softbuffer 0.3.0

// Not needed on all platforms
#![allow(dead_code)]

use crate::SoftBufferError;

/// Takes a mutable reference to a container and a function deriving a
/// reference into it, and stores both, making it possible to get back the
/// reference to the container once the other reference is no longer needed.
///
/// This should be consistent with stacked borrow rules, and miri seems to
/// accept it at least in simple cases.
pub struct BorrowStack<'a, T: 'static + ?Sized, U: 'static + ?Sized> {
    container: *mut T,
    member: *mut U,
    _phantom: std::marker::PhantomData<&'a mut T>,
}

impl<'a, T: 'static + ?Sized, U: 'static + ?Sized> BorrowStack<'a, T, U> {
    pub fn new<F>(container: &'a mut T, f: F) -> Result<Self, SoftBufferError>
    where
        F: for<'b> FnOnce(&'b mut T) -> Result<&'b mut U, SoftBufferError>,
    {
        let container = container as *mut T;
        let member = f(unsafe { &mut *container })? as *mut U;
        Ok(Self {
            container,
            member,
            _phantom: std::marker::PhantomData,
        })
    }

    pub fn member(&self) -> &U {
        unsafe { &*self.member }
    }

    pub fn member_mut(&mut self) -> &mut U {
        unsafe { &mut *self.member }
    }

    pub fn into_container(self) -> &'a mut T {
        // SAFETY: Since we consume self and no longer reference member, this
        // mutable reference is unique.
        unsafe { &mut *self.container }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_borrowstack_slice_int() {
        fn f(mut stack: BorrowStack<[u32], u32>) {
            assert_eq!(*stack.member(), 3);
            *stack.member_mut() = 42;
            assert_eq!(stack.into_container(), &[1, 2, 42, 4, 5]);
        }

        let mut v = vec![1, 2, 3, 4, 5];
        f(BorrowStack::new(v.as_mut(), |v: &mut [u32]| Ok(&mut v[2])).unwrap());
        assert_eq!(&v, &[1, 2, 42, 4, 5]);
    }
}