Comment by kibwen

Short background: in today's Rust, the lifetime of a reference is bound to a lexical scope. Sometimes this forces one to create temporary variables solely to change the lexical scope of a reference, which is an annoyance. Take the following example:

    let mut nums = Vec::new();
    nums.push(nums.len());

The above code would compile if we had non-lexical lifetimes, but today it doesn't. This is because method calls are effectively desugared to something like this:

    let mut nums = Vec::new();
    Vec::push(&mut nums, nums.len());

The arguments are evaluated left-to-right, and the mutable borrow prevents the immutable borrow later. Once non-lexical lifetimes arrives this will be possible thanks to control-flow analysis, but today we have to make a temporary:

    let mut nums = Vec::new();
    let length = nums.len();
    nums.push(length);

It doesn't involve any new user-facing features to the language and is fully backward-compatible, so it's basically just a straight quality-of-life improvement for Rust programmers.