64 lines
1.9 KiB
Rust
64 lines
1.9 KiB
Rust
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// Not needed on all platforms
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#![allow(dead_code)]
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use crate::SoftBufferError;
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/// Takes a mutable reference to a container and a function deriving a
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/// reference into it, and stores both, making it possible to get back the
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/// reference to the container once the other reference is no longer needed.
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///
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/// This should be consistent with stacked borrow rules, and miri seems to
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/// accept it at least in simple cases.
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pub struct BorrowStack<'a, T: 'static + ?Sized, U: 'static + ?Sized> {
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container: *mut T,
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member: *mut U,
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_phantom: std::marker::PhantomData<&'a mut T>,
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}
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impl<'a, T: 'static + ?Sized, U: 'static + ?Sized> BorrowStack<'a, T, U> {
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pub fn new<F>(container: &'a mut T, f: F) -> Result<Self, SoftBufferError>
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where
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F: for<'b> FnOnce(&'b mut T) -> Result<&'b mut U, SoftBufferError>,
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{
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let container = container as *mut T;
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let member = f(unsafe { &mut *container })? as *mut U;
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Ok(Self {
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container,
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member,
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_phantom: std::marker::PhantomData,
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})
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}
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pub fn member(&self) -> &U {
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unsafe { &*self.member }
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}
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pub fn member_mut(&mut self) -> &mut U {
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unsafe { &mut *self.member }
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}
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pub fn into_container(self) -> &'a mut T {
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// SAFETY: Since we consume self and no longer reference member, this
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// mutable reference is unique.
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unsafe { &mut *self.container }
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn test_borrowstack_slice_int() {
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fn f(mut stack: BorrowStack<[u32], u32>) {
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assert_eq!(*stack.member(), 3);
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*stack.member_mut() = 42;
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assert_eq!(stack.into_container(), &[1, 2, 42, 4, 5]);
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}
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let mut v = vec![1, 2, 3, 4, 5];
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f(BorrowStack::new(v.as_mut(), |v: &mut [u32]| Ok(&mut v[2])).unwrap());
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assert_eq!(&v, &[1, 2, 42, 4, 5]);
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}
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}
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