Enable the API breakage checks

Fixes #2986

Author: ahoppen

.github/workflows/pull_request.yml

@@ -16,8 +16,7 @@ jobs:
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
     with:
       license_header_check_project_name: "Swift.org"
-      # https://github.com/swiftlang/swift-syntax/issues/2986
-      api_breakage_check_enabled: false
+      api_breakage_check_allowlist_path: "Release Notes/api-breakages.txt"
       # https://github.com/swiftlang/swift-syntax/issues/2987
       docs_check_enabled: false
       # https://github.com/swiftlang/swift-syntax/issues/2988

Contributor Documentation/API Breakage Checks.md

@@ -0,0 +1,5 @@
+# API Breakage Checks
+
+We have a CI jobs that checks for API breakages. Since swift-syntax releases increase the major version number, it is possible to make API-breaking changes – but we should avoid them whenever possible to avoid churn on clients when they update to a new swift-syntax version.
+
+If your change contains an API breaking change, add the change to `api-breakages.txt`. All API breaking changes should be accompanied by an [RFC](RFC%20Process.md).

Contributor Documentation/README.md

@@ -2,6 +2,7 @@
 
 The following documentation documents are primarily intended for developers of swift-syntax.
 
+- [API Breakage Checks](API%20Breakage%20Checks.md)
 - [Changing Swift Syntax](Changing%20Swift%20Syntax.md)
 - [Filing Parser Bug Reports](Filing%20Parser%20Bug%20Reports.md)
 - [Fixing Bugs](Fixing%20Bugs.md)

api-breakages.txt

@@ -0,0 +1,2 @@
+6.2
+---