### Problem/Motivation Replace deprecated system_region_list() and system_default_region() with Theme object methods via the theme_handler service. Both functions are deprecated in drupal:11.4.0 and removed in drupal:13.0.0. ### Proposed resolution Handled automatically by drupal-rector via the `SystemRegionFunctionsRector` rule. ### How this rule was built This rule was produced with an AI-assisted workflow in which every step was verified manually by @bbrala. Its logic started from the matching drupal-digests entry, which was adapted to drupal-rector's conventions — a dedicated rule or an existing upstream rule configured into the Drupal 12 set — and covered with before/after test fixtures. Backward-compatibility was assessed and version-gated: safe rewrites are applied directly, while unsafe ones are wrapped via `DeprecationHelper` or moved to an opt-in breaking set. The rule was then live-tested against a set of contributed modules; where no contrib call site could be found, a static code sample was written in a test module to exercise it. A final automated QA pass covered type-guard, fixture coverage, BC-decision, registration and idempotency checks. ### References - Related issue(s): [#3015812](https://www.drupal.org/i/3015812), [#2479459](https://www.drupal.org/i/2479459), [#2538970](https://www.drupal.org/i/2538970), [#2941155](https://www.drupal.org/i/2941155), [#3035288](https://www.drupal.org/i/3035288), [#3112263](https://www.drupal.org/i/3112263) ### Work items - Commit(s): [38620e4786](https://github.com/palantirnet/drupal-rector/commit/38620e4786c1) ### Credit - Implemented by: @bbrala - Reviewed / helped by: @mglaman --- *Filed retroactively as part of the Drupal 12 rector coverage effort. Closed as fixed; credit to be applied to the contributors listed above.* *AI was used extensively to build this rule and to file this issue. See [Dries Buytaert: AI-generated Rector rules for Drupal](https://dri.es/ai-generated-rector-rules-for-drupal) and [Björn Brala: Drupal Rector rules are config, not code. Mostly.](https://www.linkedin.com/pulse/drupal-rector-rules-config-code-mostly-bj%C3%B6rn-brala-vfpwe/).*