References/borrows of const values don't have 'static lifetime

[mzji]

Example:

const FOO: usize = 0;

fn bar() -> &'static usize {
    &FOO
}

fn main() {
    bar();
}

Get error:

rustc 1.17.0-nightly (b1e31766d 2017-03-03)
error: borrowed value does not live long enough
 --> <anon>:5:6
  |
5 |     &FOO
  |      ^^^ does not live long enough
6 | }
  | - temporary value only lives until here
  |
  = note: borrowed value must be valid for the static lifetime...

error: aborting due to previous error

[codyps]

This is expected. From the signature of bar(), we can't know that FOO will have been borrowed afterwards.

Borrows will generally be scoped by their inner-most block (like the function body).

You can, however, do this:

const FOO_: usize = 0;
const FOO: &'static usize = &FOO_;

fn bar() -> &'static usize {
    FOO
}

fn main() {
    bar();
}

Edit: that said, it does seem like this could be made to work. We don't really need to know that FOO has been borrowed because we can't borrow it mutably, we can't assign to it, and we won't drop it.

[mzji]

@jmesmon Thanks for the solution! Actually I just solve my problem with this pattern. However I have a question: could we do inference here to give the reference a 'static lifetime?

[Stebalien]

The problem here is static promotion. You can fix this by using static, instead of const:

static FOO: usize = 0;

fn bar() -> &'static usize {
    &FOO
}

fn main() {
    bar();
}

Basically, constants are compile-time only so &FOO gets evaluated to &0 (where 0 is a temporary) at compile-time.

Note: The linked RFC has not been implemented (which is why this doesn't currently work).

[mzji]

After reading the book, I think I know the point showed by @Stebalien 's code. Thank you all!