lib.rs

//! A crate for safe and ergonomic pin-projection.
//!
//! This crate provides the following attribute macros:
//!
//! * [`pin_project`] - An attribute that creates a projection struct covering all the fields.
//! * [`pinned_drop`] - An attribute for annotating a function that implements `Drop`.
//! * [`project`] - An attribute to support pattern matching.
//!
//! ## Examples
//!
//! [`pin_project`] attribute creates a projection struct covering all the fields.
//!
//! ```rust
//! use pin_project::pin_project;
//! use std::pin::Pin;
//!
//! #[pin_project]
//! struct Struct<T, U> {
//!     #[pin]
//!     pinned: T,
//!     unpinned: U,
//! }
//!
//! impl<T, U> Struct<T, U> {
//!     fn foo(mut self: Pin<&mut Self>) {
//!         let this = self.project();
//!         let _: Pin<&mut T> = this.pinned; // Pinned reference to the field
//!         let _: &mut U = this.unpinned; // Normal reference to the field
//!     }
//! }
//! ```
//!
//! [Code like this will be generated](https://github.com/taiki-e/pin-project/blob/master/examples/struct-default-expanded.rs)
//!
//! There are examples and generated code of each feature in [examples](https://github.com/taiki-e/pin-project/blob/master/examples/README.md) directory.
//!
//! [`pin_project`]: https://docs.rs/pin-project-internal/0.4.0-alpha.11/pin_project_internal/attr.pin_project.html
//! [`pinned_drop`]: https://docs.rs/pin-project-internal/0.4.0-alpha.11/pin_project_internal/attr.pinned_drop.html
//! [`project`]: https://docs.rs/pin-project-internal/0.4.0-alpha.11/pin_project_internal/attr.project.html

#![no_std]
#![recursion_limit = "256"]
#![doc(html_root_url = "https://docs.rs/pin-project/0.4.0-alpha.11")]
#![doc(test(
    no_crate_inject,
    attr(deny(warnings, rust_2018_idioms, single_use_lifetimes), allow(dead_code))
))]
#![warn(missing_docs)]
#![warn(unsafe_code)]
#![warn(rust_2018_idioms, unreachable_pub, single_use_lifetimes)]
#![warn(clippy::all, clippy::pedantic)]
#![allow(clippy::use_self)]

#[doc(hidden)]
pub use pin_project_internal::pin_project;

#[doc(hidden)]
pub use pin_project_internal::pinned_drop;

#[cfg(feature = "project_attr")]
#[doc(hidden)]
pub use pin_project_internal::project;

/// A trait used for custom implementations of [`Unpin`].
/// This trait is used in conjunction with the `UnsafeUnpin`
/// argument to [`pin_project`]
///
/// The Rust [`Unpin`] trait is safe to implement - by itself,
/// implementing it cannot lead to undefined behavior. Undefined
/// behavior can only occur when other unsafe code is used.
///
/// It turns out that using pin projections, which requires unsafe code,
/// imposes additional requirements on an [`Unpin`] impl. Normally, all of this
/// unsafety is contained within this crate, ensuring that it's impossible for
/// you to violate any of the guarnatees required by pin projection.
///
/// However, things change if you want to provide a custom [`Unpin`] impl
/// for your `#[pin_project]` type. As stated in [the Rust
/// documentation](https://doc.rust-lang.org/beta/std/pin/index.html#projections-and-structural-pinning),
/// you must be sure to only implement [`Unpin`] when all of your `#[pin]` fields (i.e. struturally
/// pinend fields) are also [`Unpin`].
///
/// To help highlight this unsafety, the `UnsafeUnpin` trait is provided.
/// Implementing this trait is logically equivalent to implemnting [`Unpin`] -
/// this crate will generate an [`Unpin`] impl for your type that 'forwards' to
/// your `UnsafeUnpin` impl. However, this trait is `unsafe` - since your type
/// uses structural pinning (otherwise, you wouldn't be using this crate!),
/// you must be sure that your `UnsafeUnpin` impls follows all of
/// the requirements for an [`Unpin`] impl of a structurally-pinned type.
///
/// Note that if you specify `#[pin_project(UnsafeUnpin)]`, but do *not*
/// provide an impl of `UnsafeUnpin`, your type will never implement [`Unpin`].
/// This is effectly the same thing as adding a [`PhantomPinned`] to your type
///
/// Since this trait is `unsafe`, impls of it will be detected by the `unsafe_code` lint,
/// and by tools like `cargo geiger`.
///
/// ## Examples
///
/// An `UnsafeUnpin` impl which, in addition to requiring that structually pinned
/// fields be [`Unpin`], imposes an additional requirement:
///
/// ```rust
/// use pin_project::{pin_project, UnsafeUnpin};
///
/// #[pin_project(UnsafeUnpin)]
/// struct Foo<K, V> {
///     #[pin]
///     field_1: K,
///     field_2: V
/// }
///
/// unsafe impl<K, V> UnsafeUnpin for Foo<K, V> where K: Unpin + Clone {}
/// ```
///
/// [`PhantomPinned`]: core::marker::PhantomPinned
/// [`pin_project`]: https://docs.rs/pin-project-internal/0.4.0-alpha.11/pin_project_internal/attr.pin_project.html
#[allow(unsafe_code)]
pub unsafe trait UnsafeUnpin {}

// Not public API.
#[doc(hidden)]
pub mod __private {
    use super::UnsafeUnpin;
    use core::pin::Pin;

    #[doc(hidden)]
    pub use pin_project_internal::__PinProjectAutoImplUnpin;

    // This is an internal helper trait used by `pin-project-internal`.
    // This allows us to force an error if the user tries to provide
    // a regular `Drop` impl when they specify the `PinnedDrop` argument.
    //
    // Users can implement `Drop` safely using `#[pinned_drop]`.
    // **Do not call or implement this trait directly.**
    #[allow(unsafe_code)]
    #[doc(hidden)]
    pub unsafe trait UnsafePinnedDrop {
        // Since calling it twice on the same object would be UB,
        // this method is unsafe.
        #[doc(hidden)]
        unsafe fn drop(self: Pin<&mut Self>);
    }

    // This is an internal helper struct used by `pin-project-internal`.
    // This allows us to force an error if the user tries to provide
    // a regular `Unpin` impl when they specify the `UnsafeUnpin` argument.
    // This is why we need Wrapper:
    //
    // Supposed we have the following code:
    //
    // #[pin_project(UnsafeUnpin)]
    // struct MyStruct<T> {
    //     #[pin] field: T
    // }
    //
    // impl<T> Unpin for MyStruct<T> where MyStruct<T>: UnsafeUnpin {} // generated by pin-project-internal
    // impl<T> Unpin for MyStruct<T> where T: Copy // written by the user
    //
    // We want this code to be rejected - the user is completely bypassing `UnsafeUnpin`,
    // and providing an unsound Unpin impl in safe code!
    //
    // Unfortunately, the Rust compiler will accept the above code.
    // Because MyStruct is declared in the same crate as the user-provided impl,
    // the compiler will notice that 'MyStruct<T>: UnsafeUnpin' never holds.
    //
    // The solution is to introduce the 'Wrapper' struct, which is defined
    // in the 'pin-project' crate.
    //
    // We now have code that looks like this:
    //
    // impl<T> Unpin for MyStruct<T> where Wrapper<MyStruct<T>>: UnsafeUnpin {} // generated by pin-project-internal
    // impl<T> Unpin for MyStruct<T> where T: Copy // written by the user
    //
    // We also have 'unsafe impl<T> UnsafeUnpin for Wrapper<T> where T: UnsafeUnpin {}' in the
    // 'pin-project' crate.
    //
    // Now, our generated impl has a bound involving a type defined in another crate - Wrapper.
    // This will cause rust to conservatively assume that 'Wrapper<MyStruct<T>>: UnsafeUnpin'
    // holds, in the interest of preserving forwards compatibility (in case such an impl is added
    // for Wrapper<T> in a new version of the crate).
    //
    // This will cause rust to reject any other Unpin impls for MyStruct<T>, since it will
    // assume that our generated impl could potentially apply in any situation.
    //
    // This acheives the desired effect - when the user writes `#[pin_project(UnsafeUnpin)]`,
    // the user must either provide no impl of `UnsafeUnpin` (which is equivalent
    // to making the type never implement Unpin), or provide an impl of `UnsafeUnpin`.
    // It is impossible for them to provide an impl of `Unpin`
    #[doc(hidden)]
    pub struct Wrapper<T>(T);

    #[allow(unsafe_code)]
    unsafe impl<T> UnsafeUnpin for Wrapper<T> where T: UnsafeUnpin {}
}