T: 'a inference in structs - The Edition Guide

# `T: 'a` inference in structs

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)]
    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.

## More details

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