Rust lifetimes

Lifetimes are represented by the Lifetime<I> and LifetimeData<I> types. As with types, the actual representation of a lifetime is defined by the associated type I::InternedLifetime.

The `LifetimeData` variants

This section covers the variants we use to categorize lifetimes.

Variants and their equivalents in Rust syntax

Chalk variant	Example Rust types
`BoundVar`	the `'a` in a type like `for<'a> fn(&'a u8)`, before it is instantiated
`InferenceVar`	a lifetime whose value is being inferred
`Placeholder`	how we represent `'a` when type checking `fn foo<'a>() { .. }`
`Static`	the lifetime `'static`