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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
use std::sync::Once;
#[allow(clippy::module_name_repetitions)]
pub struct OnceResult<T> {
once: Once,
v: Option<T>,
}
impl<T> OnceResult<T> {
#[must_use]
pub const fn new() -> Self {
Self {
once: Once::new(),
v: None,
}
}
pub fn call_once<F: FnOnce() -> T>(&mut self, f: F) -> &T {
let v = &mut self.v;
self.once.call_once(|| {
*v = Some(f());
});
self.v.as_ref().unwrap()
}
}
#[cfg(test)]
mod test {
use super::OnceResult;
static mut STATIC_ONCE_RESULT: OnceResult<u64> = OnceResult::new();
#[test]
fn static_update() {
assert_eq!(*unsafe { STATIC_ONCE_RESULT.call_once(|| 23) }, 23);
assert_eq!(*unsafe { STATIC_ONCE_RESULT.call_once(|| 24) }, 23);
}
}
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