Like syn, some functionality are behind optional features to optimize compile-time. Currently the follow feature is available:
extra-traits— Debug, Eq, PartialEq, Hash impls for all syntax tree types.
Holds a type that is optionally annotated with key-value options.
Holds a single key value attribute, with the value being optional.
Holds an outer attribute filled with key-value options.
Holds a simple type that is optionally annotated as
Type that holds an abstract type and how it will map to a concrete type.