Summary

Temporaries live for the enclosing block when found in a let-binding. This only holds when the reference to the temporary is taken directly. This logic should be extended to extend the cleanup scope of any temporary whose lifetime ends up in the let-binding.

For example, the following doesn't work now, but should:

use std::os;

fn main() {
    let x = os::args().slice_from(1);
    println!("{}", x);
}

Motivation

Temporary lifetimes are a bit confusing right now. Sometimes you can keep references to them, and sometimes you get the dreaded "borrowed value does not live long enough" error. Sometimes one operation works but an equivalent operation errors, e.g. autoref of ~[T] to &[T] works but calling .as_slice() doesn't. In general it feels as though the compiler is simply being overly restrictive when it decides the temporary doesn't live long enough.

Drawbacks

I can't think of any drawbacks.

Detailed design

When a reference to a temporary is passed to a function (either as a regular argument or as the self argument of a method), and the function returns a value with the same lifetime as the temporary reference, the lifetime of the temporary should be extended the same way it would if the function was not invoked.

For example, ~[T].as_slice() takes &'a self and returns &'a [T]. Calling as_slice() on a temporary of type ~[T] will implicitly take a reference &'a ~[T] and return a value &'a [T] This return value should be considered to extend the lifetime of the ~[T] temporary just as taking an explicit reference (and skipping the method call) would.

Alternatives

Don't do this. We live with the surprising borrowck errors and the ugly workarounds that look like


# #![allow(unused_variables)]
#fn main() {
let x = os::args();
let x = x.slice_from(1);
#}

Unresolved questions

None that I know of.