values does not live long enough when calling as_slice() on a slice

[arjantop]

From stackoverflow question

example:

fn test<'a>(s: &'a String) -> &'a str {
    let slice = s.as_slice();
    slice.as_slice()
}

error:

rpn_calc.rs:2:5: 2:17 error: borrowed value does not live long enough
rpn_calc.rs:2     s.as_slice().as_slice()
                  ^~~~~~~~~~~~
rpn_calc.rs:1:39: 3:2 note: reference must be valid for the lifetime 'a as defined on the block at 1:38...
rpn_calc.rs:1 fn test<'a>(s: &'a String) -> &'a str {
rpn_calc.rs:2     s.as_slice().as_slice()
rpn_calc.rs:3 }
rpn_calc.rs:1:39: 3:2 note: ...but borrowed value is only valid for the block at 1:38
rpn_calc.rs:1 fn test<'a>(s: &'a String) -> &'a str {
rpn_calc.rs:2     s.as_slice().as_slice()
rpn_calc.rs:3 }
error: aborting due to previous error

[bluss]

Even this is broken the same way:

fn test<'a>(s: &'a str) -> &'a str {
    s.as_slice()
}

2:5: 2:6 error: `s` does not live long enough

but the code s.slice(0, s.len()) would compile!

This is because of the formulation of the Str trait.

[huonw]

(@arjantop just btw, the error message and code you posted don't exactly match (no temporary slice in the error messages), I'm sure the problem exists in both, though.)

[huonw]

I think the "problem" here is the definition of as_slice. It's taking a reference to whatever is being coerced (say s), and is stating that the return value lasts for as long as the s value. In this case, slice only lasts for test's stack frame (call that 'b), and thus the reference (of type &'b &'a str) created when calling as_slice can only last that long... 'b is not 'a.

Closing as working-as-intended (the fix is not calling as_slice on values that are already &strs).

(Incidentally DST will allow impl StrSlice for str directly, which will allow "pointless" as_slice's like this to work as you expect.)

(e: I expanded this over on SO.)