rustc-ap-rustc_feature 727.0.0

# Feature gates This crate declares the set of past and present unstable features in the compiler. Feature gate checking itself is done in `rustc_ast_passes/src/feature_gate.rs` at the moment. Features are enabled in programs via the crate-level attributes of `#![feature(...)]` with a comma-separated list of features. For the purpose of future feature-tracking, once a feature gate is added, even if it is stabilized or removed, *do not remove it*. Instead, move the symbol to the `accepted` or `removed` modules respectively.