// SPDX-License-Identifier: (BSD-2-Clause OR Apache-2.0) OR MIT//// 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. #![allow(missing_copy_implementations, missing_debug_implementations, missing_docs)] //! The parameterized invariants of a [`Ptr`][super::Ptr].//!//! Invariants are encoded as ([`Aliasing`], [`Alignment`], [`Validity`])//! triples implementing the [`Invariants`] trait. /// The invariants of a [`Ptr`][super::Ptr].pub trait Invariants: Sealed {    type Aliasing: Aliasing;    type Alignment: Alignment;    type Validity: Validity;} impl<A: Aliasing, AA: Alignment, V: Validity> Invariants for (A, AA, V) {    type Aliasing = A;    type Alignment = AA;    type Validity = V;} /// The aliasing invariant of a [`Ptr`][super::Ptr].////// All aliasing invariants must permit reading from the bytes of a pointer's/// referent which are not covered by [`UnsafeCell`]s.////// [`UnsafeCell`]: core::cell::UnsafeCellpub trait Aliasing: Sealed {    /// Is `Self` [`Exclusive`]?    #[doc(hidden)]    const IS_EXCLUSIVE: bool;} /// The alignment invariant of a [`Ptr`][super::Ptr].pub trait Alignment: Sealed {    #[doc(hidden)]    #[must_use]    fn read<T, I, R>(ptr: crate::Ptr<'_, T, I>) -> T    where        T: Copy + Read<I::Aliasing, R>,        I: Invariants<Alignment = Self, Validity = Valid>,        I::Aliasing: Reference;} /// The validity invariant of a [`Ptr`][super::Ptr].////// # Safety////// In this section, we will use `Ptr<T, V>` as a shorthand for `Ptr<T, I:/// Invariants<Validity = V>>` for brevity.////// Each `V: Validity` defines a set of bit values which may appear in the/// referent of a `Ptr<T, V>`, denoted `S(T, V)`. Each `V: Validity`, in its/// documentation, provides a definition of `S(T, V)` which must be valid for/// all `T: ?Sized`. Any `V: Validity` must guarantee that this set is only a/// function of the *bit validity* of the referent type, `T`, and not of any/// other property of `T`. As a consequence, given `V: Validity`, `T`, and `U`/// where `T` and `U` have the same bit validity, `S(V, T) = S(V, U)`.////// It is guaranteed that the referent of any `ptr: Ptr<T, V>` is a member of/// `S(T, V)`. Unsafe code must ensure that this guarantee will be upheld for/// any existing `Ptr`s or any `Ptr`s that that code creates.////// An important implication of this guarantee is that it restricts what/// transmutes are sound, where "transmute" is used in this context to refer to/// changing the referent type or validity invariant of a `Ptr`, as either/// change may change the set of bit values permitted to appear in the referent./// In particular, the following are necessary (but not sufficient) conditions/// in order for a transmute from `src: Ptr<T, V>` to `dst: Ptr<U, W>` to be/// sound:/// - If `S(T, V) = S(U, W)`, then no restrictions apply; otherwise,/// - If `dst` permits mutation of its referent (e.g. via `Exclusive` aliasing///   or interior mutation under `Shared` aliasing), then it must hold that///   `S(T, V) ⊇ S(U, W)` - in other words, the transmute must not expand the///   set of allowed referent bit patterns. A violation of this requirement///   would permit using `dst` to write `x` where `x ∈ S(U, W)` but `x ∉ S(T,///   V)`, which would violate the guarantee that `src`'s referent may only///   contain values in `S(T, V)`./// - If the referent may be mutated without going through `dst` while `dst` is///   live (e.g. via interior mutation on a `Shared`-aliased `Ptr` or `&`///   reference), then it must hold that `S(T, V) ⊆ S(U, W)` - in other words,///   the transmute must not shrink the set of allowed referent bit patterns. A///   violation of this requirement would permit using `src` or another///   mechanism (e.g. a `&` reference used to derive `src`) to write `x` where///   `x ∈ S(T, V)` but `x ∉ S(U, W)`, which would violate the guarantee that///   `dst`'s referent may only contain values in `S(U, W)`.pub unsafe trait Validity: Sealed {    const KIND: ValidityKind;} pub enum ValidityKind {    Uninit,    AsInitialized,    Initialized,    Valid,} /// An [`Aliasing`] invariant which is either [`Shared`] or [`Exclusive`].////// # Safety////// Given `A: Reference`, callers may assume that either `A = Shared` or `A =/// Exclusive`.pub trait Reference: Aliasing + Sealed {} /// The `Ptr<'a, T>` adheres to the aliasing rules of a `&'a T`.////// The referent of a shared-aliased `Ptr` may be concurrently referenced by any/// number of shared-aliased `Ptr` or `&T` references, or by any number of/// `Ptr<U>` or `&U` references as permitted by `T`'s library safety invariants,/// and may not be concurrently referenced by any exclusively-aliased `Ptr`s or/// `&mut` references. The referent must not be mutated, except via/// [`UnsafeCell`]s, and only when permitted by `T`'s library safety invariants.////// [`UnsafeCell`]: core::cell::UnsafeCellpub enum Shared {}impl Aliasing for Shared {    const IS_EXCLUSIVE: bool = false;}impl Reference for Shared {} /// The `Ptr<'a, T>` adheres to the aliasing rules of a `&'a mut T`.////// The referent of an exclusively-aliased `Ptr` may not be concurrently/// referenced by any other `Ptr`s or references, and may not be accessed (read/// or written) other than via this `Ptr`.pub enum Exclusive {}impl Aliasing for Exclusive {    const IS_EXCLUSIVE: bool = true;}impl Reference for Exclusive {} /// It is unknown whether the pointer is aligned.pub enum Unaligned {} impl Alignment for Unaligned {    #[inline(always)]    fn read<T, I, R>(ptr: crate::Ptr<'_, T, I>) -> T    where        T: Copy + Read<I::Aliasing, R>,        I: Invariants<Alignment = Self, Validity = Valid>,        I::Aliasing: Reference,    {        (*ptr.into_unalign().as_ref()).into_inner()    }} /// The referent is aligned: for `Ptr<T>`, the referent's address is a multiple/// of the `T`'s alignment.pub enum Aligned {}impl Alignment for Aligned {    #[inline(always)]    fn read<T, I, R>(ptr: crate::Ptr<'_, T, I>) -> T    where        T: Copy + Read<I::Aliasing, R>,        I: Invariants<Alignment = Self, Validity = Valid>,        I::Aliasing: Reference,    {        *ptr.as_ref()    }} /// Any bit pattern is allowed in the `Ptr`'s referent, including uninitialized/// bytes.pub enum Uninit {}// SAFETY: `Uninit`'s validity is well-defined for all `T: ?Sized`, and is not a// function of any property of `T` other than its bit validity (in fact, it's// not even a property of `T`'s bit validity, but this is more than we are// required to uphold).unsafe impl Validity for Uninit {    const KIND: ValidityKind = ValidityKind::Uninit;} /// The byte ranges initialized in `T` are also initialized in the referent of a/// `Ptr<T>`.////// Formally: uninitialized bytes may only be present in `Ptr<T>`'s referent/// where they are guaranteed to be present in `T`. This is a dynamic property:/// if, at a particular byte offset, a valid enum discriminant is set, the/// subsequent bytes may only have uninitialized bytes as specified by the/// corresponding enum.////// Formally, given `len = size_of_val_raw(ptr)`, at every byte offset, `b`, in/// the range `[0, len)`:/// - If, in any instance `t: T` of length `len`, the byte at offset `b` in `t`///   is initialized, then the byte at offset `b` within `*ptr` must be///   initialized./// - Let `c` be the contents of the byte range `[0, b)` in `*ptr`. Let `S` be///   the subset of valid instances of `T` of length `len` which contain `c` in///   the offset range `[0, b)`. If, in any instance of `t: T` in `S`, the byte///   at offset `b` in `t` is initialized, then the byte at offset `b` in `*ptr`///   must be initialized.//////   Pragmatically, this means that if `*ptr` is guaranteed to contain an enum///   type at a particular offset, and the enum discriminant stored in `*ptr`///   corresponds to a valid variant of that enum type, then it is guaranteed///   that the appropriate bytes of `*ptr` are initialized as defined by that///   variant's bit validity (although note that the variant may contain another///   enum type, in which case the same rules apply depending on the state of///   its discriminant, and so on recursively).pub enum AsInitialized {}// SAFETY: `AsInitialized`'s validity is well-defined for all `T: ?Sized`, and// is not a function of any property of `T` other than its bit validity.unsafe impl Validity for AsInitialized {    const KIND: ValidityKind = ValidityKind::AsInitialized;} /// The byte ranges in the referent are fully initialized. In other words, if/// the referent is `N` bytes long, then it contains a bit-valid `[u8; N]`.pub enum Initialized {}// SAFETY: `Initialized`'s validity is well-defined for all `T: ?Sized`, and is// not a function of any property of `T` other than its bit validity (in fact,// it's not even a property of `T`'s bit validity, but this is more than we are// required to uphold).unsafe impl Validity for Initialized {    const KIND: ValidityKind = ValidityKind::Initialized;} /// The referent of a `Ptr<T>` is valid for `T`, upholding bit validity and any/// library safety invariants.pub enum Valid {}// SAFETY: `Valid`'s validity is well-defined for all `T: ?Sized`, and is not a// function of any property of `T` other than its bit validity.unsafe impl Validity for Valid {    const KIND: ValidityKind = ValidityKind::Valid;} /// # Safety////// `DT: CastableFrom<ST, SV, DV>` is sound if `SV = DV = Uninit` or `SV = DV =/// Initialized`.pub unsafe trait CastableFrom<ST: ?Sized, SV, DV> {} // SAFETY: `SV = DV = Uninit`.unsafe impl<ST: ?Sized, DT: ?Sized> CastableFrom<ST, Uninit, Uninit> for DT {}// SAFETY: `SV = DV = Initialized`.unsafe impl<ST: ?Sized, DT: ?Sized> CastableFrom<ST, Initialized, Initialized> for DT {} /// [`Ptr`](crate::Ptr) referents that permit unsynchronized read operations.////// `T: Read<A, R>` implies that a pointer to `T` with aliasing `A` permits/// unsynchronized read operations. This can be because `A` is [`Exclusive`] or/// because `T` does not permit interior mutation.////// # Safety////// `T: Read<A, R>` if either of the following conditions holds:/// - `A` is [`Exclusive`]/// - `T` implements [`Immutable`](crate::Immutable)////// As a consequence, if `T: Read<A, R>`, then any `Ptr<T, (A, ...)>` is/// permitted to perform unsynchronized reads from its referent.pub trait Read<A: Aliasing, R> {} impl<A: Aliasing, T: ?Sized + crate::Immutable> Read<A, BecauseImmutable> for T {}impl<T: ?Sized> Read<Exclusive, BecauseExclusive> for T {} /// Unsynchronized reads are permitted because only one live [`Ptr`](crate::Ptr)/// or reference may exist to the referent bytes at a time.#[derive(Copy, Clone, Debug)]pub enum BecauseExclusive {} /// Unsynchronized reads are permitted because no live [`Ptr`](crate::Ptr)s or/// references permit interior mutation.#[derive(Copy, Clone, Debug)]pub enum BecauseImmutable {} use sealed::Sealed;mod sealed {    use super::*;     pub trait Sealed {}     impl Sealed for Shared {}    impl Sealed for Exclusive {}     impl Sealed for Unaligned {}    impl Sealed for Aligned {}     impl Sealed for Uninit {}    impl Sealed for AsInitialized {}    impl Sealed for Initialized {}    impl Sealed for Valid {}     impl<A: Sealed, AA: Sealed, V: Sealed> Sealed for (A, AA, V) {}     impl Sealed for BecauseImmutable {}    impl Sealed for BecauseExclusive {}}