1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3/*!
4<!-- tidy:crate-doc:start -->
5A crate for safe and ergonomic [pin-projection].
6
7## Usage
8
9Add this to your `Cargo.toml`:
10
11```toml
12[dependencies]
13pin-project = "1"
14```
15
16*Compiler support: requires rustc 1.56+*
17
18## Examples
19
20[`#[pin_project]`][`pin_project`] attribute creates projection types
21covering all the fields of struct or enum.
22
23```rust
24use std::pin::Pin;
25
26use pin_project::pin_project;
27
28#[pin_project]
29struct Struct<T, U> {
30 #[pin]
31 pinned: T,
32 unpinned: U,
33}
34
35impl<T, U> Struct<T, U> {
36 fn method(self: Pin<&mut Self>) {
37 let this = self.project();
38 let _: Pin<&mut T> = this.pinned; // Pinned reference to the field
39 let _: &mut U = this.unpinned; // Normal reference to the field
40 }
41}
42```
43
44[*code like this will be generated*][struct-default-expanded]
45
46To use `#[pin_project]` on enums, you need to name the projection type
47returned from the method.
48
49```rust
50use std::pin::Pin;
51
52use pin_project::pin_project;
53
54#[pin_project(project = EnumProj)]
55enum Enum<T, U> {
56 Pinned(#[pin] T),
57 Unpinned(U),
58}
59
60impl<T, U> Enum<T, U> {
61 fn method(self: Pin<&mut Self>) {
62 match self.project() {
63 EnumProj::Pinned(x) => {
64 let _: Pin<&mut T> = x;
65 }
66 EnumProj::Unpinned(y) => {
67 let _: &mut U = y;
68 }
69 }
70 }
71}
72```
73
74[*code like this will be generated*][enum-default-expanded]
75
76See [`#[pin_project]`][`pin_project`] attribute for more details, and
77see [examples] directory for more examples and generated code.
78
79## Related Projects
80
81- [pin-project-lite]: A lightweight version of pin-project written with declarative macros.
82
83[enum-default-expanded]: https://github.com/taiki-e/pin-project/blob/HEAD/examples/enum-default-expanded.rs
84[examples]: https://github.com/taiki-e/pin-project/blob/HEAD/examples/README.md
85[pin-project-lite]: https://github.com/taiki-e/pin-project-lite
86[pin-projection]: https://doc.rust-lang.org/std/pin/index.html#projections-and-structural-pinning
87[struct-default-expanded]: https://github.com/taiki-e/pin-project/blob/HEAD/examples/struct-default-expanded.rs
88
89<!-- tidy:crate-doc:end -->
90*/
91
92#![no_std]
93#![doc(test(
94 no_crate_inject,
95 attr(
96 deny(warnings, rust_2018_idioms, single_use_lifetimes),
97 allow(dead_code, unused_variables)
98 )
99))]
100#![warn(unsafe_op_in_unsafe_fn)]
101#![warn(
102 // Lints that may help when writing public library.
103 missing_debug_implementations,
104 missing_docs,
105 clippy::alloc_instead_of_core,
106 clippy::exhaustive_enums,
107 clippy::exhaustive_structs,
108 clippy::impl_trait_in_params,
109 // clippy::missing_inline_in_public_items,
110 clippy::std_instead_of_alloc,
111 clippy::std_instead_of_core,
112)]
113#![allow(clippy::needless_doctest_main)]
114
115#[doc(inline)]
116pub use pin_project_internal::pin_project;
117#[doc(inline)]
118pub use pin_project_internal::pinned_drop;
119
120/// A trait used for custom implementations of [`Unpin`].
121///
122/// This trait is used in conjunction with the `UnsafeUnpin` argument to
123/// the [`#[pin_project]`][macro@pin_project] attribute.
124///
125/// # Safety
126///
127/// The Rust [`Unpin`] trait is safe to implement - by itself,
128/// implementing it cannot lead to [undefined behavior][undefined-behavior].
129/// Undefined behavior can only occur when other unsafe code is used.
130///
131/// It turns out that using pin projections, which requires unsafe code,
132/// imposes additional requirements on an [`Unpin`] impl. Normally, all of this
133/// unsafety is contained within this crate, ensuring that it's impossible for
134/// you to violate any of the guarantees required by pin projection.
135///
136/// However, things change if you want to provide a custom [`Unpin`] impl
137/// for your `#[pin_project]` type. As stated in [the Rust
138/// documentation][pin-projection], you must be sure to only implement [`Unpin`]
139/// when all of your `#[pin]` fields (i.e. structurally pinned fields) are also
140/// [`Unpin`].
141///
142/// To help highlight this unsafety, the `UnsafeUnpin` trait is provided.
143/// Implementing this trait is logically equivalent to implementing [`Unpin`] -
144/// this crate will generate an [`Unpin`] impl for your type that 'forwards' to
145/// your `UnsafeUnpin` impl. However, this trait is `unsafe` - since your type
146/// uses structural pinning (otherwise, you wouldn't be using this crate!),
147/// you must be sure that your `UnsafeUnpin` impls follows all of
148/// the requirements for an [`Unpin`] impl of a structurally-pinned type.
149///
150/// Note that if you specify `#[pin_project(UnsafeUnpin)]`, but do *not*
151/// provide an impl of `UnsafeUnpin`, your type will never implement [`Unpin`].
152/// This is effectively the same thing as adding a [`PhantomPinned`] to your
153/// type.
154///
155/// Since this trait is `unsafe`, impls of it will be detected by the
156/// `unsafe_code` lint, and by tools like [`cargo geiger`][cargo-geiger].
157///
158/// # Examples
159///
160/// An `UnsafeUnpin` impl which, in addition to requiring that structurally
161/// pinned fields be [`Unpin`], imposes an additional requirement:
162///
163/// ```
164/// use pin_project::{pin_project, UnsafeUnpin};
165///
166/// #[pin_project(UnsafeUnpin)]
167/// struct Struct<K, V> {
168/// #[pin]
169/// field_1: K,
170/// field_2: V,
171/// }
172///
173/// unsafe impl<K, V> UnsafeUnpin for Struct<K, V> where K: Unpin + Clone {}
174/// ```
175///
176/// [`PhantomPinned`]: core::marker::PhantomPinned
177/// [cargo-geiger]: https://github.com/rust-secure-code/cargo-geiger
178/// [pin-projection]: core::pin#projections-and-structural-pinning
179/// [undefined-behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
180pub unsafe trait UnsafeUnpin {}
181
182// Not public API.
183#[doc(hidden)]
184pub mod __private {
185 #![allow(missing_debug_implementations)]
186
187 use core::mem::ManuallyDrop;
188 #[doc(hidden)]
189 pub use core::{
190 marker::{PhantomData, PhantomPinned, Unpin},
191 ops::Drop,
192 pin::Pin,
193 ptr,
194 };
195
196 #[doc(hidden)]
197 pub use pin_project_internal::__PinProjectInternalDerive;
198
199 use super::UnsafeUnpin;
200
201 // An internal trait used for custom implementations of [`Drop`].
202 //
203 // **Do not call or implement this trait directly.**
204 //
205 // # Why this trait is private and `#[pinned_drop]` attribute is needed?
206 //
207 // Implementing `PinnedDrop::drop` is safe, but calling it is not safe.
208 // This is because destructors can be called multiple times in safe code and
209 // [double dropping is unsound][rust-lang/rust#62360].
210 //
211 // Ideally, it would be desirable to be able to forbid manual calls in
212 // the same way as [`Drop::drop`], but the library cannot do it. So, by using
213 // macros and replacing them with private traits,
214 // this crate prevent users from calling `PinnedDrop::drop` in safe code.
215 //
216 // This allows implementing [`Drop`] safely using `#[pinned_drop]`.
217 // Also by using the [`drop`] function just like dropping a type that directly
218 // implements [`Drop`], can drop safely a type that implements `PinnedDrop`.
219 //
220 // [rust-lang/rust#62360]: https://github.com/rust-lang/rust/pull/62360
221 #[doc(hidden)]
222 pub trait PinnedDrop {
223 #[doc(hidden)]
224 unsafe fn drop(self: Pin<&mut Self>);
225 }
226
227 // This is an internal helper struct used by `pin-project-internal`.
228 // This allows us to force an error if the user tries to provide
229 // a regular `Unpin` impl when they specify the `UnsafeUnpin` argument.
230 // This is why we need Wrapper:
231 //
232 // Supposed we have the following code:
233 //
234 // ```
235 // #[pin_project(UnsafeUnpin)]
236 // struct MyStruct<T> {
237 // #[pin] field: T
238 // }
239 //
240 // impl<T> Unpin for MyStruct<T> where MyStruct<T>: UnsafeUnpin {} // generated by pin-project-internal
241 // impl<T> Unpin for MyStruct<T> where T: Copy // written by the user
242 // ```
243 //
244 // We want this code to be rejected - the user is completely bypassing
245 // `UnsafeUnpin`, and providing an unsound Unpin impl in safe code!
246 //
247 // Unfortunately, the Rust compiler will accept the above code.
248 // Because MyStruct is declared in the same crate as the user-provided impl,
249 // the compiler will notice that `MyStruct<T>: UnsafeUnpin` never holds.
250 //
251 // The solution is to introduce the `Wrapper` struct, which is defined
252 // in the `pin-project` crate.
253 //
254 // We now have code that looks like this:
255 //
256 // ```
257 // impl<T> Unpin for MyStruct<T> where Wrapper<MyStruct<T>>: UnsafeUnpin {} // generated by pin-project-internal
258 // impl<T> Unpin for MyStruct<T> where T: Copy // written by the user
259 // ```
260 //
261 // We also have `unsafe impl<T> UnsafeUnpin for Wrapper<T> where T: UnsafeUnpin {}`
262 // in the `pin-project` crate.
263 //
264 // Now, our generated impl has a bound involving a type defined in another
265 // crate - Wrapper. This will cause rust to conservatively assume that
266 // `Wrapper<MyStruct<T>>: UnsafeUnpin` holds, in the interest of preserving
267 // forwards compatibility (in case such an impl is added for Wrapper<T> in
268 // a new version of the crate).
269 //
270 // This will cause rust to reject any other `Unpin` impls for MyStruct<T>,
271 // since it will assume that our generated impl could potentially apply in
272 // any situation.
273 //
274 // This achieves the desired effect - when the user writes
275 // `#[pin_project(UnsafeUnpin)]`, the user must either provide no impl of
276 // `UnsafeUnpin` (which is equivalent to making the type never implement
277 // Unpin), or provide an impl of `UnsafeUnpin`. It is impossible for them to
278 // provide an impl of `Unpin`
279 #[doc(hidden)]
280 pub struct Wrapper<'a, T: ?Sized>(PhantomData<&'a ()>, T);
281
282 // SAFETY: `T` implements UnsafeUnpin.
283 unsafe impl<T: ?Sized + UnsafeUnpin> UnsafeUnpin for Wrapper<'_, T> {}
284
285 // This is an internal helper struct used by `pin-project-internal`.
286 //
287 // See https://github.com/taiki-e/pin-project/pull/53 for more details.
288 #[doc(hidden)]
289 pub struct AlwaysUnpin<'a, T>(PhantomData<&'a ()>, PhantomData<T>);
290
291 impl<T> Unpin for AlwaysUnpin<'_, T> {}
292
293 // This is an internal helper used to ensure a value is dropped.
294 #[doc(hidden)]
295 pub struct UnsafeDropInPlaceGuard<T: ?Sized>(*mut T);
296
297 impl<T: ?Sized> UnsafeDropInPlaceGuard<T> {
298 #[doc(hidden)]
299 pub unsafe fn new(ptr: *mut T) -> Self {
300 Self(ptr)
301 }
302 }
303
304 impl<T: ?Sized> Drop for UnsafeDropInPlaceGuard<T> {
305 fn drop(&mut self) {
306 // SAFETY: the caller of `UnsafeDropInPlaceGuard::new` must guarantee
307 // that `ptr` is valid for drop when this guard is destructed.
308 unsafe {
309 ptr::drop_in_place(self.0);
310 }
311 }
312 }
313
314 // This is an internal helper used to ensure a value is overwritten without
315 // its destructor being called.
316 #[doc(hidden)]
317 pub struct UnsafeOverwriteGuard<T> {
318 target: *mut T,
319 value: ManuallyDrop<T>,
320 }
321
322 impl<T> UnsafeOverwriteGuard<T> {
323 #[doc(hidden)]
324 pub unsafe fn new(target: *mut T, value: T) -> Self {
325 Self { target, value: ManuallyDrop::new(value) }
326 }
327 }
328
329 impl<T> Drop for UnsafeOverwriteGuard<T> {
330 fn drop(&mut self) {
331 // SAFETY: the caller of `UnsafeOverwriteGuard::new` must guarantee
332 // that `target` is valid for writes when this guard is destructed.
333 unsafe {
334 ptr::write(self.target, ptr::read(&*self.value));
335 }
336 }
337 }
338}
339