Document compiler backport nomination/review process

src/SUMMARY.md

@@ -55,6 +55,7 @@
     - [Review Policy](./compiler/reviews.md)
     - [Proposals, Approval and Stabilization](./compiler/proposals-and-stabilization.md)
         - [Adding ecosystem/integration test jobs/components to `rust-lang/rust` CI](./compiler/proposals-and-stabilization/ecosystem-integration-tests.md)
+    - [Backports](./compiler/backports.md)
     - [Third-party and Out-of-tree Crates Policy](./compiler/third-party-out-of-tree.md)
     - [Triage and Prioritization](./compiler/prioritization.md)
     - [Working Areas](./compiler/working-areas.md)

src/compiler/README.md

@@ -6,6 +6,13 @@ We use the Forge to document the team's processes, policies and working practice
 read about how the compiler works and instructions on how to set up a development environment,
 you're looking for the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/).
 
+<!--
+NOTE: please keep the following alphabetically sorted.
+-->
+
+- [Backports](./backports.md)
+  - *How do I request a beta and/or stable backport of a compiler change? How will compiler 
+    backport nominations be handled?*
 - [Calendar](./calendar.md)
   - *How do I subscribe to the compiler team's calendar?*
 - [Cross-team Collaboration](./cross-team-collaboration.md)
@@ -14,20 +21,20 @@ you're looking for the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/)
   - *What meetings do the compiler team run and how can I attend?*
 - [Membership](./membership.md)
   - *What is expected of compiler team members and how do I join?*
+- [Proposals, Approval and Stabilization](./proposals-and-stabilization.md)
+  - *How do I propose a change to the compiler team? What approval is necessary for my change?*
+- [Repositories we maintain](https://github.com/orgs/rust-lang/teams/compiler/repositories)
+  - *Various code repositories maintained by the Compiler Team*
 - [Resources](./resources.md)
   - *What useful resources are available for contributors and team members?*
 - [Review Policy](./reviews.md)
   - *How do I make a contribution which is easy to review? How do I start reviewing as a team member?*
-- [Proposals, Approval and Stabilization](./proposals-and-stabilization.md)
-  - *How do I propose a change to the compiler team? What approval is necessary for my change?*
 - [Third-party and Out-of-tree Crates Policy](./third-party-out-of-tree.md)
   - *When can I add third-party crates to the compiler? When can I create a out-of-tree crate for
      the compiler?*
 - [Triage and Prioritization](./prioritization.md)
   - *How are compiler issues triaged and prioritized?*
-- [Working Areas](./working-areas.md)
-  - *Specific areas of work around the compiler*
-- [Repository we maintain](https://github.com/orgs/rust-lang/teams/compiler/repositories)
-  - *Various code repositories maintained by the Compiler Team*
 - [Operations](./operations.md)
   - *Supporting the compiler team*
+- [Working Areas](./working-areas.md)
+  - *Specific areas of work around the compiler*

src/compiler/backports.md

@@ -0,0 +1,90 @@
+# Backports
+
+Sometimes compiler fixes need to be backported to the stable and/or beta channel to fix a
+significant regression or to revert unintentional changes that weren't noticed when originally
+merged. See also [prioritization](./prioritization.md) for how priority of issues and regressions
+are determined.
+
+In case a backport is applied to the stable channel, the Rust project will release a patch release
+(for example a 1.87.1 point release could follow the initial stable release 1.87.0).
+
+## Nominating for backport review
+
+You can propose a change be backported by applying labels to the GitHub pull request. Add
+`beta-nominated` if the patch should be backported to the **beta** channel, and also
+`stable-nominated` if it needs backported to the **stable** channel. Make sure the pull request has
+a [`T-compiler`] label as well.
+
+In any case, you should **add a comment when you nominate the pull request for backport** providing
+context for the compiler team backport reviewers about why it should be backported.
+
+**Backport nominations are not guaranteed be accepted**. Please refer to the [*Should the backport
+be approved*](#should-the-backport-be-approved) section below for the criteria on which backport
+nominations may be accepted or rejected.
+
+Beta regressions that slid to the stable channel will need a stable backport nomination (and a
+subsequent patch release, if approved).
+
+The compiler team tries to make sure that critical issues (labeled with `P-critical`) do not
+progress to the stable release.
+
+## Reviewing compiler backport nominations
+
+When one of [`beta-nominated`] or [`stable-nominated`] label is applied, a new thread is
+automatically opened on Zulip in the [#t-compiler/backports] channel. Compiler team members can use
+these Zulip threads to cast their vote asynchronously in favor of or raise concerns about
+backporting. If you are a compiler team member and want to be notified of these threads, you should
+subscribe that zulip channel.
+
+During the weekly triage meeting (happening on [#t-compiler/meetings], see [here](./meetings.md)),
+the compiler team will finalize the decision and apply the relevant `{beta,stable}-accepted` label.
+
+### Should the backport be approved?
+
+For compiler backport reviewers, here are some **non-exhaustive** considerations that they might
+consider in making a backport decision:
+
+- The nominated change is not merged in time for the stable or beta release channel (e.g. due to
+  implementation issues).
+- Has the backport enough time to be tested?
+  - (For beta backport nominations) The nominated change would be applied too close to the next
+    stable compiler release. Merging a backport at this time will mean very limited (if any) time
+    for testing.
+  - A stable point release will be *immediately* available to all users without any time to bake!
+- How complex or risky is the nominated compiler change? Is the risk of the backport introducing new
+  regressions potentially worse than the regression or issue it addresses?
+- How severe is the regression/issue being fixed?
+  - For example stable regressions are not all equal: some are not very severe or are already on
+    stable channel since multiple releases.
+
+By default, approved stable backports will cause a new point release to be issued by the [release
+team](../release/backporting.md#stable-backporting-in-rust-langrust).
+
+However, the compiler team may approve a stable backport, but additionally indicate to the [release
+team](../release/backporting.md) that the nomination does not justify a stable point release *on its
+own*. In this case, the release team will consider other approved stable backport candidates and how
+serious those are in conjunction with this candidate, to finalize a decision on whether to execute a
+stable point release.
+
+## How are approved backports handled?
+
+The [release team](../release/README.md) (`T-release`) will handle the backport at the end of the
+current development cycle (see [release backporting](../release/backporting.md)). If a beta backport
+nomination is approved too late, the release team may be unable to backport the change.
+
+Most of the time, accepted backports target the `master` branch. In rare circumstances, a beta
+backport may need to *directly* target the `beta` branch. In this case, coordinate with the release
+team before merging, by opening a new thread on the Zulip [#t-release
+channel](https://rust-lang.zulipchat.com/#narrow/channel/241545-t-release)).
+
+For complicated backports, the release team may ask the patch author for assistance.
+
+
+[`beta-nominated`]: https://github.com/rust-lang/rust/labels/beta-nominated
+[`beta-accepted`]: https://github.com/rust-lang/rust/labels/beta-accepted
+[`stable-nominated`]: https://github.com/rust-lang/rust/labels/stable-nominated
+[`stable-accepted`]: https://github.com/rust-lang/rust/labels/stable-accepted
+[#t-compiler/backports]:
+    https://rust-lang.zulipchat.com/#narrow/channel/474880-t-compiler.2Fbackports
+[#t-compiler/meetings]: https://rust-lang.zulipchat.com/#narrow/channel/238009-t-compiler.2Fmeetings
+[`T-compiler`]: https://github.com/rust-lang/rust/labels/T-compiler

src/compiler/proposals-and-stabilization.md

@@ -334,6 +334,10 @@ circumstance.
   - **Approve using:** r+ (compiler leads)
   - Open a PR with the change affecting licensing and assign it to the team leads for review
 
+### Stable/beta channel backport nominations of compiler changes
+
+See [*Backports*](./backports.md).
+
 ### Adding ecosystem/integration test jobs/components to rust-lang/rust CI
 
 See [*Adding ecosystem/integration test jobs/components to rust-lang/rust CI*](./proposals-and-stabilization/custom-test-jobs.md).