An annotation in the form of T: 'a, where T is either a type or another lifetime, is called an "outlives" requirement. Note that "outlives" also implies 'a: 'a.

One way in which edition 2018 helps you out in maintaining flow when writing programs is by removing the need to explicitly annotate these T: 'a outlives requirements in struct definitions. Instead, the requirements will be inferred from the fields present in the definitions.

Consider the following struct definitions in Rust 2015:

#![allow(unused_variables)]
fn main() {
// Rust 2015

struct Ref<'a, T: 'a> {
    field: &'a T
}

// or written with a `where` clause:

struct WhereRef<'a, T> where T: 'a {
    data: &'a T
}

// with nested references:

struct RefRef<'a, 'b: 'a, T: 'b> {
    field: &'a &'b T,
}

// using an associated type:

struct ItemRef<'a, T: Iterator>
where
    T::Item: 'a
{
    field: &'a T::Item
}
}

In Rust 2018, since the requirements are inferred, you can instead write:

// Rust 2018

struct Ref<'a, T> {
    field: &'a T
}

struct WhereRef<'a, T> {
    data: &'a T
}

struct RefRef<'a, 'b, T> {
    field: &'a &'b T,
}

struct ItemRef<'a, T: Iterator> {
    field: &'a T::Item
}

If you prefer to be more explicit in some cases, that is still possible.

For more details, see the tracking issue and the RFC.