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//! The `stackpin` crate exposes a [`StackPinned`] type that allows to represent [`!Unpin`] data that should be [pinned](https://doc.rust-lang.org/std/pin/index.html) to the stack //! at the point of declaration. //! The crate exposes a trait, [`FromUnpinned`], as well as a [`stack_let`] macro that makes safely creating [`StackPinned`] instances easier. //! The crate also exposes the [`PinStack`] type alias for `Pin<StackPinned<T>>`. //! //! This crate was inspired from the [pin-utils] crate, the main differences being: //! * [pin-utils] provides a macro to return a `Pin<&mut T>` instance, //! with a "mutable reference" semantics that includes reborrow. The `stackpin` crate promotes a //! "root handle" semantics that guarantees that a function consuming a [`PinStack<T>`] consumes //! the *only* handle to `T`, and not a reborrowed reference. //! * The syntax for the `stack_let!(mut id : ty = expr)` macro attempts to mimic a regular `let mut id : ty = expr` statement. //! * The provided [`FromUnpinned`] trait and [`Unpinned`] struct aim at separating unmovable types //! from the data that can be used to construct them. `stackpin` aims at promoting a model where //! all unmovable types are only accessible once pinned. //! * The [`StackPinned<T>`] type expresses strong guarantee about the fact that the destructor for //! `T` will be run. //! * The `stackpin` crate solely focuses on stack pinning. The [pin-utils] crate also provides //! other utilities such as pin projection. //! //! # Stack pinnable types //! //! A type T that wants to benefit from the guarantees provided by [`StackPinned`] should be //! [`!Unpin`]. This is necessary to enforce the "drop will be run" guarantee. //! //! Additionally, the `stackpin` crate promotes an idiom where "unmovable" types are strictly //! separated from movable types, and are preferably only accessible through `PinStack`. //! //! For example, let's consider the following `Unmovable` struct (from the [documentation for the //! `pin` module](https://doc.rust-lang.org/std/pin/index.html)): //! ``` //! use std::marker::PhantomPinned; //! use std::ptr::NonNull; //! struct Unmovable { //! // Owned data //! s: String, //! // Self referential pointer meant to point to `s` //! slice: NonNull<String>, //! // Obligatory marker that makes this struct `!Unpin`. //! // Without this, implementing `FromUnpinned` for `Unmovable` would not be safe. //! _pinned: PhantomPinned, //! } //! ``` //! //! It is important to note that this struct is **not** unmovable by itself, as there are no such types in Rust. //! Instead, we are going to enforce this through privacy: since the fields of the struct are private, no instance can be created //! from outside the module. //! Similarly, no public "constructor" function `pub fn new() -> Unmovable` should be provided. //! //! So, how will clients consume `Unmovable` instances? //! //! The recommended solution using `stackpin` is to implement `FromUnpinned<Data>` for `Unmovable`, where `Data` is the //! type that would normally serve as parameters in a "constructor" function. //! ``` //! # use std::marker::PhantomPinned; //! # use std::ptr::NonNull; //! # struct Unmovable { //! # s: String, //! # slice: NonNull<String>, //! # _pinned: PhantomPinned, //! # } //! use stackpin::FromUnpinned; //! // An `Unmovable` can be created from a `String` //! unsafe impl FromUnpinned<String> for Unmovable { //! // This associated type can be used to retain information between the creation of the instance and its pinning. //! // This allows for some sort of "two-steps initialization" without having to store the initialization part in the //! // type itself. //! // Here, we don't need it, so we just set it to `()`. //! type PinData = (); //! //! // Simply builds the Unmovable from the String. //! // The implementation of this function is not allowed to consider that the type won't ever move **yet**. //! // (in particular, the `Self` instance is returned by this function) //! // Note, however, that safe users of FromUnpinned will: //! // * Not do anything to with the returned `Self` instance between the call to //! // `from_unpinned` and the call to `on_pin`. //! // * Not panic between calling the two functions //! // * Always call the second function if the first has been called. //! unsafe fn from_unpinned(s: String) -> (Self, ()) { //! ( //! Self { //! s, //! // We will "fix" this dangling pointer once the data will be pinned //! // and guaranteed not to move anymore. //! slice: NonNull::dangling(), //! _pinned: PhantomPinned, //! }, //! (), //! ) //! } //! //! // Performs a second initialization step on an instance that is already guaranteed to never move again. //! // This allows to e.g. set self borrow with the guarantee that they will remain valid. //! unsafe fn on_pin(&mut self, _data: ()) { //! // Data will never move again, set the pointer to our own internal String whose address //! // will never change anymore //! self.slice = NonNull::from(&self.s); //! } //! } //! ``` //! With `FromUnpinned<Data>` implemented for `T`, one can now add a "constructor method" that would return an //! `Unpinned<Data, T>`. The `Unpinned<U, T>` struct is a simple helper struct around `U` that maintains the destination //! type `T`. This is used by the [`stack_let`] macro to infer the type of `T` that the user may want to produce. //! //! ``` //! # use std::marker::PhantomPinned; //! # use std::ptr::NonNull; //! # struct Unmovable { //! # s: String, //! # slice: NonNull<String>, //! # _pinned: PhantomPinned, //! # } //! # use stackpin::Unpinned; //! # use stackpin::FromUnpinned; //! # unsafe impl FromUnpinned<String> for Unmovable { //! # type PinData = (); //! # unsafe fn from_unpinned(s: String) -> (Self, ()) { //! # ( //! # Self { //! # s, //! # slice: NonNull::dangling(), //! # _pinned: PhantomPinned, //! # }, //! # (), //! # ) //! # } //! # unsafe fn on_pin(&mut self, _data: ()) { //! # self.slice = NonNull::from(&self.s); //! # } //! # } //! impl Unmovable { //! fn new_unpinned<T: Into<String>>(s: T) -> Unpinned<String, Unmovable> { //! Unpinned::new(s.into()) //! } //! } //! ``` //! //! Then, a user of the `Unmovable` struct can simply build an instance by using the [`stack_let`] macro: //! ``` //! # use std::marker::PhantomPinned; //! # use std::ptr::NonNull; //! # struct Unmovable { //! # s: String, //! # slice: NonNull<String>, //! # _pinned: PhantomPinned, //! # } //! # use stackpin::Unpinned; //! # use stackpin::FromUnpinned; //! # unsafe impl FromUnpinned<String> for Unmovable { //! # type PinData = (); //! # unsafe fn from_unpinned(s: String) -> (Self, ()) { //! # ( //! # Self { //! # s, //! # slice: NonNull::dangling(), //! # _pinned: PhantomPinned, //! # }, //! # (), //! # ) //! # } //! # unsafe fn on_pin(&mut self, _data: ()) { //! # self.slice = NonNull::from(&self.s); //! # } //! # } //! # impl Unmovable { //! # fn new_unpinned<T: Into<String>>(s: T) -> Unpinned<String, Unmovable> { //! # Unpinned::new(s.into()) //! # } //! # } //! use stackpin::stack_let; //! // ... //! stack_let!(unmovable = Unmovable::new_unpinned("Intel the Beagle")); // this creates the unmovable instance on the stack and binds `unmovable` with a `PinStack<Unmovable>` //! // ... //! ``` //! //! [pin-utils]: https://docs.rs/pin-utils //! [`StackPinned`]: struct.StackPinned.html //! [`StackPinned<T>`]: struct.StackPinned.html //! [`FromUnpinned`]: trait.FromUnpinned.html //! [`stack_let`]: macro.stack_let.html //! [`PinStack`]: type.PinStack.html //! [`PinStack<T>`]: type.PinStack.html //! [`Unpinned`]: struct.Unpinned.html //! [`!Unpin`]: https://doc.rust-lang.org/std/pin/index.html#unpin use std::marker::PhantomData; use std::ops::Deref; use std::ops::DerefMut; use std::pin::Pin; /// Struct that represents data that is pinned to the stack, at the point of declaration. /// /// Because this property cannot be guaranteed by safe rust, constructing an instance of a /// [`StackPinned`] directly is `unsafe`. /// Rather, one should use the [`stack_let`] macro that returns a [`PinStack`] instance. /// /// In particular, one should note the following about [`StackPinned`] instance: /// * It is impossible to safely pass a [`StackPinned`] instance to a function /// * It is impossible to safely return a [`StackPinned`] instance from a function /// * It is impossible to safely store a [`StackPinned`] instance inside of a struct /// /// Instead, one should replace [`StackPinned<T>`] with [`PinStack<T>`] in each of these situations. /// /// A [`PinStack<T>`] instance does have its benefits: /// * The underlying `T` instance is guaranteed to never move for `T: !Unpin` once pinned. /// This is useful for `T` types whose instances should never move. /// * For `T: !Unpin`, the destructor of `T` is guaranteed to run when the T leaves the stack frame it was allocated on, /// even if one uses [`std::mem::forget`](https://doc.rust-lang.org/std/mem/fn.forget.html) on /// the [`PinStack<T>`] instance. /// /// [`StackPinned`]: struct.StackPinned.html /// [`StackPinned<T>`]: struct.StackPinned.html /// [`PinStack`]: type.PinStack.html /// [`PinStack<T>`]: type.PinStack.html /// [`stack_let`]: macro.stack_let.html #[repr(transparent)] pub struct StackPinned<'pin, T>(&'pin mut T); impl<'pin, T> StackPinned<'pin, T> { /// # Safety /// Currently the only way to build a safe [`StackPinned<T>`] instance is to use the /// [`stack_let`] macro that will return a [`PinStack<T>`] instance. /// /// [`StackPinned<T>`]: struct.StackPinned.html /// [`PinStack<T>`]: type.PinStack.html /// [`stack_let`]: macro.stack_let.html #[inline(always)] pub unsafe fn new(t: &'pin mut T) -> Self { Self(t) } } impl<'pin, T> Deref for StackPinned<'pin, T> { type Target = T; fn deref(&self) -> &Self::Target { &self.0 } } impl<'pin, T> DerefMut for StackPinned<'pin, T> { fn deref_mut(&mut self) -> &mut <Self as Deref>::Target { &mut self.0 } } /// Trait to build [`StackPinned`] values from unpinned types. /// /// Implementers of `FromUnpinned<Source>` indicate that they can be built from a `Source` instance, /// to the condition that they will be pinned afterwards. /// /// # Safety /// /// This trait both exposes unsafe functions **and** is unsafe to implement. /// * Unsafe functions are exposed because the functions have the preconditions of having to be /// called from the [`stack_let`] macro. /// * The trait itself is unsafe to implement because implementers must provide implementations of /// the functions that must uphold invariants that cannot be checked by the compiler. See the /// documentation of each function for information on the invariants. /// /// [`stack_let`]: macro.stack_let.html /// [`StackPinned`]: struct.StackPinned.html pub unsafe trait FromUnpinned<Source> where Self: Sized, { /// This associated type can be used to retain information between the creation of the instance and its pinning. /// This allows for some sort of "two-steps initialization" without having to store the initialization part in the /// type itself. type PinData; /// Performs a first initialization step, resulting in the creation of the `Self` instance. /// /// # Safety /// /// * This function is used by the construction macro, it is never safe to call directly. /// * Implementers of this function are **not** allowed to consider that the type won't ever move **yet**. /// (in particular, the `Self` instance is returned by this function). The type should be /// movable at this point. unsafe fn from_unpinned(src: Source) -> (Self, Self::PinData); /// Performs a second initialization step, resulting in the pinning of the `Self` instance. /// /// # Safety /// /// * This function is used by the construction macro, it is never safe to call directly. /// * Implementers of this function **are** allowed to consider that the type won't move ever again. /// You can for instance set autoborrows safely in this function. /// * For convenience, a naked mutable borrow is directly given. /// Implementers of this function are **not** allowed to move out of this mutable borrow. unsafe fn on_pin(&mut self, pin_data: Self::PinData); } /// A helper struct around `U` that remembers the `T` destination type. /// /// This struct is typically used to build [`PinStack`] values using the [`stack_let`] macro /// without having to specify the destination type. /// /// # Example /// /// ``` /// # use std::marker::PhantomPinned; /// # use std::ptr::NonNull; /// # struct Unmovable { /// # s: String, /// # slice: NonNull<String>, /// # _pinned: PhantomPinned, /// # } /// # use stackpin::Unpinned; /// # use stackpin::FromUnpinned; /// # unsafe impl FromUnpinned<String> for Unmovable { /// # type PinData = (); /// # unsafe fn from_unpinned(s: String) -> (Self, ()) { /// # ( /// # Self { /// # s, /// # slice: NonNull::dangling(), /// # _pinned: PhantomPinned, /// # }, /// # (), /// # ) /// # } /// # unsafe fn on_pin(&mut self, _data: ()) { /// # self.slice = NonNull::from(&self.s); /// # } /// # } /// use stackpin::stack_let; /// // Without `Unpinned` /// fn new_string(s : impl Into<String>) -> String { s.into() } /// stack_let!(unmovable : Unmovable = new_string("toto")); /// // With `Unpinned` /// fn new_unpinned(s : impl Into<String>) -> Unpinned<String, Unmovable> { Unpinned::new(s.into()) } /// stack_let!(unmovable = new_unpinned("toto")); /// ``` /// /// [`stack_let`]: macro.stack_let.html /// [`PinStack`]: type.PinStack.html pub struct Unpinned<U, T: FromUnpinned<U>> { u: U, t: std::marker::PhantomData<T>, } unsafe impl<U, T: FromUnpinned<U>> FromUnpinned<Unpinned<U, T>> for T { type PinData = <T as FromUnpinned<U>>::PinData; unsafe fn from_unpinned(src: Unpinned<U, T>) -> (Self, Self::PinData) { <T as FromUnpinned<U>>::from_unpinned(src.u) } unsafe fn on_pin(&mut self, pin_data: Self::PinData) { <T as FromUnpinned<U>>::on_pin(self, pin_data) } } impl<U, T: FromUnpinned<U>> Unpinned<U, T> { pub fn new(u: U) -> Self { Self { u, t: PhantomData } } } #[doc(hidden)] #[macro_export] macro_rules! internal_pin_stack { ($id:ident) => { // Shadow the original binding so that it can't directly be accessed ever again. let $id: $crate::PinStack<_> = unsafe { let $id = $crate::StackPinned::new(&mut $id); std::pin::Pin::new_unchecked($id) }; }; (mut $id:ident) => { // Shadow the original binding so that it can't directly be accessed ever again. let mut $id: $crate::PinStack<_> = unsafe { let $id = $crate::StackPinned::new(&mut $id); std::pin::Pin::new_unchecked($id) }; }; } #[doc(hidden)] pub unsafe fn write_pinned<Source, Dest>(source: Source, pdest: *mut Dest) where Dest: FromUnpinned<Source>, { let (dest, data) = FromUnpinned::<Source>::from_unpinned(source); std::ptr::write(pdest, dest); FromUnpinned::<Source>::on_pin(&mut *pdest, data); } #[doc(hidden)] pub unsafe fn from_unpinned<Source, Dest>( source: Unpinned<Source, Dest>, ) -> (Dest, Dest::PinData, PhantomData<Unpinned<Source, Dest>>) where Dest: FromUnpinned<Source>, { let (dest, data) = FromUnpinned::from_unpinned(source); (dest, data, PhantomData) } #[doc(hidden)] pub unsafe fn from_source<Dest, Source>( source: Source, ) -> (Dest, Dest::PinData, PhantomData<Source>) where Dest: FromUnpinned<Source>, { let (dest, data) = FromUnpinned::from_unpinned(source); (dest, data, PhantomData) } #[doc(hidden)] pub unsafe fn on_pin<Source, Dest>( pdest: *mut Dest, data: Dest::PinData, _source: PhantomData<Source>, ) where Dest: FromUnpinned<Source>, { FromUnpinned::<Source>::on_pin(&mut *pdest, data); } /// `stack_let!(id = expr)` binds a [`PinStack<T>`] to `id` if `expr` is an expression of type `U` where [`T: FromUnpinned<U>`]. /// /// If `expr` is of type [`Unpinned<U, T>`] for some `U`, then no type annotation is necessary. /// If `expr` is of type `U` where [`T: FromUnpinned<U>`], use `stack_let!(id : T = expr)`. /// /// To bind `id` mutably, use `stack_let!(mut id = expr)`. /// /// [`PinStack<T>`]: type.PinStack.html /// [`T: FromUnpinned<U>`]: trait.FromUnpinned.html /// [`Unpinned<U, T>`]: struct.Unpinned.html #[macro_export] macro_rules! stack_let { ($id: ident = $expr: expr) => { let (mut $id, _stack_data, _stack_phantom) = unsafe { $crate::from_unpinned($expr) }; unsafe { $crate::on_pin(&mut $id as *mut _, _stack_data, _stack_phantom) } $crate::internal_pin_stack!($id); }; (mut $id: ident = $expr: expr) => { let (mut $id, _stack_data, _stack_phantom) = unsafe { $crate::from_unpinned($expr) }; unsafe { $crate::on_pin(&mut $id as *mut _, _stack_data, _stack_phantom) } $crate::internal_pin_stack!(mut $id); }; ($id: ident : $type:ty = $expr: expr) => { let (mut $id, _stack_data, _stack_phantom) = unsafe { $crate::from_source::<$type, _>($expr) }; unsafe { $crate::on_pin(&mut $id as *mut _, _stack_data, _stack_phantom) } $crate::internal_pin_stack!($id); }; (mut $id: ident : $type:ty = $expr: expr) => { let (mut $id, _stack_data, _stack_phantom) = unsafe { $crate::from_source::<$type, _>($expr) }; unsafe { $crate::on_pin(&mut $id as *mut _, _stack_data, _stack_phantom) } $crate::internal_pin_stack!(mut $id); }; } /// Short-hand for `Pin<StackPinned<T>>` pub type PinStack<'a, T> = Pin<StackPinned<'a, T>>; #[cfg(test)] mod tests { use super::FromUnpinned; use super::PinStack; use super::Unpinned; use std::marker::PhantomPinned; use std::ptr::NonNull; struct Unmovable { data: String, slice: NonNull<String>, _pin: PhantomPinned, } impl Unmovable { fn slice(&self) -> &str { unsafe { self.slice.as_ref() } } fn slice_mut<'a>(this: &'a mut PinStack<Unmovable>) -> &'a mut str { unsafe { this.as_mut().get_unchecked_mut().slice.as_mut() } } } impl Unmovable { fn new_unpinned(src: String) -> Unpinned<String, Unmovable> { Unpinned::new(src) } } unsafe impl FromUnpinned<String> for Unmovable { type PinData = (); unsafe fn from_unpinned(src: String) -> (Self, Self::PinData) { ( Self { data: src, slice: NonNull::dangling(), _pin: PhantomPinned, }, (), ) } unsafe fn on_pin(&mut self, _pin_data: Self::PinData) { self.slice = NonNull::from(&self.data); } } #[test] fn let_stack_unmovable() { let test_str = "Intel the Beagle is the greatest dog in existence"; stack_let!(mut unmovable = Unmovable::new_unpinned(String::from(test_str))); let slice = Unmovable::slice_mut(&mut unmovable); slice.make_ascii_uppercase(); assert_eq!(test_str.to_ascii_uppercase(), Unmovable::slice(&unmovable)); } }