src/generic_parameters_summary.md - rust-lang/rustc-dev-guide - Git at Google

 # 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 `Binder`s ][ch_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:

 ```rust,ignore
 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][ch_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.

 [ch_representing_types]: ./ty.md
 [`ty::Generics`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html
 [`GenericParamDef`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/generics/struct.GenericParamDef.html
 [ch_binders]: ./ty_module/binders.md
	# 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 `Binder`s ][ch_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:

	```rust,ignore
	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][ch_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.

	[ch_representing_types]: ./ty.md
	[`ty::Generics`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Generics.html
	[`GenericParamDef`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/generics/struct.GenericParamDef.html
	[ch_binders]: ./ty_module/binders.md