rustc-dev-guide/src/generic_arguments.md

Generic arguments

A ty::GenericArg<'tcx> represents some entity in the type system: a type (Ty<'tcx>), lifetime (ty::Region<'tcx>) or constant (ty::Const<'tcx>). GenericArg is used to perform instantiation of generic parameters to concrete arguments, such as when calling a function with generic parameters explicitly with type arguments. Instantiations are represented using the GenericArgs type as described below.

GenericArgs

ty::GenericArgs<'tcx> is intuitively simply a slice of GenericArg<'tcx>s, acting as an ordered list of generic parameters instantiated to concrete arguments (such as types, lifetimes and consts).

For example, given a HashMap<K, V> with two type parameters, K and V, an instantiation of the parameters, for example HashMap<i32, u32>, would be represented by &'tcx [tcx.types.i32, tcx.types.u32].

GenericArgs provides various convenience methods to instantiate generic arguments given item definitions, which should generally be used rather than explicitly instantiating such slices.

GenericArg

The actual GenericArg struct is optimised for space, storing the type, lifetime or const as an interned pointer containing a tag identifying its kind (in the lowest 2 bits). Unless you are working with the GenericArgs implementation specifically, you should generally not have to deal with GenericArg and instead make use of the safe GenericArgKind abstraction.

GenericArgKind

As GenericArg itself is not type-safe, the GenericArgKind enum provides a more convenient and safe interface for dealing with generic arguments. An GenericArgKind can be converted to a raw GenericArg using GenericArg::from() (or simply .into() when the context is clear). As mentioned earlier, instantiation lists store raw GenericArgs, so before dealing with them, it is preferable to convert them to GenericArgKinds first. This is done by calling the .unpack() method.

// An example of unpacking and packing a generic argument.
fn deal_with_generic_arg<'tcx>(generic_arg: GenericArg<'tcx>) -> GenericArg<'tcx> {
    // Unpack a raw `GenericArg` to deal with it safely.
    let new_generic_arg: GenericArgKind<'tcx> = match generic_arg.unpack() {
        GenericArgKind::Type(ty) => { /* ... */ }
        GenericArgKind::Lifetime(lt) => { /* ... */ }
        GenericArgKind::Const(ct) => { /* ... */ }
    };
    // Pack the `GenericArgKind` to store it in a generic args list.
    new_generic_arg.into()
}