Add CODEOWNERS for review assignment (#6153)

This disables the autoassign action so that the codeowners approach can
be tested without interference.

Trying this out because it might be a path for vacation handling. See
[GitHub
docs](https://docs.github.com/en/organizations/organizing-members-into-teams/managing-code-review-settings-for-your-team)
and
[#infra](https://discord.com/channels/655572317891461132/707150492370862090/1422985757311635620)

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

.github/workflows/auto_assign_prs.yaml

@@ -1,63 +0,0 @@
-# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-# Exceptions. See /LICENSE for license information.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-name: 'Auto assign PRs'
-on:
-  pull_request_target:
-    types: [opened, ready_for_review]
-
-permissions:
-  pull-requests: write # For gh to edit assignees.
-
-jobs:
-  assign_reviewer:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Harden Runner
-        uses: step-security/harden-runner@17d0e2bd7d51742c71671bd19fa12bdc9d40a3d6 # v2.8.1
-        with:
-          disable-sudo: true
-          egress-policy: block
-          # prettier-ignore
-          allowed-endpoints: >
-            api.github.com:443
-
-      - id: filter
-        uses: dorny/paths-filter@de90cc6fb38fc0963ad72b210f1f284cd68cea36 # v3.0.2
-        with:
-          filters: |
-            leads:
-              - '*.md'
-              - 'LICENSE'
-              - 'docs/project/principles/*'
-              - 'docs/project/evolution.md'
-              - 'docs/project/goals.md'
-              - 'docs/project/roadmap.md'
-              - 'proposals/*.md'
-            toolchain:
-              - 'toolchain/**'
-
-      - id: assign-leads
-        if: steps.filter.outputs.leads == 'true'
-        uses: hkusu/review-assign-action@5bee595fdb9765d4a0bd35724b6302fa15569158 # v1.4.0
-        with:
-          reviewers: KateGregory, chandlerc, zygoloid
-          max-num-of-reviewers: 1
-
-      - id: assign-toolchain
-        if: steps.filter.outputs.toolchain == 'true'
-        uses: hkusu/review-assign-action@5bee595fdb9765d4a0bd35724b6302fa15569158 # v1.4.0
-        with:
-          reviewers:
-            chandlerc, danakj, dwblaikie, geoffromer, jonmeow, josh11b, zygoloid
-          max-num-of-reviewers: 1
-
-      - id: assign-fallback
-        if: |
-          steps.filter.outputs.leads != 'true' &&
-          steps.filter.outputs.toolchain != 'true'
-        uses: hkusu/review-assign-action@5bee595fdb9765d4a0bd35724b6302fa15569158 # v1.4.0
-        with:
-          reviewers: chandlerc, danakj, jonmeow, josh11b, zygoloid
-          max-num-of-reviewers: 1

CODEOWNERS

@@ -0,0 +1,24 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# This file is only used for PR autoassignment. Branch protections don't enforce
+# it.
+#
+# Syntax:
+# https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#codeowners-syntax
+
+# Toolchain reviewers are used as a fallback.
+*                          @carbon-language/toolchain-reviewers
+
+# Key project documents should be reviewed by leads.
+/*.md                      @carbon-language/leads
+/LICENSE                   @carbon-language/leads
+/docs/project/evolution.md @carbon-language/leads
+/docs/project/goals.md     @carbon-language/leads
+/docs/project/principles/* @carbon-language/leads
+/docs/project/roadmap.md   @carbon-language/leads
+/proposals/*.md            @carbon-language/leads
+
+# Toolchain code.
+/toolchain                 @carbon-language/toolchain-reviewers

docs/project/code_review.md

@@ -86,11 +86,11 @@ In Carbon, developers will focus on particular areas, loosely broken down as:
     -   We split out auto-assignment by explorer, toolchain, and other files
         (including documentation).
 
-[Auto-assignment](/.github/workflows/auto_assign_prs.yaml) will help find
-owners, but won't always be perfect -- developers may take a PR they weren't
-auto-assigned in order to help review go quickly. Contributors can also request
-multiple reviewers, but it can be daunting to get feedback from a large number
-of reviewers, so we suggest keeping the number of reviewers reasonably small.
+[Auto-assignment](/CODEOWNERS) will help find owners, but won't always be
+perfect -- developers may take a PR they weren't auto-assigned in order to help
+review go quickly. Contributors can also request multiple reviewers, but it can
+be daunting to get feedback from a large number of reviewers, so we suggest
+keeping the number of reviewers reasonably small.
 
 Any reviews that explicitly request changes should be addressed, either with the
 changes or an explanation of why not, before a pull request is merged. Further,

proposals/p1367.md

@@ -110,8 +110,9 @@ group access controls are the last word on who can commit PRs.
 
 ### Auto-assignment
 
-This PR [introduces auto-assignment](/.github/workflows/auto_assign_prs.yaml) in
-order to ensure PRs aren't lost. It provides categories of assignment, and a
+This PR
+[introduces auto-assignment](https://github.com/carbon-language/carbon-lang/pull/1367/files#diff-5e5db53c34ec04ba9da95c70d5c797f1be9f390f7e2bf18ca3149cbd99c277df)
+in order to ensure PRs aren't lost. It provides categories of assignment, and a
 fallback for other PRs that don't have explicit assignment.
 
 ### Stacked PRs