Function rustc_passes::stability::check_features

fn check_features<'tcx>(
    tcx: TyCtxt<'tcx>,
    remaining_lib_features: &mut FxIndexMap<&Symbol, Span>,
    remaining_implications: &mut FxHashMap<Symbol, Symbol>,
    defined_features: &[(Symbol, Option<Symbol>)],
    all_implications: &FxHashMap<Symbol, Symbol>
)

For each feature in defined_features..

• If it is in remaining_lib_features (those features with #![feature(..)] attributes in the current crate), check if it is stable (or partially stable) and thus an unnecessary attribute.
• If it is in remaining_implications (a feature that is referenced by an implied_by from the current crate), then remove it from the remaining implications.

Once this function has been invoked for every feature (local crate and all extern crates), then..

• If features remain in remaining_lib_features, then the user has enabled a feature that does not exist.
• If features remain in remaining_implications, the implied_by refers to a feature that does not exist.

By structuring the code in this way: checking the features defined from each crate one at a time, less loading from metadata is performed and thus compiler performance is improved.