comparison rust/hg-core/src/dirstate_tree/owning.rs @ 48567:d1210d56008b

rust: Fix outdated comments in OwningDirstateMap

OwningDirstateMap used to own a PyBytes, but was generalized to be
more generic when it was moved from hg-cpython to hg-core.
This fixes some comments that were still referencing PyBytes.

Differential Revision: https://phab.mercurial-scm.org/D12003

48566:50de08904c63

    /// Pointer for `Box<DirstateMap<'on_disk>>`, typed-erased because the
    /// language cannot represent a lifetime referencing a sibling field.
    /// This is not quite a self-referencial struct (moving this struct is not
    /// a problem as it doesn’t change the address of the bytes buffer owned
    /// by `PyBytes`) but touches similar borrow-checker limitations.
    ptr: *mut (),
}

impl OwningDirstateMap {
    pub fn new_empty<OnDisk>(on_disk: OnDisk) -> Self

        &'a mut self,
    ) -> (&'a [u8], &'a mut DirstateMap<'a>) {
        // SAFETY: We cast the type-erased pointer back to the same type it had
        // in `new`, except with a different lifetime parameter. This time we
        // connect the lifetime to that of `self`. This cast is valid because
        // `self` owns the same `PyBytes` whose buffer `DirstateMap`
        // references. That buffer has a stable memory address because the byte
        // string value of a `PyBytes` is immutable.
        let ptr: *mut DirstateMap<'a> = self.ptr.cast();
        // SAFETY: we dereference that pointer, connecting the lifetime of the
        // new   `&mut` to that of `self`. This is valid because the
        // raw pointer is   to a boxed value, and `self` owns that box.
        (&self.on_disk, unsafe { &mut *ptr })
    }

    pub fn get_map_mut<'a>(&'a mut self) -> &'a mut DirstateMap<'a> {
        self.get_pair_mut().1
    }

    pub fn get_map<'a>(&'a self) -> &'a DirstateMap<'a> {
        // SAFETY: same reasoning as in `get_mut` above.
        let ptr: *mut DirstateMap<'a> = self.ptr.cast();
        unsafe { &*ptr }
    }

    pub fn on_disk<'a>(&'a self) -> &'a [u8] {

impl Drop for OwningDirstateMap {
    fn drop(&mut self) {
        // Silence a "field is never read" warning, and demonstrate that this
        // value is still alive.
        let _ = &self.on_disk;
        // SAFETY: this cast is the same as in `get_mut`, and is valid for the
        // same reason. `self.on_disk` still exists at this point, drop glue
        // will drop it implicitly after this `drop` method returns.
        let ptr: *mut DirstateMap<'_> = self.ptr.cast();
        // SAFETY: `Box::from_raw` takes ownership of the box away from `self`.
        // This is fine because drop glue does nothig for `*mut ()` and we’re
        // in `drop`, so `get` and `get_mut` cannot be called again.
        unsafe { drop(Box::from_raw(ptr)) }
    }
}

48567:d1210d56008b

    /// Pointer for `Box<DirstateMap<'on_disk>>`, typed-erased because the
    /// language cannot represent a lifetime referencing a sibling field.
    /// This is not quite a self-referencial struct (moving this struct is not
    /// a problem as it doesn’t change the address of the bytes buffer owned
    /// by `on_disk`) but touches similar borrow-checker limitations.
    ptr: *mut (),
}

impl OwningDirstateMap {
    pub fn new_empty<OnDisk>(on_disk: OnDisk) -> Self

        &'a mut self,
    ) -> (&'a [u8], &'a mut DirstateMap<'a>) {
        // SAFETY: We cast the type-erased pointer back to the same type it had
        // in `new`, except with a different lifetime parameter. This time we
        // connect the lifetime to that of `self`. This cast is valid because
        // `self` owns the same `on_disk` whose buffer `DirstateMap`
        // references. That buffer has a stable memory address because our
        // `Self::new_empty` counstructor requires `StableDeref`.
        let ptr: *mut DirstateMap<'a> = self.ptr.cast();
        // SAFETY: we dereference that pointer, connecting the lifetime of the
        // new `&mut` to that of `self`. This is valid because the
        // raw pointer is to a boxed value, and `self` owns that box.
        (&self.on_disk, unsafe { &mut *ptr })
    }

    pub fn get_map_mut<'a>(&'a mut self) -> &'a mut DirstateMap<'a> {
        self.get_pair_mut().1
    }

    pub fn get_map<'a>(&'a self) -> &'a DirstateMap<'a> {
        // SAFETY: same reasoning as in `get_pair_mut` above.
        let ptr: *mut DirstateMap<'a> = self.ptr.cast();
        unsafe { &*ptr }
    }

    pub fn on_disk<'a>(&'a self) -> &'a [u8] {

impl Drop for OwningDirstateMap {
    fn drop(&mut self) {
        // Silence a "field is never read" warning, and demonstrate that this
        // value is still alive.
        let _: &Box<dyn Deref<Target = [u8]> + Send> = &self.on_disk;
        // SAFETY: this cast is the same as in `get_mut`, and is valid for the
        // same reason. `self.on_disk` still exists at this point, drop glue
        // will drop it implicitly after this `drop` method returns.
        let ptr: *mut DirstateMap<'_> = self.ptr.cast();
        // SAFETY: `Box::from_raw` takes ownership of the box away from `self`.
        // This is fine because drop glue does nothing for `*mut ()` and we’re
        // in `drop`, so `get` and `get_mut` cannot be called again.
        unsafe { drop(Box::from_raw(ptr)) }
    }
}