Order of code affects lifetime resolution

[chlai88]

This playpen code snippet now fails in compile : playpen

with error

<anon>:23:26: 23:46 error: `long_enough_lifetime` does not live long enough
<anon>:23         handle.set_data(&long_enough_lifetime); // This is OK
                                 ^~~~~~~~~~~~~~~~~~~~
<anon>:17:34: 27:2 note: reference must be valid for the block suffix following statement 0 at 17:33...

But if you just swap the following 2 lines, line 17 with line 18, it compiles fine.

let long_enough_lifetime = DropCanary{label:"long"};
let mut handle = Handle(None);

[Thiez]

I think this is intended. Because items are destroyed in the opposite order that they are created when they go out of scope, the DropCanary must live strictly longer than the Handle, or otherwise the Handle destructor could see a dropped canary :)

Edit: If you put them on the same line, e.g.:

let (mut handle, long_enough_lifetime) = (Handle(None), DropCanary{label:"long"});

it also works. I'm not sure if this is correct because I'm not aware of the order in which they are dropped in this case.

[pnkfelix]

This is all by design. See #21657 and rust-lang/rfcs#769

[pnkfelix]

(If we had a way to attach a lifetime to a struct Handle <'a> where the lifetime did not have to outlive the struct value itself, then this code could be made to work. There are not plans currently to add such a feature as of right now, though.)

[pnkfelix]

Note also that the reason it is okay to put them on the same line is because Handle does not implement Drop.