// Copyright 2024 The Fuchsia Authors

//

// Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0

// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT

// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.

// This file may not be copied, modified, or distributed except according to

// those terms.

//! Traits for types that encapsulate a `[u8]`.

//!

//! These traits are used to bound the `B` parameter of [`Ref`].

use core::{

    cell,

    ops::{Deref, DerefMut},

};

// For each trait polyfill, as soon as the corresponding feature is stable, the

// polyfill import will be unused because method/function resolution will prefer

// the inherent method/function over a trait method/function. Thus, we suppress

// the `unused_imports` warning.

//

// See the documentation on `util::polyfills` for more information.

#[allow(unused_imports)]

use crate::util::polyfills::{self, NonNullExt as _, NumExt as _};

#[cfg(doc)]

use crate::Ref;

/// A mutable or immutable reference to a byte slice.

///

/// `ByteSlice` abstracts over the mutability of a byte slice reference, and is

/// implemented for various special reference types such as

/// [`Ref<[u8]>`](core::cell::Ref) and [`RefMut<[u8]>`](core::cell::RefMut).

///

/// # Safety

///

/// Implementations of `ByteSlice` must promise that their implementations of

/// [`Deref`] and [`DerefMut`] are "stable". In particular, given `B: ByteSlice`

/// and `b: B`, two calls, each to either `b.deref()` or `b.deref_mut()`, must

/// return a byte slice with the same address and length. This must hold even if

/// the two calls are separated by an arbitrary sequence of calls to methods on

/// `ByteSlice`, [`ByteSliceMut`], [`IntoByteSlice`], or [`IntoByteSliceMut`],

/// or on their super-traits. This does *not* need to hold if the two calls are

/// separated by any method calls, field accesses, or field modifications *other

/// than* those from these traits.

///

/// Note that this also implies that, given `b: B`, the address and length

/// cannot be modified via objects other than `b`, either on the same thread or

/// on another thread.

pub unsafe trait ByteSlice: Deref<Target = [u8]> + Sized {}

/// A mutable reference to a byte slice.

///

/// `ByteSliceMut` abstracts over various ways of storing a mutable reference to

/// a byte slice, and is implemented for various special reference types such as

/// `RefMut<[u8]>`.

///

/// `ByteSliceMut` is a shorthand for [`ByteSlice`] and [`DerefMut`].

pub trait ByteSliceMut: ByteSlice + DerefMut {}

impl<B: ByteSlice + DerefMut> ByteSliceMut for B {}

/// A [`ByteSlice`] which can be copied without violating dereference stability.

///

/// # Safety

///

/// If `B: CopyableByteSlice`, then the dereference stability properties

/// required by [`ByteSlice`] (see that trait's safety documentation) do not

/// only hold regarding two calls to `b.deref()` or `b.deref_mut()`, but also

/// hold regarding `c.deref()` or `c.deref_mut()`, where `c` is produced by

/// copying `b`.

pub unsafe trait CopyableByteSlice: ByteSlice + Copy + CloneableByteSlice {}

/// A [`ByteSlice`] which can be cloned without violating dereference stability.

///

/// # Safety

///

/// If `B: CloneableByteSlice`, then the dereference stability properties

/// required by [`ByteSlice`] (see that trait's safety documentation) do not

/// only hold regarding two calls to `b.deref()` or `b.deref_mut()`, but also

/// hold regarding `c.deref()` or `c.deref_mut()`, where `c` is produced by

/// `b.clone()`, `b.clone().clone()`, etc.

pub unsafe trait CloneableByteSlice: ByteSlice + Clone {}

/// A [`ByteSlice`] that can be split in two.

///

/// # Safety

///

/// Unsafe code may depend for its soundness on the assumption that `split_at`

/// and `split_at_unchecked` are implemented correctly. In particular, given `B:

/// SplitByteSlice` and `b: B`, if `b.deref()` returns a byte slice with address

/// `addr` and length `len`, then if `split <= len`, both of these

/// invocations:

/// - `b.split_at(split)`

/// - `b.split_at_unchecked(split)`

///

/// ...will return `(first, second)` such that:

/// - `first`'s address is `addr` and its length is `split`

/// - `second`'s address is `addr + split` and its length is `len - split`

pub unsafe trait SplitByteSlice: ByteSlice {

    /// Attempts to split `self` at the midpoint.

    ///

    /// `s.split_at(mid)` returns `Ok((s[..mid], s[mid..]))` if `mid <=

    /// s.deref().len()` and otherwise returns `Err(s)`.

    ///

    /// # Safety

    ///

    /// Unsafe code may rely on this function correctly implementing the above

    /// functionality.

    #[inline]

    fn split_at(self, mid: usize) -> Result<(Self, Self), Self> {

        if mid <= self.deref().len() {

            // SAFETY: Above, we ensure that `mid <= self.deref().len()`. By

            // invariant on `ByteSlice`, a supertrait of `SplitByteSlice`,

            // `.deref()` is guaranteed to be "stable"; i.e., it will always

            // dereference to a byte slice of the same address and length. Thus,

            // we can be sure that the above precondition remains satisfied

            // through the call to `split_at_unchecked`.

            unsafe { Ok(self.split_at_unchecked(mid)) }

        } else {

            Err(self)

        }

    }

    /// Splits the slice at the midpoint, possibly omitting bounds checks.

    ///

    /// `s.split_at_unchecked(mid)` returns `s[..mid]` and `s[mid..]`.

    ///

    /// # Safety

    ///

    /// `mid` must not be greater than `self.deref().len()`.

    ///

    /// # Panics

    ///

    /// Implementations of this method may choose to perform a bounds check and

    /// panic if `mid > self.deref().len()`. They may also panic for any other

    /// reason. Since it is optional, callers must not rely on this behavior for

    /// soundness.

    #[must_use]

    unsafe fn split_at_unchecked(self, mid: usize) -> (Self, Self);

}

/// A shorthand for [`SplitByteSlice`] and [`ByteSliceMut`].

pub trait SplitByteSliceMut: SplitByteSlice + ByteSliceMut {}

impl<B: SplitByteSlice + ByteSliceMut> SplitByteSliceMut for B {}

#[allow(clippy::missing_safety_doc)] // There's a `Safety` section on `into_byte_slice`.

/// A [`ByteSlice`] that conveys no ownership, and so can be converted into a

/// byte slice.

///

/// Some `ByteSlice` types (notably, the standard library's [`Ref`] type) convey

/// ownership, and so they cannot soundly be moved by-value into a byte slice

/// type (`&[u8]`). Some methods in this crate's API (such as [`Ref::into_ref`])

/// are only compatible with `ByteSlice` types without these ownership

/// semantics.

///

/// [`Ref`]: core::cell::Ref

pub unsafe trait IntoByteSlice<'a>: ByteSlice {

    /// Coverts `self` into a `&[u8]`.

    ///

    /// # Safety

    ///

    /// The returned reference has the same address and length as `self.deref()`

    /// and `self.deref_mut()`.

    ///

    /// Note that, combined with the safety invariant on [`ByteSlice`], this

    /// safety invariant implies that the returned reference is "stable" in the

    /// sense described in the `ByteSlice` docs.

    fn into_byte_slice(self) -> &'a [u8];

}

#[allow(clippy::missing_safety_doc)] // There's a `Safety` section on `into_byte_slice_mut`.

/// A [`ByteSliceMut`] that conveys no ownership, and so can be converted into a

/// mutable byte slice.

///

/// Some `ByteSliceMut` types (notably, the standard library's [`RefMut`] type)

/// convey ownership, and so they cannot soundly be moved by-value into a byte

/// slice type (`&mut [u8]`). Some methods in this crate's API (such as

/// [`Ref::into_mut`]) are only compatible with `ByteSliceMut` types without

/// these ownership semantics.

///

/// [`RefMut`]: core::cell::RefMut

pub unsafe trait IntoByteSliceMut<'a>: IntoByteSlice<'a> + ByteSliceMut {

    /// Coverts `self` into a `&mut [u8]`.

    ///

    /// # Safety

    ///

    /// The returned reference has the same address and length as `self.deref()`

    /// and `self.deref_mut()`.

    ///

    /// Note that, combined with the safety invariant on [`ByteSlice`], this

    /// safety invariant implies that the returned reference is "stable" in the

    /// sense described in the `ByteSlice` docs.

    fn into_byte_slice_mut(self) -> &'a mut [u8];

}

// FIXME(#429): Add a "SAFETY" comment and remove this `allow`.

#[allow(clippy::undocumented_unsafe_blocks)]

unsafe impl ByteSlice for &[u8] {}

// FIXME(#429): Add a "SAFETY" comment and remove this `allow`.

#[allow(clippy::undocumented_unsafe_blocks)]

unsafe impl CopyableByteSlice for &[u8] {}

// FIXME(#429): Add a "SAFETY" comment and remove this `allow`.

#[allow(clippy::undocumented_unsafe_blocks)]

unsafe impl CloneableByteSlice for &[u8] {}

// SAFETY: This delegates to `polyfills:split_at_unchecked`, which is documented

// to correctly split `self` into two slices at the given `mid` point.

unsafe impl SplitByteSlice for &[u8] {

    #[inline]

    unsafe fn split_at_unchecked(self, mid: usize) -> (Self, Self) {

        // SAFETY: By contract on caller, `mid` is not greater than

        // `bytes.len()`.

        #[allow(clippy::multiple_unsafe_ops_per_block)]

        unsafe {

            (<[u8]>::get_unchecked(self, ..mid), <[u8]>::get_unchecked(self, mid..))

        }

    }

}

// SAFETY: See inline.

unsafe impl<'a> IntoByteSlice<'a> for &'a [u8] {

    #[inline(always)]

    fn into_byte_slice(self) -> &'a [u8] {

        // SAFETY: It would be patently insane to implement `<Deref for

        // &[u8]>::deref` as anything other than `fn deref(&self) -> &[u8] {

        // *self }`. Assuming this holds, then `self` is stable as required by

        // `into_byte_slice`.

        self

    }

}

// FIXME(#429): Add a "SAFETY" comment and remove this `allow`.

#[allow(clippy::undocumented_unsafe_blocks)]

unsafe impl ByteSlice for &mut [u8] {}

// SAFETY: This delegates to `polyfills:split_at_mut_unchecked`, which is

// documented to correctly split `self` into two slices at the given `mid`

// point.

unsafe impl SplitByteSlice for &mut [u8] {

    #[inline]

    unsafe fn split_at_unchecked(self, mid: usize) -> (Self, Self) {

        use core::slice::from_raw_parts_mut;

        // `l_ptr` is non-null, because `self` is non-null, by invariant on

        // `&mut [u8]`.

        let l_ptr = self.as_mut_ptr();

        // SAFETY: By contract on caller, `mid` is not greater than

        // `self.len()`.

        let r_ptr = unsafe { l_ptr.add(mid) };

        let l_len = mid;

        // SAFETY: By contract on caller, `mid` is not greater than

        // `self.len()`.

        //

        // FIXME(#67): Remove this allow. See NumExt for more details.

        #[allow(unstable_name_collisions)]

        let r_len = unsafe { self.len().unchecked_sub(mid) };

        // SAFETY: These invocations of `from_raw_parts_mut` satisfy its

        // documented safety preconditions [1]:

        // - The data `l_ptr` and `r_ptr` are valid for both reads and writes of

        //   `l_len` and `r_len` bytes, respectively, and they are trivially

        //   aligned. In particular:

        //   - The entire memory range of each slice is contained within a

        //     single allocated object, since `l_ptr` and `r_ptr` are both

        //     derived from within the address range of `self`.

        //   - Both `l_ptr` and `r_ptr` are non-null and trivially aligned.

        //     `self` is non-null by invariant on `&mut [u8]`, and the

        //     operations that derive `l_ptr` and `r_ptr` from `self` do not

        //     nullify either pointer.

        // - The data `l_ptr` and `r_ptr` point to `l_len` and `r_len`,

        //   respectively, consecutive properly initialized values of type `u8`.

        //   This is true for `self` by invariant on `&mut [u8]`, and remains

        //   true for these two sub-slices of `self`.

        // - The memory referenced by the returned slice cannot be accessed

        //   through any other pointer (not derived from the return value) for

        //   the duration of lifetime `'a``, because:

        //   - `split_at_unchecked` consumes `self` (which is not `Copy`),

        //   - `split_at_unchecked` does not exfiltrate any references to this

        //     memory, besides those references returned below,

        //   - the returned slices are non-overlapping.

        // - The individual sizes of the sub-slices of `self` are no larger than

        //   `isize::MAX`, because their combined sizes are no larger than

        //   `isize::MAX`, by invariant on `self`.

        //

        // [1] https://doc.rust-lang.org/std/slice/fn.from_raw_parts_mut.html#safety

        #[allow(clippy::multiple_unsafe_ops_per_block)]

        unsafe {

            (from_raw_parts_mut(l_ptr, l_len), from_raw_parts_mut(r_ptr, r_len))

        }

    }

}

// SAFETY: See inline.

unsafe impl<'a> IntoByteSlice<'a> for &'a mut [u8] {

    #[inline(always)]

    fn into_byte_slice(self) -> &'a [u8] {

        // SAFETY: It would be patently insane to implement `<Deref for &mut

        // [u8]>::deref` as anything other than `fn deref(&self) -> &[u8] {

        // *self }`. Assuming this holds, then `self` is stable as required by

        // `into_byte_slice`.

        self

    }

}

// SAFETY: See inline.

unsafe impl<'a> IntoByteSliceMut<'a> for &'a mut [u8] {

    #[inline(always)]

    fn into_byte_slice_mut(self) -> &'a mut [u8] {

        // SAFETY: It would be patently insane to implement `<DerefMut for &mut

        // [u8]>::deref` as anything other than `fn deref_mut(&mut self) -> &mut

        // [u8] { *self }`. Assuming this holds, then `self` is stable as

        // required by `into_byte_slice_mut`.

        self

    }

}

// FIXME(#429): Add a "SAFETY" comment and remove this `allow`.

#[allow(clippy::undocumented_unsafe_blocks)]

unsafe impl ByteSlice for cell::Ref<'_, [u8]> {}

// SAFETY: This delegates to stdlib implementation of `Ref::map_split`, which is

// assumed to be correct, and `SplitByteSlice::split_at_unchecked`, which is

// documented to correctly split `self` into two slices at the given `mid`

// point.

unsafe impl SplitByteSlice for cell::Ref<'_, [u8]> {

    #[inline]

    unsafe fn split_at_unchecked(self, mid: usize) -> (Self, Self) {

        cell::Ref::map_split(self, |slice|

            // SAFETY: By precondition on caller, `mid` is not greater than

            // `slice.len()`.

            unsafe {

                SplitByteSlice::split_at_unchecked(slice, mid)

            })

    }

}

// FIXME(#429): Add a "SAFETY" comment and remove this `allow`.

#[allow(clippy::undocumented_unsafe_blocks)]

unsafe impl ByteSlice for cell::RefMut<'_, [u8]> {}

// SAFETY: This delegates to stdlib implementation of `RefMut::map_split`, which

// is assumed to be correct, and `SplitByteSlice::split_at_unchecked`, which is

// documented to correctly split `self` into two slices at the given `mid`

// point.

unsafe impl SplitByteSlice for cell::RefMut<'_, [u8]> {

    #[inline]

    unsafe fn split_at_unchecked(self, mid: usize) -> (Self, Self) {

        cell::RefMut::map_split(self, |slice|

            // SAFETY: By precondition on caller, `mid` is not greater than

            // `slice.len()`

            unsafe {

                SplitByteSlice::split_at_unchecked(slice, mid)

            })

    }

}

#[cfg(kani)]

mod proofs {

    use super::*;

    fn any_vec() -> Vec<u8> {

        let len = kani::any();

        kani::assume(len <= isize::MAX as usize);

        vec![0u8; len]

    }

    #[kani::proof]

    fn prove_split_at_unchecked() {

        let v = any_vec();

        let slc = v.as_slice();

        let mid = kani::any();

        kani::assume(mid <= slc.len());

        let (l, r) = unsafe { slc.split_at_unchecked(mid) };

        assert_eq!(l.len() + r.len(), slc.len());

        let slc: *const _ = slc;

        let l: *const _ = l;

        let r: *const _ = r;

        assert_eq!(slc.cast::<u8>(), l.cast::<u8>());

        assert_eq!(unsafe { slc.cast::<u8>().add(mid) }, r.cast::<u8>());

        let mut v = any_vec();

        let slc = v.as_mut_slice();

        let len = slc.len();

        let mid = kani::any();

        kani::assume(mid <= slc.len());

        let (l, r) = unsafe { slc.split_at_unchecked(mid) };

        assert_eq!(l.len() + r.len(), len);

        let l: *mut _ = l;

        let r: *mut _ = r;

        let slc: *mut _ = slc;

        assert_eq!(slc.cast::<u8>(), l.cast::<u8>());

        assert_eq!(unsafe { slc.cast::<u8>().add(mid) }, r.cast::<u8>());

    }

}

#[cfg(test)]

mod tests {

    use core::cell::RefCell;

    use super::*;

    #[test]

    fn test_ref_split_at_unchecked() {

        let cell = RefCell::new([1, 2, 3, 4]);

        let borrow = cell.borrow();

        let slice_ref: cell::Ref<'_, [u8]> = cell::Ref::map(borrow, |a| &a[..]);

        // SAFETY: 2 is within bounds of [1, 2, 3, 4]

        let (l, r) = unsafe { slice_ref.split_at_unchecked(2) };

        assert_eq!(*l, [1, 2]);

        assert_eq!(*r, [3, 4]);

    }

    #[test]

    fn test_ref_mut_split_at_unchecked() {

        let cell = RefCell::new([1, 2, 3, 4]);

        let borrow_mut = cell.borrow_mut();

        let slice_ref_mut: cell::RefMut<'_, [u8]> = cell::RefMut::map(borrow_mut, |a| &mut a[..]);

        // SAFETY: 2 is within bounds of [1, 2, 3, 4]

        let (l, r) = unsafe { slice_ref_mut.split_at_unchecked(2) };

        assert_eq!(*l, [1, 2]);

        assert_eq!(*r, [3, 4]);

    }

}