Generic parameter definitions

This chapter will discuss how rustc tracks what generic parameters are introduced. For example given some struct Foo<T> how does rustc track that Foo defines some type parameter T (and no other generic parameters).

This will not cover how we track generic parameters introduced via for<'a> syntax (e.g. in where clauses or fn types), which is covered elsewhere in the chapter on Binders .

ty::Generics

The generic parameters introduced by an item are tracked by the ty::Generics struct. Sometimes items allow usage of generics defined on parent items, 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 parameters.