rc-new-cyclic-fallible 1.0.0

Fallible variant of Rc::new_cyclic
Documentation 
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use std::mem::MaybeUninit;
use std::{rc::Rc, rc::Weak};

/// Helps building a synchronous Rc cyclic reference, from a closure that can fail.
///
/// Will be replaced by [UniqueRc](https://doc.rust-lang.org/stable/alloc/rc/struct.UniqueRc.html), once it gets stabilized.
pub fn rc_new_cyclic_fallible<T, E, F>(f: F) -> Result<Rc<T>, E>
where
    F: FnOnce(&Weak<T>) -> Result<T, E>,
{
    let mut result: Result<(), E> = Ok(());
    let maybe_uninit_rc = Rc::<MaybeUninit<T>>::new_cyclic(|weak_uninit| unsafe {
        // transmute guaranteed to be ok, because MaybeUninit has repr(transparent),
        // additionally, the reference is not going to be used in case of error
        let weak: &Weak<T> = core::mem::transmute(weak_uninit);

        match f(weak) {
            Ok(t) => MaybeUninit::<T>::new(t),
            Err(err) => {
                result = Err(err);
                MaybeUninit::<T>::uninit()
            }
        }
    });
    result?;

    // transmute guaranteed to be ok, because MaybeUninit has repr(transparent)
    let converted: Rc<T> = unsafe { core::mem::transmute(maybe_uninit_rc) };

    Ok(converted)
}