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    //! A crate for safe and ergonomic [pin-projection].
    //!
    //! # Examples
    //!
    //! [`#[pin_project]`][`pin_project`] attribute creates projection types
    //! covering all the fields of struct or enum.
    //!
    //! ```rust
    //! use std::pin::Pin;
    //!
    //! use pin_project::pin_project;
    //!
    //! #[pin_project]
    //! struct Struct<T, U> {
    //!     #[pin]
    //!     pinned: T,
    //!     unpinned: U,
    //! }
    //!
    //! impl<T, U> Struct<T, U> {
    //!     fn method(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*][struct-default-expanded]
    //!
    //! To use `#[pin_project]` on enums, you need to name the projection type
    //! returned from the method.
    //!
    //! ```rust
    //! use std::pin::Pin;
    //!
    //! use pin_project::pin_project;
    //!
    //! #[pin_project(project = EnumProj)]
    //! enum Enum<T, U> {
    //!     Pinned(#[pin] T),
    //!     Unpinned(U),
    //! }
    //!
    //! impl<T, U> Enum<T, U> {
    //!     fn method(self: Pin<&mut Self>) {
    //!         match self.project() {
    //!             EnumProj::Pinned(x) => {
    //!                 let _: Pin<&mut T> = x;
    //!             }
    //!             EnumProj::Unpinned(y) => {
    //!                 let _: &mut U = y;
    //!             }
    //!         }
    //!     }
    //! }
    //! ```
    //!
    //! [*code like this will be generated*][enum-default-expanded]
    //!
    //! See [`#[pin_project]`][`pin_project`] attribute for more details, and
    //! see [examples] directory for more examples and generated code.
    //!
    //! [examples]: https://github.com/taiki-e/pin-project/blob/master/examples/README.md
    //! [enum-default-expanded]: https://github.com/taiki-e/pin-project/blob/master/examples/enum-default-expanded.rs
    //! [pin-projection]: core::pin#projections-and-structural-pinning
    //! [struct-default-expanded]: https://github.com/taiki-e/pin-project/blob/master/examples/struct-default-expanded.rs

    #![no_std]
    #![doc(test(
        no_crate_inject,
        attr(
            deny(warnings, rust_2018_idioms, single_use_lifetimes),
            allow(dead_code, unused_variables)
        )
    ))]
    #![warn(future_incompatible, rust_2018_idioms, single_use_lifetimes, unreachable_pub)]
    #![warn(missing_docs)]
    #![warn(clippy::all, clippy::default_trait_access)]
    #![allow(clippy::needless_doctest_main)]

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

    /// A trait used for custom implementations of [`Unpin`].
    ///
    /// This trait is used in conjunction with the `UnsafeUnpin` argument to
    /// the [`#[pin_project]`][macro@pin_project] attribute.
    ///
    /// The Rust [`Unpin`] trait is safe to implement - by itself,
    /// implementing it cannot lead to [undefined behavior][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 guarantees 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][pin-projection], you must be sure to only implement [`Unpin`]
    /// when all of your `#[pin]` fields (i.e. structurally pinned fields) are also
    /// [`Unpin`].
    ///
    /// To help highlight this unsafety, the `UnsafeUnpin` trait is provided.
    /// Implementing this trait is logically equivalent to implementing [`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 effectively 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`][cargo-geiger].
    ///
    /// # Examples
    ///
    /// An `UnsafeUnpin` impl which, in addition to requiring that structurally
    /// pinned fields be [`Unpin`], imposes an additional requirement:
    ///
    /// ```rust
    /// use pin_project::{pin_project, UnsafeUnpin};
    ///
    /// #[pin_project(UnsafeUnpin)]
    /// struct Struct<K, V> {
    ///     #[pin]
    ///     field_1: K,
    ///     field_2: V,
    /// }
    ///
    /// unsafe impl<K, V> UnsafeUnpin for Struct<K, V> where K: Unpin + Clone {}
    /// ```
    ///
    /// [`PhantomPinned`]: core::marker::PhantomPinned
    /// [cargo-geiger]: https://github.com/rust-secure-code/cargo-geiger
    /// [pin-projection]: core::pin#projections-and-structural-pinning
    /// [undefined-behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    pub unsafe trait UnsafeUnpin {}

    // Not public API.
    #[doc(hidden)]
    pub mod __private {
        #[doc(hidden)]
        pub use core::{
            marker::{PhantomData, PhantomPinned, Unpin},
            mem::ManuallyDrop,
            ops::Drop,
            pin::Pin,
            ptr,
        };

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

        use super::UnsafeUnpin;

        // An internal trait used for custom implementations of [`Drop`].
        //
        // **Do not call or implement this trait directly.**
        //
        // # Why this trait is private and `#[pinned_drop]` attribute is needed?
        //
        // Implementing `PinnedDrop::drop` is safe, but calling it is not safe.
        // This is because destructors can be called multiple times in safe code and
        // [double dropping is unsound][rust-lang/rust#62360].
        //
        // Ideally, it would be desirable to be able to forbid manual calls in
        // the same way as [`Drop::drop`], but the library cannot do it. So, by using
        // macros and replacing them with private traits,
        // this crate prevent users from calling `PinnedDrop::drop` in safe code.
        //
        // This allows implementing [`Drop`] safely using `#[pinned_drop]`.
        // Also by using the [`drop`] function just like dropping a type that directly
        // implements [`Drop`], can drop safely a type that implements `PinnedDrop`.
        //
        // [rust-lang/rust#62360]: https://github.com/rust-lang/rust/pull/62360
        #[doc(hidden)]
        pub trait PinnedDrop {
            #[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:
        //
        // ```rust
        // #[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:
        //
        // ```rust
        // 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 achieves 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<'a, T: ?Sized>(PhantomData<&'a ()>, T);

        unsafe impl<T: ?Sized> UnsafeUnpin for Wrapper<'_, T> where T: UnsafeUnpin {}

        // This is an internal helper struct used by `pin-project-internal`.
        //
        // See https://github.com/taiki-e/pin-project/pull/53 for more details.
        #[doc(hidden)]
        pub struct AlwaysUnpin<'a, T>(PhantomData<&'a ()>, PhantomData<T>);

        impl<T> Unpin for AlwaysUnpin<'_, T> {}

        // This is an internal helper used to ensure a value is dropped.
        #[doc(hidden)]
        pub struct UnsafeDropInPlaceGuard<T: ?Sized>(pub *mut T);

        impl<T: ?Sized> Drop for UnsafeDropInPlaceGuard<T> {
            fn drop(&mut self) {
                unsafe {
                    ptr::drop_in_place(self.0);
                }
            }
        }

        // This is an internal helper used to ensure a value is overwritten without
        // its destructor being called.
        #[doc(hidden)]
        pub struct UnsafeOverwriteGuard<T> {
            pub value: ManuallyDrop<T>,
            pub target: *mut T,
        }

        impl<T> Drop for UnsafeOverwriteGuard<T> {
            fn drop(&mut self) {
                unsafe {
                    ptr::write(self.target, ptr::read(&*self.value));
                }
            }
        }
    }