1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
//! 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/HEAD/examples/README.md
//! [enum-default-expanded]: https://github.com/taiki-e/pin-project/blob/HEAD/examples/enum-default-expanded.rs
//! [pin-projection]: core::pin#projections-and-structural-pinning
//! [struct-default-expanded]: https://github.com/taiki-e/pin-project/blob/HEAD/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(missing_docs, rust_2018_idioms, single_use_lifetimes, unreachable_pub)]
#![warn(clippy::default_trait_access, clippy::wildcard_imports)]
#![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.
///
/// # Safety
///
/// 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 {
    use core::mem::ManuallyDrop;
    #[doc(hidden)]
    pub use core::{
        marker::{PhantomData, PhantomPinned, Unpin},
        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>(*mut T);

    impl<T: ?Sized> UnsafeDropInPlaceGuard<T> {
        #[doc(hidden)]
        pub unsafe fn new(ptr: *mut T) -> Self {
            Self(ptr)
        }
    }

    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> {
        target: *mut T,
        value: ManuallyDrop<T>,
    }

    impl<T> UnsafeOverwriteGuard<T> {
        #[doc(hidden)]
        pub unsafe fn new(target: *mut T, value: T) -> Self {
            Self { target, value: ManuallyDrop::new(value) }
        }
    }

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