migration is reduced to 2 weeks.

What Bazel versions are supported?

rules_rust support the current Bazel LTS at the time of a rules_rust release.

Support for the current Bazel rolling release is on the best effort basis. If the CI build of rules_rust against the current Bazel rolling release is green, it has to stay green. If the build is already red (because the new Bazel rolling release had incompatible changes that broke rules_rust), it is acceptable to merge PRs leaving the build red as long as the reason for failure remains the same. We hope red CI with the current rolling release will be rare.

rules_rust don't promise all new rules_rust features available to the current Bazel rolling release to be be available to the current Bazel LTS release (because Bazel compatibility policy doesn't allow us to make that promise, and some new features of rules_rust require new Bazel features that are only available in Bazel rolling releases). rules_rust will aim that new features available to the current Bazel rolling release will be available to the next Bazel LTS release at latest.

Whenever there is a new Bazel LTS release, all releases of rules_rust will maintain support for the older LTS version for at least 3 months unless Bazel doesn't allow this.

What host platforms are supported?

Platforms subject to backwards compatibility policy are x86_64-unknown-linux-gnu and x86_64-apple-darwin (platforms supported by rules_rust). Process for moving a best effort platform to a supported platform is consensus-based.

What are the stable APIs of rules_rust?

//rust:defs.bzl is subject to the backwards compatibility policy. That means that anything directly accessible from this file is considered stable.

//rust/private/… is not subject to the backwards compatibility policy. Content of this package is an implementation detail.

//cargo/... is subject to the backwards compatibility policy.

//util, //tools, //test, //examples, //extensions, //ffi, //nix and any packages not mentioned by this document are by default not subject to the backwards compatibility policy.

Experimental build settings are not subject to the backward compatibility policy. They should be added to //rust:experimental.bzl.

Incompatible build settings are subject to the backward compatibility policy, meaning the behavior of the flag cannot change in a backwards incompatible way. They should be added to //rust:incompatible.bzl.

Bug fixes are not a breaking change by default. We'll use Common Sense (and we will pull in more maintainers and the community to discuss) if we see a certain bug fix is controversial. Incompatible changes to //cargo:defs.bzl that make cargo_build_script more accurately follow cargo's behavior are considered bug fixes.

How to make a backwards incompatible change?

  1. Create a GitHub issue (example: Split rust_library into smaller rules#591).
  2. Describe the change, motivation for the change, provide migration instructions/tooling.
  3. Add a build setting into //rust:incompatible.bzl that removes the old behavior (whenever possible) or changes the current behavior (when just removing the old behavior is not possible). Ideally, users should not need to manually flip incompatible flags.
  4. Mention the link to the GitHub issue in error messages. Do not add a deprecation warning (warnings make the deprecation visible to every user building a project, not only to the maintainers of the project or the rules).
  5. Mention the issue in the CHANGELOG file.
  6. Give the community 3 months from the first release mentioning the issue until the flag flip to migrate.