Fix `tidy` fallout due to `feature_gate.rs` refactoring.
Fix `tidy` fallout due to `feature_gate.rs` refactoring.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust