# Parameter `Ty`/`Const`/`Region`s

When inside of generic items, types can be written that use in scope generic parameters, for example `fn foo<'a, T>(_: &'a Vec<T>)`. In this specific case
the `&'a Vec<T>` type would be represented internally as:
```
TyKind::Ref(
  RegionKind::LateParam(DefId(foo), DefId(foo::'a), "'a"),
  TyKind::Adt(Vec, &[TyKind::Param("T", 0)])
)
```

There are three separate ways we represent usages of generic parameters:
- [`TyKind::Param`]/[`ConstKind::Param`]/[`RegionKind::EarlyParam`] for early bound generic parameters (note: all type and const parameters are considered early bound, see the [chapter on early vs late bound parameters][ch_early_late_bound] for more information)
- [`TyKind::Bound`]/[`ConstKind::Bound`]/[`RegionKind::Bound`] for references to parameters introduced via higher ranked bounds or higher ranked types i.e. `for<'a> fn(&'a u32)` or `for<'a> T: Trait<'a>`. This is discussed in the [chapter on `Binder`s][ch_binders].
- [`RegionKind::LateParam`] for late bound lifetime parameters, `LateParam` is discussed in the [chapter on instantiating `Binder`s][ch_instantiating_binders].

This chapter only covers `TyKind::Param` `ConstKind::Param` and `RegionKind::EarlyParam`.

## Ty/Const Parameters

As `TyKind::Param` and `ConstKind::Param` are implemented identically this section only refers to `TyKind::Param` for simplicity.
However you should keep in mind that everything here also is true of `ConstKind::Param`

Each `TyKind::Param` contains two things: the name of the parameter and an index.

See the following concrete example of a usage of `TyKind::Param`:
```rust,ignore
struct Foo<T>(Vec<T>);
```
The `Vec<T>` type is represented as `TyKind::Adt(Vec, &[GenericArgKind::Type(Param("T", 0))])`.

The name is somewhat self explanatory, it's the name of the type parameter. The index of the type parameter is an integer indicating
its order in the list of generic parameters in scope (note: this includes parameters defined on items on outer scopes than the item the parameter is defined on). Consider the following examples:

```rust,ignore
struct Foo<A, B> {
  // A would have index 0
  // B would have index 1

  .. // some fields
}
impl<X, Y> Foo<X, Y> {
  fn method<Z>() {
    // inside here, X, Y and Z are all in scope
    // X has index 0
    // Y has index 1
    // Z has index 2
  }
}
```

Concretely given the `ty::Generics` for the item the parameter is defined on, if the index is `2` then starting from the root `parent`, it will be the third parameter to be introduced. For example in the above example, `Z` has index `2` and is the third generic parameter to be introduced, starting from the `impl` block.