What is ty::Generics

The generic parameters introduced by an item are tracked the ty::Generics struct. Sometimes items allows usage of generics of parent items inside of them, this is accomplished via the ty::Generics struct having an optional field to specify a parent item to inherit generic parameters of. For example given the following code:

trait Trait<T> {
    fn foo<U>(&self);

The ty::Generics used for foo would contain [U] and a parent of Some(Trait). Trait would have a ty::Generics containing [Self, T] with a parent of None.

The GenericParamDef struct is used to represent each individual generic parameter in a ty::Generics listing. The GenericParamDef struct contains information about the generic parameter, for example its name, defid, what kind of parameter it is (i.e. type, const, lifetime).

GenericParamDef also contains a u32 index representing what position the parameter is (starting from the outermost parent), this is the value used to represent usages of generic parameters (more on this in the chapter on representing types).

Interestingly, ty::Generics does not currently contain every generic parameter defined on an item. In the case of functions it only contains the early bound lifetime parameters. See the next chapter for information on what "early bound" and "late bound" parameters are.