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    SubjectRe: [PATCH 2/5] rust: types: introduce `ForeignOwnable`
    On Fri, 20 Jan 2023 at 16:59, Boqun Feng <boqun.feng@gmail.com> wrote:
    >
    > On Thu, Jan 19, 2023 at 02:40:33PM -0300, Wedson Almeida Filho wrote:
    > > It was originally called `PointerWrapper`. It is used to convert
    > > a Rust object to a pointer representation (void *) that can be
    > > stored on the C side, used, and eventually returned to Rust.
    > >
    > > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
    > > ---
    > > rust/kernel/lib.rs | 1 +
    > > rust/kernel/types.rs | 54 ++++++++++++++++++++++++++++++++++++++++++++
    > > 2 files changed, 55 insertions(+)
    > >
    > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
    > > index e0b0e953907d..223564f9f0cc 100644
    > > --- a/rust/kernel/lib.rs
    > > +++ b/rust/kernel/lib.rs
    > > @@ -16,6 +16,7 @@
    > > #![feature(coerce_unsized)]
    > > #![feature(core_ffi_c)]
    > > #![feature(dispatch_from_dyn)]
    > > +#![feature(generic_associated_types)]
    > > #![feature(receiver_trait)]
    > > #![feature(unsize)]
    > >
    > > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
    > > index f0ad4472292d..5475f6163002 100644
    > > --- a/rust/kernel/types.rs
    > > +++ b/rust/kernel/types.rs
    > > @@ -9,6 +9,60 @@ use core::{
    > > ops::{Deref, DerefMut},
    > > };
    > >
    > > +/// Used to transfer ownership to and from foreign (non-Rust) languages.
    > > +///
    > > +/// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
    > > +/// later may be transferred back to Rust by calling [`Self::from_foreign`].
    > > +///
    > > +/// This trait is meant to be used in cases when Rust objects are stored in C objects and
    > > +/// eventually "freed" back to Rust.
    > > +pub trait ForeignOwnable {
    > > + /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and
    > > + /// [`ForeignOwnable::from_foreign`].
    > > + type Borrowed<'a>;
    > > +
    > > + /// Converts a Rust-owned object to a foreign-owned one.
    > > + ///
    > > + /// The foreign representation is a pointer to void.
    > > + fn into_foreign(self) -> *const core::ffi::c_void;
    > > +
    > > + /// Borrows a foreign-owned object.
    > > + ///
    > > + /// # Safety
    > > + ///
    > > + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
    > > + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
    > > + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow_mut`]
    > > + /// for this object must have been dropped.
    > > + unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;
    > > +
    > > + /// Mutably borrows a foreign-owned object.
    > > + ///
    > > + /// # Safety
    > > + ///
    > > + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
    > > + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
    > > + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
    > > + /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
    > > + unsafe fn borrow_mut<T: ForeignOwnable>(ptr: *const core::ffi::c_void) -> ScopeGuard<T, fn(T)> {
    > > + // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
    > > + // `into_foreign`.
    > > + ScopeGuard::new_with_data(unsafe { T::from_foreign(ptr) }, |d| {
    > > + d.into_foreign();
    > > + })
    >
    > I kinda want to suggest borrow_mut() to be implemented as:
    >
    > pub trait ForeignOwnable {
    > ...
    > unsafe fn borrow_mut(ptr: *const core::ffi::c_void) -> ScopeGuard<Self, fn(Self)> {
    > // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
    > // `into_foreign`.
    > ScopeGuard::new_with_data(unsafe { Self::from_foreign(ptr) }, |d| {
    > d.into_foreign();
    > })
    > }

    Oh, eliminate the generic type `T` and just use `Self`. Yes, I think
    that's a good simplification.

    >
    > to avoid funny code as follow:
    >
    > let a = Box::new(0).into_foreign();
    > // Using an irrelevant `impl ForeignOwnable` to `borrow_mut`
    > let borrowed_a: ScopeGuard<Box<i32>, ...> = unsafe { Arc::<u64>::borrow_mut(a) };
    >
    > but that requires `Self: Sized`. Is it too restrictive?

    It isn't. Since we want the raw pointer to fit in a single pointer
    (from C's perspective), we already require `Self: Sized`.

    I'll make this change in v2.

    >
    > Regards,
    > Boqun
    >
    > > + }
    > > +
    > > + /// Converts a foreign-owned object back to a Rust-owned one.
    > > + ///
    > > + /// # Safety
    > > + ///
    > > + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for
    > > + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet.
    > > + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
    > > + /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
    > > + unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
    > > +}
    > > +
    > > /// Runs a cleanup function/closure when dropped.
    > > ///
    > > /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
    > > --
    > > 2.34.1
    > >

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