swift

2fbe2021 - Introduce FUTURE_FEATURE definitions for features staged in Swift 6.

Introduce FUTURE_FEATURE definitions for features staged in Swift 6. Using the same feature set logic as experimental features, provide feature names for "future" features, which are changes that will become available with Swift 6. Use the feature check when determining whether to implementation the feature instead of a language version check, and map existing flags for these features (when available) over to the feature set. As an internal implementation detail, this makes it easier to reason about when specific features are enabled (or not). If we decide to go with piecemeal adoption support for features, it can provide an alternative path to enabling features that feeds this mechanism.