pin_project_internal/
lib.rs

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
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
// SPDX-License-Identifier: Apache-2.0 OR MIT

//! Implementation detail of the `pin-project` crate. - **do not use directly**

#![doc(test(
    no_crate_inject,
    attr(
        deny(warnings, rust_2018_idioms, single_use_lifetimes),
        allow(dead_code, unused_variables)
    )
))]
#![forbid(unsafe_code)]
#![allow(clippy::needless_doctest_main)]

#[macro_use]
mod error;

#[macro_use]
mod utils;

mod pin_project;
mod pinned_drop;

use proc_macro::TokenStream;

/// An attribute that creates projection types covering all the fields of
/// struct or enum.
///
/// This attribute creates projection types according to the following rules:
///
/// - For the fields that use `#[pin]` attribute, create the pinned reference to
///   the field.
/// - For the other fields, create a normal reference to the field.
///
/// And the following methods are implemented on the original type:
///
/// ```
/// # use std::pin::Pin;
/// # type Projection<'a> = &'a ();
/// # type ProjectionRef<'a> = &'a ();
/// # trait Dox {
/// fn project(self: Pin<&mut Self>) -> Projection<'_>;
/// fn project_ref(self: Pin<&Self>) -> ProjectionRef<'_>;
/// # }
/// ```
///
/// By passing an argument with the same name as the method to the attribute,
/// you can name the projection type returned from the method. This allows you
/// to use pattern matching on the projected types.
///
/// ```
/// # use pin_project::pin_project;
/// # use std::pin::Pin;
/// #[pin_project(project = EnumProj)]
/// enum Enum<T> {
///     Variant(#[pin] T),
/// }
///
/// impl<T> Enum<T> {
///     fn method(self: Pin<&mut Self>) {
///         let this: EnumProj<'_, T> = self.project();
///         match this {
///             EnumProj::Variant(x) => {
///                 let _: Pin<&mut T> = x;
///             }
///         }
///     }
/// }
/// ```
///
/// Note that the projection types returned by `project` and `project_ref` have
/// an additional lifetime at the beginning of generics.
///
/// ```text
/// let this: EnumProj<'_, T> = self.project();
///                    ^^
/// ```
///
/// The visibility of the projected types and projection methods is based on the
/// original type. However, if the visibility of the original type is `pub`, the
/// visibility of the projected types and the projection methods is downgraded
/// to `pub(crate)`.
///
/// # Safety
///
/// This attribute is completely safe. In the absence of other `unsafe` code
/// *that you write*, it is impossible to cause [undefined
/// behavior][undefined-behavior] with this attribute.
///
/// This is accomplished by enforcing the four requirements for pin projection
/// stated in [the Rust documentation][pin-projection]:
///
/// 1. The struct must only be [`Unpin`] if all the structural fields are
///    [`Unpin`].
///
///    To enforce this, this attribute will automatically generate an [`Unpin`]
///    implementation for you, which will require that all structurally pinned
///    fields be [`Unpin`].
///
///    If you attempt to provide an [`Unpin`] impl, the blanket impl will then
///    apply to your type, causing a compile-time error due to the conflict with
///    the second impl.
///
///    If you wish to provide a manual [`Unpin`] impl, you can do so via the
///    [`UnsafeUnpin`][unsafe-unpin] argument.
///
/// 2. The destructor of the struct must not move structural fields out of its
///    argument.
///
///    To enforce this, this attribute will generate code like this:
///
///    ```
///    struct MyStruct {}
///    trait MyStructMustNotImplDrop {}
///    # #[allow(unknown_lints, drop_bounds)]
///    impl<T: Drop> MyStructMustNotImplDrop for T {}
///    impl MyStructMustNotImplDrop for MyStruct {}
///    ```
///
///    If you attempt to provide an [`Drop`] impl, the blanket impl will then
///    apply to your type, causing a compile-time error due to the conflict with
///    the second impl.
///
///    If you wish to provide a custom [`Drop`] impl, you can annotate an impl
///    with [`#[pinned_drop]`][pinned-drop]. This impl takes a pinned version of
///    your struct - that is, [`Pin`]`<&mut MyStruct>` where `MyStruct` is the
///    type of your struct.
///
///    You can call `.project()` on this type as usual, along with any other
///    methods you have defined. Because your code is never provided with
///    a `&mut MyStruct`, it is impossible to move out of pin-projectable
///    fields in safe code in your destructor.
///
/// 3. You must make sure that you uphold the [`Drop`
///    guarantee][drop-guarantee]: once your struct is pinned, the memory that
///    contains the content is not overwritten or deallocated without calling
///    the content's destructors.
///
///    Safe code doesn't need to worry about this - the only way to violate
///    this requirement is to manually deallocate memory (which is `unsafe`),
///    or to overwrite a field with something else.
///    Because your custom destructor takes [`Pin`]`<&mut MyStruct>`, it's
///    impossible to obtain a mutable reference to a pin-projected field in safe
///    code.
///
/// 4. You must not offer any other operations that could lead to data being
///    moved out of the structural fields when your type is pinned.
///
///    As with requirement 3, it is impossible for safe code to violate this.
///    This crate ensures that safe code can never obtain a mutable reference to
///    `#[pin]` fields, which prevents you from ever moving out of them in safe
///    code.
///
/// Pin projections are also incompatible with [`#[repr(packed)]`][repr-packed]
/// types. Attempting to use this attribute on a `#[repr(packed)]` type results
/// in a compile-time error.
///
/// # Examples
///
/// `#[pin_project]` can be used on structs and enums.
///
/// ```
/// 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;
///         let _: &mut U = this.unpinned;
///     }
/// }
/// ```
///
/// ```
/// use std::pin::Pin;
///
/// use pin_project::pin_project;
///
/// #[pin_project]
/// struct TupleStruct<T, U>(#[pin] T, U);
///
/// impl<T, U> TupleStruct<T, U> {
///     fn method(self: Pin<&mut Self>) {
///         let this = self.project();
///         let _: Pin<&mut T> = this.0;
///         let _: &mut U = this.1;
///     }
/// }
/// ```
///
/// To use `#[pin_project]` on enums, you need to name the projection type
/// returned from the method.
///
/// ```
/// use std::pin::Pin;
///
/// use pin_project::pin_project;
///
/// #[pin_project(project = EnumProj)]
/// enum Enum<T, U> {
///     Tuple(#[pin] T),
///     Struct { field: U },
///     Unit,
/// }
///
/// impl<T, U> Enum<T, U> {
///     fn method(self: Pin<&mut Self>) {
///         match self.project() {
///             EnumProj::Tuple(x) => {
///                 let _: Pin<&mut T> = x;
///             }
///             EnumProj::Struct { field } => {
///                 let _: &mut U = field;
///             }
///             EnumProj::Unit => {}
///         }
///     }
/// }
/// ```
///
/// When `#[pin_project]` is used on enums, only named projection types and
/// methods are generated because there is no way to access variants of
/// projected types without naming it.
/// For example, in the above example, only the `project` method is generated,
/// and the `project_ref` method is not generated.
/// (When `#[pin_project]` is used on structs, both methods are always generated.)
///
/// ```compile_fail,E0599
/// # use pin_project::pin_project;
/// # use std::pin::Pin;
/// #
/// # #[pin_project(project = EnumProj)]
/// # enum Enum<T, U> {
/// #     Tuple(#[pin] T),
/// #     Struct { field: U },
/// #     Unit,
/// # }
/// #
/// impl<T, U> Enum<T, U> {
///     fn call_project_ref(self: Pin<&Self>) {
///         let _this = self.project_ref();
///         //~^ ERROR no method named `project_ref` found for struct `Pin<&Enum<T, U>>` in the current scope
///     }
/// }
/// ```
///
/// If you want to call `.project()` multiple times or later use the
/// original [`Pin`] type, it needs to use [`.as_mut()`][`Pin::as_mut`] to avoid
/// consuming the [`Pin`].
///
/// ```
/// use std::pin::Pin;
///
/// use pin_project::pin_project;
///
/// #[pin_project]
/// struct Struct<T> {
///     #[pin]
///     field: T,
/// }
///
/// impl<T> Struct<T> {
///     fn call_project_twice(mut self: Pin<&mut Self>) {
///         // `project` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`.
///         self.as_mut().project();
///         self.as_mut().project();
///     }
/// }
/// ```
///
/// # `!Unpin`
///
/// If you want to ensure that [`Unpin`] is not implemented, use the `!Unpin`
/// argument to `#[pin_project]`.
///
/// ```
/// use pin_project::pin_project;
///
/// #[pin_project(!Unpin)]
/// struct Struct<T> {
///     field: T,
/// }
/// ```
///
/// This is equivalent to using `#[pin]` attribute for the [`PhantomPinned`]
/// field.
///
/// ```
/// use std::marker::PhantomPinned;
///
/// use pin_project::pin_project;
///
/// #[pin_project]
/// struct Struct<T> {
///     field: T,
///     #[pin] // <------ This `#[pin]` is required to make `Struct` to `!Unpin`.
///     _pin: PhantomPinned,
/// }
/// ```
///
/// Note that using [`PhantomPinned`] without `#[pin]` attribute has no effect.
///
/// # `UnsafeUnpin`
///
/// If you want to implement [`Unpin`] manually, you must use the `UnsafeUnpin`
/// argument to `#[pin_project]`.
///
/// ```
/// use pin_project::{pin_project, UnsafeUnpin};
///
/// #[pin_project(UnsafeUnpin)]
/// struct Struct<T, U> {
///     #[pin]
///     pinned: T,
///     unpinned: U,
/// }
///
/// unsafe impl<T: Unpin, U> UnsafeUnpin for Struct<T, U> {}
/// ```
///
/// Note the usage of the unsafe [`UnsafeUnpin`] trait, instead of the usual
/// [`Unpin`] trait. [`UnsafeUnpin`] behaves exactly like [`Unpin`], except that
/// is unsafe to implement. This unsafety comes from the fact that pin
/// projections are being used. If you implement [`UnsafeUnpin`], you must
/// ensure that it is only implemented when all pin-projected fields implement
/// [`Unpin`].
///
/// See [`UnsafeUnpin`] trait for more details.
///
/// # `#[pinned_drop]`
///
/// In order to correctly implement pin projections, a type's [`Drop`] impl must
/// not move out of any structurally pinned fields. Unfortunately,
/// [`Drop::drop`] takes `&mut Self`, not [`Pin`]`<&mut Self>`.
///
/// To ensure that this requirement is upheld, the `#[pin_project]` attribute
/// will provide a [`Drop`] impl for you. This [`Drop`] impl will delegate to
/// an impl block annotated with `#[pinned_drop]` if you use the `PinnedDrop`
/// argument to `#[pin_project]`.
///
/// This impl block acts just like a normal [`Drop`] impl,
/// except for the following two:
///
/// - `drop` method takes [`Pin`]`<&mut Self>`
/// - Name of the trait is `PinnedDrop`.
///
/// ```
/// # use std::pin::Pin;
/// pub trait PinnedDrop {
///     fn drop(self: Pin<&mut Self>);
/// }
/// ```
///
/// `#[pin_project]` implements the actual [`Drop`] trait via `PinnedDrop` you
/// implemented. To drop a type that implements `PinnedDrop`, use the [`drop`]
/// function just like dropping a type that directly implements [`Drop`].
///
/// In particular, it will never be called more than once, just like
/// [`Drop::drop`].
///
/// For example:
///
/// ```
/// use std::{fmt::Debug, pin::Pin};
///
/// use pin_project::{pin_project, pinned_drop};
///
/// #[pin_project(PinnedDrop)]
/// struct PrintOnDrop<T: Debug, U: Debug> {
///     #[pin]
///     pinned_field: T,
///     unpin_field: U,
/// }
///
/// #[pinned_drop]
/// impl<T: Debug, U: Debug> PinnedDrop for PrintOnDrop<T, U> {
///     fn drop(self: Pin<&mut Self>) {
///         println!("Dropping pinned field: {:?}", self.pinned_field);
///         println!("Dropping unpin field: {:?}", self.unpin_field);
///     }
/// }
///
/// fn main() {
///     let _x = PrintOnDrop { pinned_field: true, unpin_field: 40 };
/// }
/// ```
///
/// See also [`#[pinned_drop]`][macro@pinned_drop] attribute.
///
/// # `project_replace` method
///
/// In addition to the `project` and `project_ref` methods which are always
/// provided when you use the `#[pin_project]` attribute, there is a third
/// method, `project_replace` which can be useful in some situations. It is
/// equivalent to [`Pin::set`], except that the unpinned fields are moved and
/// returned, instead of being dropped in-place.
///
/// ```
/// # use std::pin::Pin;
/// # type ProjectionOwned = ();
/// # trait Dox {
/// fn project_replace(self: Pin<&mut Self>, other: Self) -> ProjectionOwned;
/// # }
/// ```
///
/// The `ProjectionOwned` type is identical to the `Self` type, except that
/// all pinned fields have been replaced by equivalent [`PhantomData`] types.
///
/// This method is opt-in, because it is only supported for [`Sized`] types, and
/// because it is incompatible with the [`#[pinned_drop]`][pinned-drop]
/// attribute described above. It can be enabled by using
/// `#[pin_project(project_replace)]`.
///
/// For example:
///
/// ```
/// use std::{marker::PhantomData, pin::Pin};
///
/// use pin_project::pin_project;
///
/// #[pin_project(project_replace)]
/// struct Struct<T, U> {
///     #[pin]
///     pinned_field: T,
///     unpinned_field: U,
/// }
///
/// impl<T, U> Struct<T, U> {
///     fn method(self: Pin<&mut Self>, other: Self) {
///         let this = self.project_replace(other);
///         let _: U = this.unpinned_field;
///         let _: PhantomData<T> = this.pinned_field;
///     }
/// }
/// ```
///
/// By passing the value to the `project_replace` argument, you can name the
/// returned type of the `project_replace` method. This is necessary whenever
/// destructuring the return type of the `project_replace` method, and work in exactly
/// the same way as the `project` and `project_ref` arguments.
///
/// ```
/// use pin_project::pin_project;
///
/// #[pin_project(project_replace = EnumProjOwn)]
/// enum Enum<T, U> {
///     A {
///         #[pin]
///         pinned_field: T,
///         unpinned_field: U,
///     },
///     B,
/// }
///
/// let mut x = Box::pin(Enum::A { pinned_field: 42, unpinned_field: "hello" });
///
/// match x.as_mut().project_replace(Enum::B) {
///     EnumProjOwn::A { unpinned_field, .. } => assert_eq!(unpinned_field, "hello"),
///     EnumProjOwn::B => unreachable!(),
/// }
/// ```
///
/// [`PhantomData`]: core::marker::PhantomData
/// [`PhantomPinned`]: core::marker::PhantomPinned
/// [`Pin::as_mut`]: core::pin::Pin::as_mut
/// [`Pin::set`]: core::pin::Pin::set
/// [`Pin`]: core::pin::Pin
/// [`UnsafeUnpin`]: https://docs.rs/pin-project/latest/pin_project/trait.UnsafeUnpin.html
/// [drop-guarantee]: core::pin#drop-guarantee
/// [pin-projection]: core::pin#projections-and-structural-pinning
/// [pinned-drop]: macro@pin_project#pinned_drop
/// [repr-packed]: https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacked
/// [undefined-behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
/// [unsafe-unpin]: macro@pin_project#unsafeunpin
#[proc_macro_attribute]
pub fn pin_project(args: TokenStream, input: TokenStream) -> TokenStream {
    pin_project::attribute(&args.into(), input.into()).into()
}

/// An attribute used for custom implementations of [`Drop`].
///
/// This attribute is used in conjunction with the `PinnedDrop` argument to
/// the [`#[pin_project]`][macro@pin_project] attribute.
///
/// The impl block annotated with this attribute acts just like a normal
/// [`Drop`] impl, except for the following two:
///
/// - `drop` method takes [`Pin`]`<&mut Self>`
/// - Name of the trait is `PinnedDrop`.
///
/// ```
/// # use std::pin::Pin;
/// pub trait PinnedDrop {
///     fn drop(self: Pin<&mut Self>);
/// }
/// ```
///
/// `#[pin_project]` implements the actual [`Drop`] trait via `PinnedDrop` you
/// implemented. To drop a type that implements `PinnedDrop`, use the [`drop`]
/// function just like dropping a type that directly implements [`Drop`].
///
/// In particular, it will never be called more than once, just like
/// [`Drop::drop`].
///
/// # Examples
///
/// ```
/// use std::pin::Pin;
///
/// use pin_project::{pin_project, pinned_drop};
///
/// #[pin_project(PinnedDrop)]
/// struct PrintOnDrop {
///     #[pin]
///     field: u8,
/// }
///
/// #[pinned_drop]
/// impl PinnedDrop for PrintOnDrop {
///     fn drop(self: Pin<&mut Self>) {
///         println!("Dropping: {}", self.field);
///     }
/// }
///
/// fn main() {
///     let _x = PrintOnDrop { field: 50 };
/// }
/// ```
///
/// See also ["pinned-drop" section of `#[pin_project]` attribute][pinned-drop].
///
/// # Why `#[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 like the following,
/// this crate prevent users from calling `PinnedDrop::drop` in safe code.
///
/// ```
/// # use std::pin::Pin;
/// pub trait PinnedDrop {
///     unsafe fn drop(self: Pin<&mut Self>);
/// }
/// ```
///
/// 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
/// [`Pin`]: core::pin::Pin
/// [pinned-drop]: macro@pin_project#pinned_drop
#[proc_macro_attribute]
pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(input);
    pinned_drop::attribute(&args.into(), input).into()
}

// Not public API.
#[doc(hidden)]
#[proc_macro_derive(__PinProjectInternalDerive, attributes(pin))]
pub fn __pin_project_internal_derive(input: TokenStream) -> TokenStream {
    pin_project::derive(input.into()).into()
}