std::local_data broken with zero-sized types

[SimonSapin]

Test case:

use std::local_data;

local_data_key!(silence_errors: ())

fn main() {
    local_data::get(silence_errors, |silenced| println!("{}", silenced));
    local_data::set(silence_errors, ());
    local_data::get(silence_errors, |silenced| println!("{}", silenced));
}

Expected output:

None
Some(())

Actual output:

None
None

Replacing () with bool/true fixes the issue.

[SimonSapin]

Sounds kinda possibly maybe related to #11998?

[alexcrichton]

Fixed by 898669c (working on nightly)