[gnuvince]

I admit that I'm very unexcited about non-lexical lifetimes—the cost-to-benefit ratio seems worse than the current model. The current lifetime system is simple to understand: a value is live until its owning variable goes out of scope. It can be a bit restrictive sometimes (though it seems that after a while you learn how to work around those issues easily enough), but it's easy to keep in your head and reason about code. With non-lexical lifetimes, I think that we'll have a harder time understanding lifetimes, it'll be more difficult to teach them and to debug them.

[wwright]

My understanding is that the semantics are exactly the same, but it’s just possible for the checker to match your intent more tightly.

For example, in lexical lifetimes, this fails:

    let x = &mut foo;
    
    if x.bar() {
        baz()
    } else {
        foo.consume() // takes foo by value
    }

It will complain that you’ve used `foo` while it is borrowed by `x`, even though a human can easily tell that `x` is already irrelevant by that point. NLL would accept this (I believe).