rustc-dev-guide/src/feature-gates.md

Feature Gates

This chapter is intended to provide basic help for modifying feature gates. See "Stability in code" for help with adding feature gates.

Removing a feature gate

To remove a feature gate, follow these steps:

1. Remove the feature gate declaration in rustc_feature/src/active.rs. It will look like this:

   /// description of feature
   (active, $feature_name, "$version", Some($tracking_issue_number), $edition)
   

2. Add a modified version of the feature gate declaration that you just removed to rustc_feature/src/removed.rs:

   /// description of feature
   (removed, $old_feature_name, "$version", Some($tracking_issue_number), $edition,
    Some("$why_it_was_removed"))
   

Renaming a feature gate

To rename a feature gate, follow these steps (the first two are the same steps to follow when removing a feature gate):

1. Remove the old feature gate declaration in rustc_feature/src/active.rs. It will look like this:

   /// description of feature
   (active, $old_feature_name, "$version", Some($tracking_issue_number), $edition)
   

2. Add a modified version of the old feature gate declaration that you just removed to rustc_feature/src/removed.rs:

   /// description of feature
   /// Renamed to `$new_feature_name`
   (removed, $old_feature_name, "$version", Some($tracking_issue_number), $edition,
    Some("renamed to `$new_feature_name`"))
   

3. Add a feature gate declaration with the new name to rustc_feature/src/active.rs. It should look very similar to the old declaration:

   /// description of feature
   (active, $new_feature_name, "$version", Some($tracking_issue_number), $edition)