Box::into_inner() is double freeing

[wada314]

As the title says. When I run the following code, it fails with the error message saying "Exited with signal 6 (SIGABRT): abort program" (in playground) or "process didn't exit successfully: C:\Users\wada3\Documents\projects\foo\target\debug\deps\foo-c5fce5ee08e1c1dc.exe (exit code: 0xc0000374, STATUS_HEAP_CORRUPTION)" in my Windows.

Please note that the Box::into_inner() is an unstable function and it's guarded behind box_into_inner feature flag in the std::boxed::Box for now.
So this might be something should not be supported by this library. Anyway, just for someone else who noticed this and came here from google search...

Minimum reproducing sample:

#![feature(allocator_api)]

use ::allocator_api2::boxed::Box;

pub fn main() {
    let b = Box::new(3);
    Box::into_inner(b);
}

allocator_api2 version: 0.2.20

thanks!

[wada314]

Note: This issue happened even in 0.2.18.