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# Feature Gates
This chapter is intended to provide basic help for adding, removing, and
modifying feature gates.
## Adding a feature gate
See ["Stability in code"] for help with adding a new feature; this section just
covers how to add the feature gate *declaration*.
Add a feature gate declaration to `rustc_feature/src/active.rs` in the active
`declare_features` block:
```rust,ignore
/// description of feature
(active, $feature_name, "$current_nightly_version", Some($tracking_issue_number), $edition)
```
where `$edition` has the type `Option<Edition>`, and is typically
just `None`.
For example:
```rust,ignore
/// Allows defining identifiers beyond ASCII.
(active, non_ascii_idents, "1.0.0", Some(55467), None),
```
Features can be marked as incomplete, and trigger the warn-by-default [`incomplete_features` lint]
by setting their type to `incomplete`:
```rust,ignore
/// Allows unsized rvalues at arguments and parameters.
(incomplete, unsized_locals, "1.30.0", Some(48055), None),
```
When added, the current version should be the one for the current nightly.
Once the feature is moved to `accepted.rs`, the version is changed to that
nightly version.
## Removing a feature gate
[removing]: #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:
```rust,ignore
/// 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`:
```rust,ignore
/// description of feature
(removed, $old_feature_name, "$version", Some($tracking_issue_number), $edition,
Some("$why_it_was_removed"))
```
## Renaming a feature gate
[renaming]: #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][removing]):
1. Remove the old feature gate declaration in `rustc_feature/src/active.rs`.
It will look like this:
```rust,ignore
/// description of feature
(active, $old_feature_name, "$version", Some($tracking_issue_number), $edition)
```
2. Add a modified version of the old feature gate declaration that you just
removed to `rustc_feature/src/removed.rs`:
```rust,ignore
/// description of feature
/// Renamed to `$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:
```rust,ignore
/// description of feature
(active, $new_feature_name, "$version", Some($tracking_issue_number), $edition)
```
## Stabilizing a feature
See ["Updating the feature-gate listing"] in the "Stabilizing Features" chapter
for instructions. There are additional steps you will need to take beyond just
updating the declaration!
["Stability in code"]: ./implementing_new_features.md#stability-in-code
[`incomplete_features` lint]: https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html#incomplete-features
["Updating the feature-gate listing"]: ./stabilization_guide.md#updating-the-feature-gate-listing
|
