<span id="upvar">upvar</span> &nbsp; | A variable captured by a closure from outside the closure.

<span id="variance">variance</span> &nbsp; | Determines how changes to a generic type/lifetime parameter affect subtyping; for example, if `T` is a subtype of `U`, then `Vec<T>` is a subtype `Vec<U>` because `Vec` is *covariant* in its generic parameter. See [the background chapter](./background.md#variance) for a more general explanation. See the [variance chapter](../variance.md) for an explanation of how type checking handles variance.

<span id="variant-idx">variant index</span> &nbsp; | In an enum, identifies a variant by assigning them indices starting at 0. This is purely internal and not to be confused with the ["discriminant"](#discriminant) which can be overwritten by the user (e.g. `enum Bool { True = 42, False = 0 }`).

<span id="wide-ptr">wide pointer</span> &nbsp; | A pointer with additional metadata. See "fat pointer" for more.

<span id="zst">ZST</span> &nbsp; | Zero-Sized Type. A type whose values have size 0 bytes. Since `2^0 = 1`, such types can have exactly one value. For example, `()` (unit) is a ZST. `struct Foo;` is also a ZST. The compiler can do some nice optimizations around ZSTs.