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//! Crate providing [`Zc`] for self-referential zero-copy structures. #![no_std] #![forbid( clippy::pedantic, rust_2018_idioms, anonymous_parameters, unused_qualifications, missing_docs, trivial_casts, trivial_numeric_casts, unstable_features, unused_extern_crates, unused_import_braces, unused_results, warnings )] #[cfg(feature = "std")] extern crate std; #[cfg(feature = "alloc")] extern crate alloc; mod r#impl; // FIXME: Remove the need for macros. mod macros; mod private; use core::fmt::{self, Debug, Display}; use core::ops::Deref; #[cfg(feature = "alloc")] pub use aliasable; #[cfg(feature = "derive")] pub use zc_derive::Dependant; use self::private::{Construct, TryConstruct}; /// Zero-copy structure consisting of an [`Owner`] and a [`Dependant`]. pub struct Zc<O: Owner, D> { // SAFETY: Order of fields is important for preventing dropping the storage // before the value that references it. value: D, storage: O::Storage, } impl<O, D> Zc<O, D> where O: Owner, D: Dependant<'static>, { /// Construct a new zero-copied structure given an [`Owner`] and a /// function for constructing the [`Dependant`]. /// /// # Example /// ``` /// use zc::{Zc, Dependant}; /// /// #[derive(Dependant)] /// struct MyStruct<'a>(&'a [u8]); /// /// impl<'a> From<&'a [u8]> for MyStruct<'a> { /// fn from(bytes: &'a [u8]) -> Self { /// Self(&bytes[1..]) /// } /// } /// /// let owner = vec![1, 2, 3]; /// let _ = zc::from!(owner, MyStruct, [u8]); /// ``` pub fn new<C>(owner: O, constructor: C) -> Self where C: for<'o> Construct<'o, <O::Storage as Deref>::Target, Dependant = D>, { let storage = Owner::into_storage(owner); // Create a temporary dependant given the target reference. let value = unsafe { constructor.construct(storage.deref()) }; // Construct the zero-copy structure given the raw parts. Self { storage, value } } /// Try construct a new zero-copied structure given an [`Owner`] and a /// function for constructing the [`Dependant`]. /// /// # Example /// ``` /// use zc::{Zc, Dependant}; /// use core::convert::TryFrom; /// /// #[derive(Dependant)] /// struct MyStruct<'a>(&'a [u8]); /// /// impl<'a> TryFrom<&'a [u8]> for MyStruct<'a> { /// type Error = (); /// /// fn try_from(bytes: &'a [u8]) -> Result<Self, Self::Error> { /// Ok(Self(&bytes[1..])) /// } /// } /// /// let owner = vec![1, 2, 3]; /// let _ = zc::try_from!(owner, MyStruct, [u8]); /// ``` /// /// # Errors /// Returns `E` if the constructor failed. pub fn try_new<C, E>(owner: O, constructor: C) -> Result<Self, (E, O)> where E: 'static, C: for<'o> TryConstruct<'o, <O::Storage as Deref>::Target, Error = E, Dependant = D>, { let storage = Owner::into_storage(owner); // Try create a temporary dependant given the target reference. match unsafe { constructor.try_construct(storage.deref()) } { Ok(value) => Ok(Self { storage, value }), Err(err) => Err((err, Owner::from_storage(storage))), } } /// Return a reference to the [`Dependant`]. /// /// The dependant type `T` must be supplied (eg. /// `Self::dependant::<MyStruct>(&self)`). /// /// # Example /// ``` /// use zc::{Zc, Dependant}; /// /// #[derive(Debug, PartialEq, Dependant)] /// struct MyStruct<'a>(&'a [u8]); /// /// impl<'a> From<&'a [u8]> for MyStruct<'a> { /// fn from(bytes: &'a [u8]) -> Self { /// Self(&bytes[1..]) /// } /// } /// /// let owner = vec![1, 2, 3]; /// let data = zc::from!(owner, MyStruct, [u8]); /// /// assert_eq!( /// data.get::<MyStruct>(), /// &MyStruct(&[2, 3]) /// ); /// ``` // FIXME: This interface isn't the nicest as you have to specific the // dependant again to retrieve it. GATs should provide us a way to make this // nicer with a generic associated lifetime. // See: https://github.com/rust-lang/rust/issues/44265 pub fn get<'a, T>(&'a self) -> &T where T: Dependant<'a, Static = D>, { let value_ptr: *const D = &self.value; unsafe { &*value_ptr.cast::<T>() } } } impl<O, D> Zc<O, D> where O: Owner, { /// Return a reference to the data [`Owner`] provides. /// /// # Example /// ``` /// use zc::{Zc, Dependant}; /// /// #[derive(Debug, PartialEq, Dependant)] /// struct MyStruct<'a>(&'a [u8]); /// /// impl<'a> From<&'a [u8]> for MyStruct<'a> { /// fn from(bytes: &'a [u8]) -> Self { /// Self(&bytes[1..]) /// } /// } /// /// let owner = vec![1, 2, 3]; /// let data = zc::from!(owner, MyStruct, [u8]); /// /// assert_eq!(data.as_owned(), &[1, 2, 3]); /// ``` pub fn as_owned(&self) -> &<O::Storage as Deref>::Target { &*self.storage } /// Consumes `self` into the [`Owner`]. /// /// # Example /// ``` /// use zc::{Zc, Dependant}; /// /// #[derive(Debug, PartialEq, Dependant)] /// struct MyStruct<'a>(&'a [u8]); /// /// impl<'a> From<&'a [u8]> for MyStruct<'a> { /// fn from(bytes: &'a [u8]) -> Self { /// Self(&bytes[1..]) /// } /// } /// /// let owner = vec![1, 2, 3]; /// let data = zc::from!(owner, MyStruct, [u8]); /// /// assert_eq!(data.into_owner(), vec![1, 2, 3]); /// ``` pub fn into_owner(self) -> O { Owner::from_storage(self.storage) } /// Map the stored [`Dependant`] to another. /// /// # Safety /// /// The [`Dependant`] passed to the function has its lifetime erased to /// `'static` and must be handled appropriately. Nothing within the /// [`Dependant`] passed can be referenced from outside of the closure. #[inline] pub unsafe fn map_unchecked<F, U>(self, f: F) -> Zc<O, U> where F: FnOnce(D) -> U, { let Self { value, storage } = self; let value = f(value); Zc { value, storage } } /// Try to map the stored [`Dependant`] to another. /// /// # Errors /// /// Returns any error the provided function returns. /// /// # Safety /// /// The [`Dependant`] passed to the function has its lifetime erased to /// `'static` and must be handled appropriately. Nothing within the /// [`Dependant`] passed can be referenced from outside of the closure, /// this includes the error returned. #[inline] pub unsafe fn try_map_unchecked<F, U, E>(self, f: F) -> Result<Zc<O, U>, E> where F: FnOnce(D) -> Result<U, E>, { let Self { value, storage } = self; f(value).map(|value| Zc { value, storage }) } } impl<O, T> Zc<O, Option<T>> where O: Owner, { /// Decomposes `self` into an option. #[inline] pub fn into_option(self) -> Option<Zc<O, T>> { self.into() } } impl<O, Ok, Err> Zc<O, Result<Ok, Err>> where O: Owner, { /// Decomposes `self` into a result. /// /// # Errors /// /// Returns `Zc<O, Err>` if the inner dependant is `Result::Err`. #[inline] pub fn into_result(self) -> Result<Zc<O, Ok>, Zc<O, Err>> { self.into() } } impl<O, T> From<Zc<O, Option<T>>> for Option<Zc<O, T>> where O: Owner, { #[inline] fn from(zc: Zc<O, Option<T>>) -> Self { match zc.value { None => None, Some(value) => Some(Zc { value, storage: zc.storage, }), } } } impl<O, Ok, Err> From<Zc<O, Result<Ok, Err>>> for Result<Zc<O, Ok>, Zc<O, Err>> where O: Owner, { #[inline] fn from(zc: Zc<O, Result<Ok, Err>>) -> Self { match zc.value { Ok(value) => Ok(Zc { value, storage: zc.storage, }), Err(value) => Err(Zc { value, storage: zc.storage, }), } } } impl<O, D> Display for Zc<O, D> where O: Owner, D: Display, { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { Display::fmt(&self.value, f) } } impl<O, D> Debug for Zc<O, D> where O: Owner, O::Storage: Debug, D: Debug, { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.debug_struct("Zc") .field("storage", &self.storage) .field("value", &self.value) .finish() } } /// Implemented for types that use data provided by an [`Owner`] and guarantee /// that internal state is protected. /// /// # Derive implementations (recommended) /// /// It is recommended not to implement this manually and instead use the /// provided proc-macro as show below. /// /// ``` /// use zc::Dependant; /// /// #[derive(Dependant)] /// pub struct MyStruct<'a> { /// value: &'a str, /// } /// ``` /// /// # Derive implementations for `Copy` /// /// If a type implements [`Copy`] it cannot support interior mutability and /// therefore is a valid `Dependant` type. /// /// To use a [`Copy`] type without having to implement `Dependant` you can tell /// the derive implementation to check based on a [`Copy`] bound for a specific /// field or all fields. /// /// ``` /// use zc::Dependant; /// /// #[derive(Copy, Clone)] /// pub struct CopyType; /// /// #[derive(Dependant)] /// pub struct StructWithCopy<'a> { /// // This field has a `Copy` bound. /// #[zc(check = "Copy")] /// field_a: &'a CopyType, /// // This field has the standard `Dependant` bound. /// field_b: u8, /// } /// /// // All fields in this struct have the `Copy` bound. /// #[derive(Dependant)] /// #[zc(check = "Copy")] /// pub struct StructWithAllCopy<'a> { /// field_a: &'a CopyType, /// field_b: u8, /// } /// ``` /// /// # Manual implementations /// /// If you wish not to use the provided proc-macro you implement as shown: /// /// ``` /// struct MyStruct<'a>(&'a [u8]); /// /// unsafe impl<'o> zc::Dependant<'o> for MyStruct<'o> { /// type Static = MyStruct<'static>; /// } /// ``` /// /// # Safety /// /// Implementer must guarantee: /// /// - the structure only requires a single lifetime. /// - `Self::Static` must be the same type but with a `'static` lifetime. /// /// And in addition the structure: /// /// - has no interior mutability. /// /// **OR** /// /// - can safely be stored with it's lifetime erased (ie. as `'static`). /// - does not provided an interface that will accept data with non-`'static` /// lifetime though a interior mutable interface. /// /// # Interior Mutability /// /// Types that provide interior mutability include both `!Sync` types (eg. /// [`RefCell<T>`]) and `Sync` types (eg. [`Mutex<T>`]). /// /// See the [Rust Language Book] on interior mutability. /// /// [`Mutex<T>`]: std::sync::Mutex /// [`RefCell<T>`]: std::cell::RefCell /// [Rust Language Book]: https://doc.rust-lang.org/book/ch15-05-interior-mutability.html pub unsafe trait Dependant<'o>: Sized + 'o { /// Always the exact same structure as `Self` but instead with a `'static` /// lifetime. type Static: Dependant<'static>; } /// Represents the owner of data with an associated storage type. /// /// An `Owner` is a convenience trait that can be implemented without the need /// of `unsafe` that returns a [`Storage`] that does require an `unsafe` /// implementation. See the notes on [`Storage`] to see why this it is required. pub trait Owner: Sized + 'static { /// The [`Storage`] type the owner uses. type Storage: Storage; /// Consumes the `Owner` into the associated [`Storage`] type. fn into_storage(self) -> Self::Storage; /// Consumes the associated [`Storage`] into the `Owner` type. fn from_storage(storage: Self::Storage) -> Self; } impl<T> Owner for T where T: Storage, { type Storage = T; fn into_storage(self) -> Self::Storage { self } fn from_storage(storage: Self::Storage) -> Self { storage } } /// Implemented for types that can safely provide a stable, aliasable reference /// to data they own. /// /// # `noalias` /// /// The pointers behind common allocation types (`Box<T>`, `Vec<T>`, etc), are /// stored via `core::ptr::Unique<T>`, which passes to the compilier a `noalias` /// attribute. This attribute allows the compiler to make optimisations with the /// guarantee that no other pointers are referencing the same data. /// /// We want to both own the data and provide a reference to it, outside of /// Rust's normal lifetime guarantees, which can break with some of the /// optimisations the compiler can make. To achieve this, we need to remove the /// `noalias` attribute of the underlying pointer to let the compiler know that /// there will exist multiple pointers referencing the same owned data, which is /// also known as aliasing. /// /// # Safety /// /// The implementer must guarantee that the reference it provides via [`Deref`] /// will be **both stable and aliasable** for the lifetime of `self`. Stable in /// this context meaning that the pointer to the data referenced will not /// change. /// /// `Box<T>` provides a stable pointer (the location of the data being pointed /// to will not change) but is not aliasable (see `noalias` above). Instead we /// can use the basic wrapper types provided by the [`aliasable`] crate. pub unsafe trait Storage: Sized + Deref + 'static {}