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diff --git a/library/core/src/pin.rs b/library/core/src/pin.rs new file mode 100644 index 000000000..ccef35b45 --- /dev/null +++ b/library/core/src/pin.rs @@ -0,0 +1,1159 @@ +//! Types that pin data to its location in memory. +//! +//! It is sometimes useful to have objects that are guaranteed not to move, +//! in the sense that their placement in memory does not change, and can thus be relied upon. +//! A prime example of such a scenario would be building self-referential structs, +//! as moving an object with pointers to itself will invalidate them, which could cause undefined +//! behavior. +//! +//! At a high level, a <code>[Pin]\<P></code> ensures that the pointee of any pointer type +//! `P` has a stable location in memory, meaning it cannot be moved elsewhere +//! and its memory cannot be deallocated until it gets dropped. We say that the +//! pointee is "pinned". Things get more subtle when discussing types that +//! combine pinned with non-pinned data; [see below](#projections-and-structural-pinning) +//! for more details. +//! +//! By default, all types in Rust are movable. Rust allows passing all types by-value, +//! and common smart-pointer types such as <code>[Box]\<T></code> and <code>[&mut] T</code> allow +//! replacing and moving the values they contain: you can move out of a <code>[Box]\<T></code>, +//! or you can use [`mem::swap`]. <code>[Pin]\<P></code> wraps a pointer type `P`, so +//! <code>[Pin]<[Box]\<T>></code> functions much like a regular <code>[Box]\<T></code>: +//! when a <code>[Pin]<[Box]\<T>></code> gets dropped, so do its contents, and the memory gets +//! deallocated. Similarly, <code>[Pin]<[&mut] T></code> is a lot like <code>[&mut] T</code>. +//! However, <code>[Pin]\<P></code> does not let clients actually obtain a <code>[Box]\<T></code> +//! or <code>[&mut] T</code> to pinned data, which implies that you cannot use operations such +//! as [`mem::swap`]: +//! +//! ``` +//! use std::pin::Pin; +//! fn swap_pins<T>(x: Pin<&mut T>, y: Pin<&mut T>) { +//! // `mem::swap` needs `&mut T`, but we cannot get it. +//! // We are stuck, we cannot swap the contents of these references. +//! // We could use `Pin::get_unchecked_mut`, but that is unsafe for a reason: +//! // we are not allowed to use it for moving things out of the `Pin`. +//! } +//! ``` +//! +//! It is worth reiterating that <code>[Pin]\<P></code> does *not* change the fact that a Rust +//! compiler considers all types movable. [`mem::swap`] remains callable for any `T`. Instead, +//! <code>[Pin]\<P></code> prevents certain *values* (pointed to by pointers wrapped in +//! <code>[Pin]\<P></code>) from being moved by making it impossible to call methods that require +//! <code>[&mut] T</code> on them (like [`mem::swap`]). +//! +//! <code>[Pin]\<P></code> can be used to wrap any pointer type `P`, and as such it interacts with +//! [`Deref`] and [`DerefMut`]. A <code>[Pin]\<P></code> where <code>P: [Deref]</code> should be +//! considered as a "`P`-style pointer" to a pinned <code>P::[Target]</code> – so, a +//! <code>[Pin]<[Box]\<T>></code> is an owned pointer to a pinned `T`, and a +//! <code>[Pin]<[Rc]\<T>></code> is a reference-counted pointer to a pinned `T`. +//! For correctness, <code>[Pin]\<P></code> relies on the implementations of [`Deref`] and +//! [`DerefMut`] not to move out of their `self` parameter, and only ever to +//! return a pointer to pinned data when they are called on a pinned pointer. +//! +//! # `Unpin` +//! +//! Many types are always freely movable, even when pinned, because they do not +//! rely on having a stable address. This includes all the basic types (like +//! [`bool`], [`i32`], and references) as well as types consisting solely of these +//! types. Types that do not care about pinning implement the [`Unpin`] +//! auto-trait, which cancels the effect of <code>[Pin]\<P></code>. For <code>T: [Unpin]</code>, +//! <code>[Pin]<[Box]\<T>></code> and <code>[Box]\<T></code> function identically, as do +//! <code>[Pin]<[&mut] T></code> and <code>[&mut] T</code>. +//! +//! Note that pinning and [`Unpin`] only affect the pointed-to type <code>P::[Target]</code>, +//! not the pointer type `P` itself that got wrapped in <code>[Pin]\<P></code>. For example, +//! whether or not <code>[Box]\<T></code> is [`Unpin`] has no effect on the behavior of +//! <code>[Pin]<[Box]\<T>></code> (here, `T` is the pointed-to type). +//! +//! # Example: self-referential struct +//! +//! Before we go into more details to explain the guarantees and choices +//! associated with <code>[Pin]\<P></code>, we discuss some examples for how it might be used. +//! Feel free to [skip to where the theoretical discussion continues](#drop-guarantee). +//! +//! ```rust +//! use std::pin::Pin; +//! use std::marker::PhantomPinned; +//! use std::ptr::NonNull; +//! +//! // This is a self-referential struct because the slice field points to the data field. +//! // We cannot inform the compiler about that with a normal reference, +//! // as this pattern cannot be described with the usual borrowing rules. +//! // Instead we use a raw pointer, though one which is known not to be null, +//! // as we know it's pointing at the string. +//! struct Unmovable { +//! data: String, +//! slice: NonNull<String>, +//! _pin: PhantomPinned, +//! } +//! +//! impl Unmovable { +//! // To ensure the data doesn't move when the function returns, +//! // we place it in the heap where it will stay for the lifetime of the object, +//! // and the only way to access it would be through a pointer to it. +//! fn new(data: String) -> Pin<Box<Self>> { +//! let res = Unmovable { +//! data, +//! // we only create the pointer once the data is in place +//! // otherwise it will have already moved before we even started +//! slice: NonNull::dangling(), +//! _pin: PhantomPinned, +//! }; +//! let mut boxed = Box::pin(res); +//! +//! let slice = NonNull::from(&boxed.data); +//! // we know this is safe because modifying a field doesn't move the whole struct +//! unsafe { +//! let mut_ref: Pin<&mut Self> = Pin::as_mut(&mut boxed); +//! Pin::get_unchecked_mut(mut_ref).slice = slice; +//! } +//! boxed +//! } +//! } +//! +//! let unmoved = Unmovable::new("hello".to_string()); +//! // The pointer should point to the correct location, +//! // so long as the struct hasn't moved. +//! // Meanwhile, we are free to move the pointer around. +//! # #[allow(unused_mut)] +//! let mut still_unmoved = unmoved; +//! assert_eq!(still_unmoved.slice, NonNull::from(&still_unmoved.data)); +//! +//! // Since our type doesn't implement Unpin, this will fail to compile: +//! // let mut new_unmoved = Unmovable::new("world".to_string()); +//! // std::mem::swap(&mut *still_unmoved, &mut *new_unmoved); +//! ``` +//! +//! # Example: intrusive doubly-linked list +//! +//! In an intrusive doubly-linked list, the collection does not actually allocate +//! the memory for the elements itself. Allocation is controlled by the clients, +//! and elements can live on a stack frame that lives shorter than the collection does. +//! +//! To make this work, every element has pointers to its predecessor and successor in +//! the list. Elements can only be added when they are pinned, because moving the elements +//! around would invalidate the pointers. Moreover, the [`Drop`][Drop] implementation of a linked +//! list element will patch the pointers of its predecessor and successor to remove itself +//! from the list. +//! +//! Crucially, we have to be able to rely on [`drop`] being called. If an element +//! could be deallocated or otherwise invalidated without calling [`drop`], the pointers into it +//! from its neighboring elements would become invalid, which would break the data structure. +//! +//! Therefore, pinning also comes with a [`drop`]-related guarantee. +//! +//! # `Drop` guarantee +//! +//! The purpose of pinning is to be able to rely on the placement of some data in memory. +//! To make this work, not just moving the data is restricted; deallocating, repurposing, or +//! otherwise invalidating the memory used to store the data is restricted, too. +//! Concretely, for pinned data you have to maintain the invariant +//! that *its memory will not get invalidated or repurposed from the moment it gets pinned until +//! when [`drop`] is called*. Only once [`drop`] returns or panics, the memory may be reused. +//! +//! Memory can be "invalidated" by deallocation, but also by +//! replacing a <code>[Some]\(v)</code> by [`None`], or calling [`Vec::set_len`] to "kill" some +//! elements off of a vector. It can be repurposed by using [`ptr::write`] to overwrite it without +//! calling the destructor first. None of this is allowed for pinned data without calling [`drop`]. +//! +//! This is exactly the kind of guarantee that the intrusive linked list from the previous +//! section needs to function correctly. +//! +//! Notice that this guarantee does *not* mean that memory does not leak! It is still +//! completely okay to not ever call [`drop`] on a pinned element (e.g., you can still +//! call [`mem::forget`] on a <code>[Pin]<[Box]\<T>></code>). In the example of the doubly-linked +//! list, that element would just stay in the list. However you must not free or reuse the storage +//! *without calling [`drop`]*. +//! +//! # `Drop` implementation +//! +//! If your type uses pinning (such as the two examples above), you have to be careful +//! when implementing [`Drop`][Drop]. The [`drop`] function takes <code>[&mut] self</code>, but this +//! is called *even if your type was previously pinned*! It is as if the +//! compiler automatically called [`Pin::get_unchecked_mut`]. +//! +//! This can never cause a problem in safe code because implementing a type that +//! relies on pinning requires unsafe code, but be aware that deciding to make +//! use of pinning in your type (for example by implementing some operation on +//! <code>[Pin]<[&]Self></code> or <code>[Pin]<[&mut] Self></code>) has consequences for your +//! [`Drop`][Drop] implementation as well: if an element of your type could have been pinned, +//! you must treat [`Drop`][Drop] as implicitly taking <code>[Pin]<[&mut] Self></code>. +//! +//! For example, you could implement [`Drop`][Drop] as follows: +//! +//! ```rust,no_run +//! # use std::pin::Pin; +//! # struct Type { } +//! impl Drop for Type { +//! fn drop(&mut self) { +//! // `new_unchecked` is okay because we know this value is never used +//! // again after being dropped. +//! inner_drop(unsafe { Pin::new_unchecked(self)}); +//! fn inner_drop(this: Pin<&mut Type>) { +//! // Actual drop code goes here. +//! } +//! } +//! } +//! ``` +//! +//! The function `inner_drop` has the type that [`drop`] *should* have, so this makes sure that +//! you do not accidentally use `self`/`this` in a way that is in conflict with pinning. +//! +//! Moreover, if your type is `#[repr(packed)]`, the compiler will automatically +//! move fields around to be able to drop them. It might even do +//! that for fields that happen to be sufficiently aligned. As a consequence, you cannot use +//! pinning with a `#[repr(packed)]` type. +//! +//! # Projections and Structural Pinning +//! +//! When working with pinned structs, the question arises how one can access the +//! fields of that struct in a method that takes just <code>[Pin]<[&mut] Struct></code>. +//! The usual approach is to write helper methods (so called *projections*) +//! that turn <code>[Pin]<[&mut] Struct></code> into a reference to the field, but what type should +//! that reference have? Is it <code>[Pin]<[&mut] Field></code> or <code>[&mut] Field</code>? +//! The same question arises with the fields of an `enum`, and also when considering +//! container/wrapper types such as <code>[Vec]\<T></code>, <code>[Box]\<T></code>, +//! or <code>[RefCell]\<T></code>. (This question applies to both mutable and shared references, +//! we just use the more common case of mutable references here for illustration.) +//! +//! It turns out that it is actually up to the author of the data structure to decide whether +//! the pinned projection for a particular field turns <code>[Pin]<[&mut] Struct></code> +//! into <code>[Pin]<[&mut] Field></code> or <code>[&mut] Field</code>. There are some +//! constraints though, and the most important constraint is *consistency*: +//! every field can be *either* projected to a pinned reference, *or* have +//! pinning removed as part of the projection. If both are done for the same field, +//! that will likely be unsound! +//! +//! As the author of a data structure you get to decide for each field whether pinning +//! "propagates" to this field or not. Pinning that propagates is also called "structural", +//! because it follows the structure of the type. +//! In the following subsections, we describe the considerations that have to be made +//! for either choice. +//! +//! ## Pinning *is not* structural for `field` +//! +//! It may seem counter-intuitive that the field of a pinned struct might not be pinned, +//! but that is actually the easiest choice: if a <code>[Pin]<[&mut] Field></code> is never created, +//! nothing can go wrong! So, if you decide that some field does not have structural pinning, +//! all you have to ensure is that you never create a pinned reference to that field. +//! +//! Fields without structural pinning may have a projection method that turns +//! <code>[Pin]<[&mut] Struct></code> into <code>[&mut] Field</code>: +//! +//! ```rust,no_run +//! # use std::pin::Pin; +//! # type Field = i32; +//! # struct Struct { field: Field } +//! impl Struct { +//! fn pin_get_field(self: Pin<&mut Self>) -> &mut Field { +//! // This is okay because `field` is never considered pinned. +//! unsafe { &mut self.get_unchecked_mut().field } +//! } +//! } +//! ``` +//! +//! You may also <code>impl [Unpin] for Struct</code> *even if* the type of `field` +//! is not [`Unpin`]. What that type thinks about pinning is not relevant +//! when no <code>[Pin]<[&mut] Field></code> is ever created. +//! +//! ## Pinning *is* structural for `field` +//! +//! The other option is to decide that pinning is "structural" for `field`, +//! meaning that if the struct is pinned then so is the field. +//! +//! This allows writing a projection that creates a <code>[Pin]<[&mut] Field></code>, thus +//! witnessing that the field is pinned: +//! +//! ```rust,no_run +//! # use std::pin::Pin; +//! # type Field = i32; +//! # struct Struct { field: Field } +//! impl Struct { +//! fn pin_get_field(self: Pin<&mut Self>) -> Pin<&mut Field> { +//! // This is okay because `field` is pinned when `self` is. +//! unsafe { self.map_unchecked_mut(|s| &mut s.field) } +//! } +//! } +//! ``` +//! +//! However, structural pinning comes with a few extra requirements: +//! +//! 1. The struct must only be [`Unpin`] if all the structural fields are +//! [`Unpin`]. This is the default, but [`Unpin`] is a safe trait, so as the author of +//! the struct it is your responsibility *not* to add something like +//! <code>impl\<T> [Unpin] for Struct\<T></code>. (Notice that adding a projection operation +//! requires unsafe code, so the fact that [`Unpin`] is a safe trait does not break +//! the principle that you only have to worry about any of this if you use [`unsafe`].) +//! 2. The destructor of the struct must not move structural fields out of its argument. This +//! is the exact point that was raised in the [previous section][drop-impl]: [`drop`] takes +//! <code>[&mut] self</code>, but the struct (and hence its fields) might have been pinned +//! before. You have to guarantee that you do not move a field inside your [`Drop`][Drop] +//! implementation. In particular, as explained previously, this means that your struct +//! must *not* be `#[repr(packed)]`. +//! See that section for how to write [`drop`] in a way that the compiler can help you +//! not accidentally break pinning. +//! 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. +//! This can be tricky, as witnessed by <code>[VecDeque]\<T></code>: the destructor of +//! <code>[VecDeque]\<T></code> can fail to call [`drop`] on all elements if one of the +//! destructors panics. This violates the [`Drop`][Drop] guarantee, because it can lead to +//! elements being deallocated without their destructor being called. +//! (<code>[VecDeque]\<T></code> has no pinning projections, so this +//! does not cause unsoundness.) +//! 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. For example, if the struct contains an +//! <code>[Option]\<T></code> and there is a [`take`][Option::take]-like operation with type +//! <code>fn([Pin]<[&mut] Struct\<T>>) -> [Option]\<T></code>, +//! that operation can be used to move a `T` out of a pinned `Struct<T>` – which means +//! pinning cannot be structural for the field holding this data. +//! +//! For a more complex example of moving data out of a pinned type, +//! imagine if <code>[RefCell]\<T></code> had a method +//! <code>fn get_pin_mut(self: [Pin]<[&mut] Self>) -> [Pin]<[&mut] T></code>. +//! Then we could do the following: +//! ```compile_fail +//! fn exploit_ref_cell<T>(rc: Pin<&mut RefCell<T>>) { +//! { let p = rc.as_mut().get_pin_mut(); } // Here we get pinned access to the `T`. +//! let rc_shr: &RefCell<T> = rc.into_ref().get_ref(); +//! let b = rc_shr.borrow_mut(); +//! let content = &mut *b; // And here we have `&mut T` to the same data. +//! } +//! ``` +//! This is catastrophic, it means we can first pin the content of the +//! <code>[RefCell]\<T></code> (using <code>[RefCell]::get_pin_mut</code>) and then move that +//! content using the mutable reference we got later. +//! +//! ## Examples +//! +//! For a type like <code>[Vec]\<T></code>, both possibilities (structural pinning or not) make +//! sense. A <code>[Vec]\<T></code> with structural pinning could have `get_pin`/`get_pin_mut` +//! methods to get pinned references to elements. However, it could *not* allow calling +//! [`pop`][Vec::pop] on a pinned <code>[Vec]\<T></code> because that would move the (structurally +//! pinned) contents! Nor could it allow [`push`][Vec::push], which might reallocate and thus also +//! move the contents. +//! +//! A <code>[Vec]\<T></code> without structural pinning could +//! <code>impl\<T> [Unpin] for [Vec]\<T></code>, because the contents are never pinned +//! and the <code>[Vec]\<T></code> itself is fine with being moved as well. +//! At that point pinning just has no effect on the vector at all. +//! +//! In the standard library, pointer types generally do not have structural pinning, +//! and thus they do not offer pinning projections. This is why <code>[Box]\<T>: [Unpin]</code> +//! holds for all `T`. It makes sense to do this for pointer types, because moving the +//! <code>[Box]\<T></code> does not actually move the `T`: the <code>[Box]\<T></code> can be freely +//! movable (aka [`Unpin`]) even if the `T` is not. In fact, even <code>[Pin]<[Box]\<T>></code> and +//! <code>[Pin]<[&mut] T></code> are always [`Unpin`] themselves, for the same reason: +//! their contents (the `T`) are pinned, but the pointers themselves can be moved without moving +//! the pinned data. For both <code>[Box]\<T></code> and <code>[Pin]<[Box]\<T>></code>, +//! whether the content is pinned is entirely independent of whether the +//! pointer is pinned, meaning pinning is *not* structural. +//! +//! When implementing a [`Future`] combinator, you will usually need structural pinning +//! for the nested futures, as you need to get pinned references to them to call [`poll`]. +//! But if your combinator contains any other data that does not need to be pinned, +//! you can make those fields not structural and hence freely access them with a +//! mutable reference even when you just have <code>[Pin]<[&mut] Self></code> (such as in your own +//! [`poll`] implementation). +//! +//! [Deref]: crate::ops::Deref "ops::Deref" +//! [`Deref`]: crate::ops::Deref "ops::Deref" +//! [Target]: crate::ops::Deref::Target "ops::Deref::Target" +//! [`DerefMut`]: crate::ops::DerefMut "ops::DerefMut" +//! [`mem::swap`]: crate::mem::swap "mem::swap" +//! [`mem::forget`]: crate::mem::forget "mem::forget" +//! [Vec]: ../../std/vec/struct.Vec.html "Vec" +//! [`Vec::set_len`]: ../../std/vec/struct.Vec.html#method.set_len "Vec::set_len" +//! [Box]: ../../std/boxed/struct.Box.html "Box" +//! [Vec::pop]: ../../std/vec/struct.Vec.html#method.pop "Vec::pop" +//! [Vec::push]: ../../std/vec/struct.Vec.html#method.push "Vec::push" +//! [Rc]: ../../std/rc/struct.Rc.html "rc::Rc" +//! [RefCell]: crate::cell::RefCell "cell::RefCell" +//! [`drop`]: Drop::drop +//! [VecDeque]: ../../std/collections/struct.VecDeque.html "collections::VecDeque" +//! [`ptr::write`]: crate::ptr::write "ptr::write" +//! [`Future`]: crate::future::Future "future::Future" +//! [drop-impl]: #drop-implementation +//! [drop-guarantee]: #drop-guarantee +//! [`poll`]: crate::future::Future::poll "future::Future::poll" +//! [&]: reference "shared reference" +//! [&mut]: reference "mutable reference" +//! [`unsafe`]: ../../std/keyword.unsafe.html "keyword unsafe" + +#![stable(feature = "pin", since = "1.33.0")] + +use crate::cmp::{self, PartialEq, PartialOrd}; +use crate::fmt; +use crate::hash::{Hash, Hasher}; +use crate::marker::{Sized, Unpin}; +use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver}; + +/// A pinned pointer. +/// +/// This is a wrapper around a kind of pointer which makes that pointer "pin" its +/// value in place, preventing the value referenced by that pointer from being moved +/// unless it implements [`Unpin`]. +/// +/// *See the [`pin` module] documentation for an explanation of pinning.* +/// +/// [`pin` module]: self +// +// Note: the `Clone` derive below causes unsoundness as it's possible to implement +// `Clone` for mutable references. +// See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311> for more details. +#[stable(feature = "pin", since = "1.33.0")] +#[lang = "pin"] +#[fundamental] +#[repr(transparent)] +#[derive(Copy, Clone)] +pub struct Pin<P> { + // FIXME(#93176): this field is made `#[unstable] #[doc(hidden)] pub` to: + // - deter downstream users from accessing it (which would be unsound!), + // - let the `pin!` macro access it (such a macro requires using struct + // literal syntax in order to benefit from lifetime extension). + // Long-term, `unsafe` fields or macro hygiene are expected to offer more robust alternatives. + #[unstable(feature = "unsafe_pin_internals", issue = "none")] + #[doc(hidden)] + pub pointer: P, +} + +// The following implementations aren't derived in order to avoid soundness +// issues. `&self.pointer` should not be accessible to untrusted trait +// implementations. +// +// See <https://internals.rust-lang.org/t/unsoundness-in-pin/11311/73> for more details. + +#[stable(feature = "pin_trait_impls", since = "1.41.0")] +impl<P: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<P> +where + P::Target: PartialEq<Q::Target>, +{ + fn eq(&self, other: &Pin<Q>) -> bool { + P::Target::eq(self, other) + } + + fn ne(&self, other: &Pin<Q>) -> bool { + P::Target::ne(self, other) + } +} + +#[stable(feature = "pin_trait_impls", since = "1.41.0")] +impl<P: Deref<Target: Eq>> Eq for Pin<P> {} + +#[stable(feature = "pin_trait_impls", since = "1.41.0")] +impl<P: Deref, Q: Deref> PartialOrd<Pin<Q>> for Pin<P> +where + P::Target: PartialOrd<Q::Target>, +{ + fn partial_cmp(&self, other: &Pin<Q>) -> Option<cmp::Ordering> { + P::Target::partial_cmp(self, other) + } + + fn lt(&self, other: &Pin<Q>) -> bool { + P::Target::lt(self, other) + } + + fn le(&self, other: &Pin<Q>) -> bool { + P::Target::le(self, other) + } + + fn gt(&self, other: &Pin<Q>) -> bool { + P::Target::gt(self, other) + } + + fn ge(&self, other: &Pin<Q>) -> bool { + P::Target::ge(self, other) + } +} + +#[stable(feature = "pin_trait_impls", since = "1.41.0")] +impl<P: Deref<Target: Ord>> Ord for Pin<P> { + fn cmp(&self, other: &Self) -> cmp::Ordering { + P::Target::cmp(self, other) + } +} + +#[stable(feature = "pin_trait_impls", since = "1.41.0")] +impl<P: Deref<Target: Hash>> Hash for Pin<P> { + fn hash<H: Hasher>(&self, state: &mut H) { + P::Target::hash(self, state); + } +} + +impl<P: Deref<Target: Unpin>> Pin<P> { + /// Construct a new `Pin<P>` around a pointer to some data of a type that + /// implements [`Unpin`]. + /// + /// Unlike `Pin::new_unchecked`, this method is safe because the pointer + /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees. + #[inline(always)] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + #[stable(feature = "pin", since = "1.33.0")] + pub const fn new(pointer: P) -> Pin<P> { + // SAFETY: the value pointed to is `Unpin`, and so has no requirements + // around pinning. + unsafe { Pin::new_unchecked(pointer) } + } + + /// Unwraps this `Pin<P>` returning the underlying pointer. + /// + /// This requires that the data inside this `Pin` is [`Unpin`] so that we + /// can ignore the pinning invariants when unwrapping it. + #[inline(always)] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + #[stable(feature = "pin_into_inner", since = "1.39.0")] + pub const fn into_inner(pin: Pin<P>) -> P { + pin.pointer + } +} + +impl<P: Deref> Pin<P> { + /// Construct a new `Pin<P>` around a reference to some data of a type that + /// may or may not implement `Unpin`. + /// + /// If `pointer` dereferences to an `Unpin` type, `Pin::new` should be used + /// instead. + /// + /// # Safety + /// + /// This constructor is unsafe because we cannot guarantee that the data + /// pointed to by `pointer` is pinned, meaning that the data will not be moved or + /// its storage invalidated until it gets dropped. If the constructed `Pin<P>` does + /// not guarantee that the data `P` points to is pinned, that is a violation of + /// the API contract and may lead to undefined behavior in later (safe) operations. + /// + /// By using this method, you are making a promise about the `P::Deref` and + /// `P::DerefMut` implementations, if they exist. Most importantly, they + /// must not move out of their `self` arguments: `Pin::as_mut` and `Pin::as_ref` + /// will call `DerefMut::deref_mut` and `Deref::deref` *on the pinned pointer* + /// and expect these methods to uphold the pinning invariants. + /// Moreover, by calling this method you promise that the reference `P` + /// dereferences to will not be moved out of again; in particular, it + /// must not be possible to obtain a `&mut P::Target` and then + /// move out of that reference (using, for example [`mem::swap`]). + /// + /// For example, calling `Pin::new_unchecked` on an `&'a mut T` is unsafe because + /// while you are able to pin it for the given lifetime `'a`, you have no control + /// over whether it is kept pinned once `'a` ends: + /// ``` + /// use std::mem; + /// use std::pin::Pin; + /// + /// fn move_pinned_ref<T>(mut a: T, mut b: T) { + /// unsafe { + /// let p: Pin<&mut T> = Pin::new_unchecked(&mut a); + /// // This should mean the pointee `a` can never move again. + /// } + /// mem::swap(&mut a, &mut b); + /// // The address of `a` changed to `b`'s stack slot, so `a` got moved even + /// // though we have previously pinned it! We have violated the pinning API contract. + /// } + /// ``` + /// A value, once pinned, must remain pinned forever (unless its type implements `Unpin`). + /// + /// Similarly, calling `Pin::new_unchecked` on an `Rc<T>` is unsafe because there could be + /// aliases to the same data that are not subject to the pinning restrictions: + /// ``` + /// use std::rc::Rc; + /// use std::pin::Pin; + /// + /// fn move_pinned_rc<T>(mut x: Rc<T>) { + /// let pinned = unsafe { Pin::new_unchecked(Rc::clone(&x)) }; + /// { + /// let p: Pin<&T> = pinned.as_ref(); + /// // This should mean the pointee can never move again. + /// } + /// drop(pinned); + /// let content = Rc::get_mut(&mut x).unwrap(); + /// // Now, if `x` was the only reference, we have a mutable reference to + /// // data that we pinned above, which we could use to move it as we have + /// // seen in the previous example. We have violated the pinning API contract. + /// } + /// ``` + /// + /// [`mem::swap`]: crate::mem::swap + #[lang = "new_unchecked"] + #[inline(always)] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + #[stable(feature = "pin", since = "1.33.0")] + pub const unsafe fn new_unchecked(pointer: P) -> Pin<P> { + Pin { pointer } + } + + /// Gets a pinned shared reference from this pinned pointer. + /// + /// This is a generic method to go from `&Pin<Pointer<T>>` to `Pin<&T>`. + /// It is safe because, as part of the contract of `Pin::new_unchecked`, + /// the pointee cannot move after `Pin<Pointer<T>>` got created. + /// "Malicious" implementations of `Pointer::Deref` are likewise + /// ruled out by the contract of `Pin::new_unchecked`. + #[stable(feature = "pin", since = "1.33.0")] + #[inline(always)] + pub fn as_ref(&self) -> Pin<&P::Target> { + // SAFETY: see documentation on this function + unsafe { Pin::new_unchecked(&*self.pointer) } + } + + /// Unwraps this `Pin<P>` returning the underlying pointer. + /// + /// # Safety + /// + /// This function is unsafe. You must guarantee that you will continue to + /// treat the pointer `P` as pinned after you call this function, so that + /// the invariants on the `Pin` type can be upheld. If the code using the + /// resulting `P` does not continue to maintain the pinning invariants that + /// is a violation of the API contract and may lead to undefined behavior in + /// later (safe) operations. + /// + /// If the underlying data is [`Unpin`], [`Pin::into_inner`] should be used + /// instead. + #[inline(always)] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + #[stable(feature = "pin_into_inner", since = "1.39.0")] + pub const unsafe fn into_inner_unchecked(pin: Pin<P>) -> P { + pin.pointer + } +} + +impl<P: DerefMut> Pin<P> { + /// Gets a pinned mutable reference from this pinned pointer. + /// + /// This is a generic method to go from `&mut Pin<Pointer<T>>` to `Pin<&mut T>`. + /// It is safe because, as part of the contract of `Pin::new_unchecked`, + /// the pointee cannot move after `Pin<Pointer<T>>` got created. + /// "Malicious" implementations of `Pointer::DerefMut` are likewise + /// ruled out by the contract of `Pin::new_unchecked`. + /// + /// This method is useful when doing multiple calls to functions that consume the pinned type. + /// + /// # Example + /// + /// ``` + /// use std::pin::Pin; + /// + /// # struct Type {} + /// impl Type { + /// fn method(self: Pin<&mut Self>) { + /// // do something + /// } + /// + /// fn call_method_twice(mut self: Pin<&mut Self>) { + /// // `method` consumes `self`, so reborrow the `Pin<&mut Self>` via `as_mut`. + /// self.as_mut().method(); + /// self.as_mut().method(); + /// } + /// } + /// ``` + #[stable(feature = "pin", since = "1.33.0")] + #[inline(always)] + pub fn as_mut(&mut self) -> Pin<&mut P::Target> { + // SAFETY: see documentation on this function + unsafe { Pin::new_unchecked(&mut *self.pointer) } + } + + /// Assigns a new value to the memory behind the pinned reference. + /// + /// This overwrites pinned data, but that is okay: its destructor gets + /// run before being overwritten, so no pinning guarantee is violated. + #[stable(feature = "pin", since = "1.33.0")] + #[inline(always)] + pub fn set(&mut self, value: P::Target) + where + P::Target: Sized, + { + *(self.pointer) = value; + } +} + +impl<'a, T: ?Sized> Pin<&'a T> { + /// Constructs a new pin by mapping the interior value. + /// + /// For example, if you wanted to get a `Pin` of a field of something, + /// you could use this to get access to that field in one line of code. + /// However, there are several gotchas with these "pinning projections"; + /// see the [`pin` module] documentation for further details on that topic. + /// + /// # Safety + /// + /// This function is unsafe. You must guarantee that the data you return + /// will not move so long as the argument value does not move (for example, + /// because it is one of the fields of that value), and also that you do + /// not move out of the argument you receive to the interior function. + /// + /// [`pin` module]: self#projections-and-structural-pinning + #[stable(feature = "pin", since = "1.33.0")] + pub unsafe fn map_unchecked<U, F>(self, func: F) -> Pin<&'a U> + where + U: ?Sized, + F: FnOnce(&T) -> &U, + { + let pointer = &*self.pointer; + let new_pointer = func(pointer); + + // SAFETY: the safety contract for `new_unchecked` must be + // upheld by the caller. + unsafe { Pin::new_unchecked(new_pointer) } + } + + /// Gets a shared reference out of a pin. + /// + /// This is safe because it is not possible to move out of a shared reference. + /// It may seem like there is an issue here with interior mutability: in fact, + /// it *is* possible to move a `T` out of a `&RefCell<T>`. However, this is + /// not a problem as long as there does not also exist a `Pin<&T>` pointing + /// to the same data, and `RefCell<T>` does not let you create a pinned reference + /// to its contents. See the discussion on ["pinning projections"] for further + /// details. + /// + /// Note: `Pin` also implements `Deref` to the target, which can be used + /// to access the inner value. However, `Deref` only provides a reference + /// that lives for as long as the borrow of the `Pin`, not the lifetime of + /// the `Pin` itself. This method allows turning the `Pin` into a reference + /// with the same lifetime as the original `Pin`. + /// + /// ["pinning projections"]: self#projections-and-structural-pinning + #[inline(always)] + #[must_use] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + #[stable(feature = "pin", since = "1.33.0")] + pub const fn get_ref(self) -> &'a T { + self.pointer + } +} + +impl<'a, T: ?Sized> Pin<&'a mut T> { + /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime. + #[inline(always)] + #[must_use = "`self` will be dropped if the result is not used"] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + #[stable(feature = "pin", since = "1.33.0")] + pub const fn into_ref(self) -> Pin<&'a T> { + Pin { pointer: self.pointer } + } + + /// Gets a mutable reference to the data inside of this `Pin`. + /// + /// This requires that the data inside this `Pin` is `Unpin`. + /// + /// Note: `Pin` also implements `DerefMut` to the data, which can be used + /// to access the inner value. However, `DerefMut` only provides a reference + /// that lives for as long as the borrow of the `Pin`, not the lifetime of + /// the `Pin` itself. This method allows turning the `Pin` into a reference + /// with the same lifetime as the original `Pin`. + #[inline(always)] + #[must_use = "`self` will be dropped if the result is not used"] + #[stable(feature = "pin", since = "1.33.0")] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + pub const fn get_mut(self) -> &'a mut T + where + T: Unpin, + { + self.pointer + } + + /// Gets a mutable reference to the data inside of this `Pin`. + /// + /// # Safety + /// + /// This function is unsafe. You must guarantee that you will never move + /// the data out of the mutable reference you receive when you call this + /// function, so that the invariants on the `Pin` type can be upheld. + /// + /// If the underlying data is `Unpin`, `Pin::get_mut` should be used + /// instead. + #[inline(always)] + #[must_use = "`self` will be dropped if the result is not used"] + #[stable(feature = "pin", since = "1.33.0")] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + pub const unsafe fn get_unchecked_mut(self) -> &'a mut T { + self.pointer + } + + /// Construct a new pin by mapping the interior value. + /// + /// For example, if you wanted to get a `Pin` of a field of something, + /// you could use this to get access to that field in one line of code. + /// However, there are several gotchas with these "pinning projections"; + /// see the [`pin` module] documentation for further details on that topic. + /// + /// # Safety + /// + /// This function is unsafe. You must guarantee that the data you return + /// will not move so long as the argument value does not move (for example, + /// because it is one of the fields of that value), and also that you do + /// not move out of the argument you receive to the interior function. + /// + /// [`pin` module]: self#projections-and-structural-pinning + #[must_use = "`self` will be dropped if the result is not used"] + #[stable(feature = "pin", since = "1.33.0")] + pub unsafe fn map_unchecked_mut<U, F>(self, func: F) -> Pin<&'a mut U> + where + U: ?Sized, + F: FnOnce(&mut T) -> &mut U, + { + // SAFETY: the caller is responsible for not moving the + // value out of this reference. + let pointer = unsafe { Pin::get_unchecked_mut(self) }; + let new_pointer = func(pointer); + // SAFETY: as the value of `this` is guaranteed to not have + // been moved out, this call to `new_unchecked` is safe. + unsafe { Pin::new_unchecked(new_pointer) } + } +} + +impl<T: ?Sized> Pin<&'static T> { + /// Get a pinned reference from a static reference. + /// + /// This is safe, because `T` is borrowed for the `'static` lifetime, which + /// never ends. + #[stable(feature = "pin_static_ref", since = "1.61.0")] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + pub const fn static_ref(r: &'static T) -> Pin<&'static T> { + // SAFETY: The 'static borrow guarantees the data will not be + // moved/invalidated until it gets dropped (which is never). + unsafe { Pin::new_unchecked(r) } + } +} + +impl<'a, P: DerefMut> Pin<&'a mut Pin<P>> { + /// Gets a pinned mutable reference from this nested pinned pointer. + /// + /// This is a generic method to go from `Pin<&mut Pin<Pointer<T>>>` to `Pin<&mut T>`. It is + /// safe because the existence of a `Pin<Pointer<T>>` ensures that the pointee, `T`, cannot + /// move in the future, and this method does not enable the pointee to move. "Malicious" + /// implementations of `P::DerefMut` are likewise ruled out by the contract of + /// `Pin::new_unchecked`. + #[unstable(feature = "pin_deref_mut", issue = "86918")] + #[must_use = "`self` will be dropped if the result is not used"] + #[inline(always)] + pub fn as_deref_mut(self) -> Pin<&'a mut P::Target> { + // SAFETY: What we're asserting here is that going from + // + // Pin<&mut Pin<P>> + // + // to + // + // Pin<&mut P::Target> + // + // is safe. + // + // We need to ensure that two things hold for that to be the case: + // + // 1) Once we give out a `Pin<&mut P::Target>`, an `&mut P::Target` will not be given out. + // 2) By giving out a `Pin<&mut P::Target>`, we do not risk of violating `Pin<&mut Pin<P>>` + // + // The existence of `Pin<P>` is sufficient to guarantee #1: since we already have a + // `Pin<P>`, it must already uphold the pinning guarantees, which must mean that + // `Pin<&mut P::Target>` does as well, since `Pin::as_mut` is safe. We do not have to rely + // on the fact that P is _also_ pinned. + // + // For #2, we need to ensure that code given a `Pin<&mut P::Target>` cannot cause the + // `Pin<P>` to move? That is not possible, since `Pin<&mut P::Target>` no longer retains + // any access to the `P` itself, much less the `Pin<P>`. + unsafe { self.get_unchecked_mut() }.as_mut() + } +} + +impl<T: ?Sized> Pin<&'static mut T> { + /// Get a pinned mutable reference from a static mutable reference. + /// + /// This is safe, because `T` is borrowed for the `'static` lifetime, which + /// never ends. + #[stable(feature = "pin_static_ref", since = "1.61.0")] + #[rustc_const_unstable(feature = "const_pin", issue = "76654")] + pub const fn static_mut(r: &'static mut T) -> Pin<&'static mut T> { + // SAFETY: The 'static borrow guarantees the data will not be + // moved/invalidated until it gets dropped (which is never). + unsafe { Pin::new_unchecked(r) } + } +} + +#[stable(feature = "pin", since = "1.33.0")] +impl<P: Deref> Deref for Pin<P> { + type Target = P::Target; + fn deref(&self) -> &P::Target { + Pin::get_ref(Pin::as_ref(self)) + } +} + +#[stable(feature = "pin", since = "1.33.0")] +impl<P: DerefMut<Target: Unpin>> DerefMut for Pin<P> { + fn deref_mut(&mut self) -> &mut P::Target { + Pin::get_mut(Pin::as_mut(self)) + } +} + +#[unstable(feature = "receiver_trait", issue = "none")] +impl<P: Receiver> Receiver for Pin<P> {} + +#[stable(feature = "pin", since = "1.33.0")] +impl<P: fmt::Debug> fmt::Debug for Pin<P> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Debug::fmt(&self.pointer, f) + } +} + +#[stable(feature = "pin", since = "1.33.0")] +impl<P: fmt::Display> fmt::Display for Pin<P> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Display::fmt(&self.pointer, f) + } +} + +#[stable(feature = "pin", since = "1.33.0")] +impl<P: fmt::Pointer> fmt::Pointer for Pin<P> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt::Pointer::fmt(&self.pointer, f) + } +} + +// Note: this means that any impl of `CoerceUnsized` that allows coercing from +// a type that impls `Deref<Target=impl !Unpin>` to a type that impls +// `Deref<Target=Unpin>` is unsound. Any such impl would probably be unsound +// for other reasons, though, so we just need to take care not to allow such +// impls to land in std. +#[stable(feature = "pin", since = "1.33.0")] +impl<P, U> CoerceUnsized<Pin<U>> for Pin<P> where P: CoerceUnsized<U> {} + +#[stable(feature = "pin", since = "1.33.0")] +impl<P, U> DispatchFromDyn<Pin<U>> for Pin<P> where P: DispatchFromDyn<U> {} + +/// Constructs a <code>[Pin]<[&mut] T></code>, by pinning[^1] a `value: T` _locally_[^2]. +/// +/// Unlike [`Box::pin`], this does not involve a heap allocation. +/// +/// [^1]: If the (type `T` of the) given value does not implement [`Unpin`], then this +/// effectively pins the `value` in memory, where it will be unable to be moved. +/// Otherwise, <code>[Pin]<[&mut] T></code> behaves like <code>[&mut] T</code>, and operations such +/// as [`mem::replace()`][crate::mem::replace] will allow extracting that value, and therefore, +/// moving it. +/// See [the `Unpin` section of the `pin` module][self#unpin] for more info. +/// +/// [^2]: This is usually dubbed "stack"-pinning. And whilst local values are almost always located +/// in the stack (_e.g._, when within the body of a non-`async` function), the truth is that inside +/// the body of an `async fn` or block —more generally, the body of a generator— any locals crossing +/// an `.await` point —a `yield` point— end up being part of the state captured by the `Future` —by +/// the `Generator`—, and thus will be stored wherever that one is. +/// +/// ## Examples +/// +/// ### Basic usage +/// +/// ```rust +/// #![feature(pin_macro)] +/// # use core::marker::PhantomPinned as Foo; +/// use core::pin::{pin, Pin}; +/// +/// fn stuff(foo: Pin<&mut Foo>) { +/// // … +/// # let _ = foo; +/// } +/// +/// let pinned_foo = pin!(Foo { /* … */ }); +/// stuff(pinned_foo); +/// // or, directly: +/// stuff(pin!(Foo { /* … */ })); +/// ``` +/// +/// ### Manually polling a `Future` (without `Unpin` bounds) +/// +/// ```rust +/// #![feature(pin_macro)] +/// use std::{ +/// future::Future, +/// pin::pin, +/// task::{Context, Poll}, +/// thread, +/// }; +/// # use std::{sync::Arc, task::Wake, thread::Thread}; +/// +/// # /// A waker that wakes up the current thread when called. +/// # struct ThreadWaker(Thread); +/// # +/// # impl Wake for ThreadWaker { +/// # fn wake(self: Arc<Self>) { +/// # self.0.unpark(); +/// # } +/// # } +/// # +/// /// Runs a future to completion. +/// fn block_on<Fut: Future>(fut: Fut) -> Fut::Output { +/// let waker_that_unparks_thread = // … +/// # Arc::new(ThreadWaker(thread::current())).into(); +/// let mut cx = Context::from_waker(&waker_that_unparks_thread); +/// // Pin the future so it can be polled. +/// let mut pinned_fut = pin!(fut); +/// loop { +/// match pinned_fut.as_mut().poll(&mut cx) { +/// Poll::Pending => thread::park(), +/// Poll::Ready(res) => return res, +/// } +/// } +/// } +/// # +/// # assert_eq!(42, block_on(async { 42 })); +/// ``` +/// +/// ### With `Generator`s +/// +/// ```rust +/// #![feature(generators, generator_trait, pin_macro)] +/// use core::{ +/// ops::{Generator, GeneratorState}, +/// pin::pin, +/// }; +/// +/// fn generator_fn() -> impl Generator<Yield = usize, Return = ()> /* not Unpin */ { +/// // Allow generator to be self-referential (not `Unpin`) +/// // vvvvvv so that locals can cross yield points. +/// static || { +/// let foo = String::from("foo"); +/// let foo_ref = &foo; // ------+ +/// yield 0; // | <- crosses yield point! +/// println!("{foo_ref}"); // <--+ +/// yield foo.len(); +/// } +/// } +/// +/// fn main() { +/// let mut generator = pin!(generator_fn()); +/// match generator.as_mut().resume(()) { +/// GeneratorState::Yielded(0) => {}, +/// _ => unreachable!(), +/// } +/// match generator.as_mut().resume(()) { +/// GeneratorState::Yielded(3) => {}, +/// _ => unreachable!(), +/// } +/// match generator.resume(()) { +/// GeneratorState::Yielded(_) => unreachable!(), +/// GeneratorState::Complete(()) => {}, +/// } +/// } +/// ``` +/// +/// ## Remarks +/// +/// Precisely because a value is pinned to local storage, the resulting <code>[Pin]<[&mut] T></code> +/// reference ends up borrowing a local tied to that block: it can't escape it. +/// +/// The following, for instance, fails to compile: +/// +/// ```rust,compile_fail +/// #![feature(pin_macro)] +/// use core::pin::{pin, Pin}; +/// # use core::{marker::PhantomPinned as Foo, mem::drop as stuff}; +/// +/// let x: Pin<&mut Foo> = { +/// let x: Pin<&mut Foo> = pin!(Foo { /* … */ }); +/// x +/// }; // <- Foo is dropped +/// stuff(x); // Error: use of dropped value +/// ``` +/// +/// <details><summary>Error message</summary> +/// +/// ```console +/// error[E0716]: temporary value dropped while borrowed +/// --> src/main.rs:9:28 +/// | +/// 8 | let x: Pin<&mut Foo> = { +/// | - borrow later stored here +/// 9 | let x: Pin<&mut Foo> = pin!(Foo { /* … */ }); +/// | ^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use +/// 10 | x +/// 11 | }; // <- Foo is dropped +/// | - temporary value is freed at the end of this statement +/// | +/// = note: consider using a `let` binding to create a longer lived value +/// ``` +/// +/// </details> +/// +/// This makes [`pin!`] **unsuitable to pin values when intending to _return_ them**. Instead, the +/// value is expected to be passed around _unpinned_ until the point where it is to be consumed, +/// where it is then useful and even sensible to pin the value locally using [`pin!`]. +/// +/// If you really need to return a pinned value, consider using [`Box::pin`] instead. +/// +/// On the other hand, pinning to the stack[<sup>2</sup>](#fn2) using [`pin!`] is likely to be +/// cheaper than pinning into a fresh heap allocation using [`Box::pin`]. Moreover, by virtue of not +/// even needing an allocator, [`pin!`] is the main non-`unsafe` `#![no_std]`-compatible [`Pin`] +/// constructor. +/// +/// [`Box::pin`]: ../../std/boxed/struct.Box.html#method.pin +#[unstable(feature = "pin_macro", issue = "93178")] +#[rustc_macro_transparency = "semitransparent"] +#[allow_internal_unstable(unsafe_pin_internals)] +pub macro pin($value:expr $(,)?) { + // This is `Pin::new_unchecked(&mut { $value })`, so, for starters, let's + // review such a hypothetical macro (that any user-code could define): + // + // ```rust + // macro_rules! pin {( $value:expr ) => ( + // match &mut { $value } { at_value => unsafe { // Do not wrap `$value` in an `unsafe` block. + // $crate::pin::Pin::<&mut _>::new_unchecked(at_value) + // }} + // )} + // ``` + // + // Safety: + // - `type P = &mut _`. There are thus no pathological `Deref{,Mut}` impls + // that would break `Pin`'s invariants. + // - `{ $value }` is braced, making it a _block expression_, thus **moving** + // the given `$value`, and making it _become an **anonymous** temporary_. + // By virtue of being anonymous, it can no longer be accessed, thus + // preventing any attempts to `mem::replace` it or `mem::forget` it, _etc._ + // + // This gives us a `pin!` definition that is sound, and which works, but only + // in certain scenarios: + // - If the `pin!(value)` expression is _directly_ fed to a function call: + // `let poll = pin!(fut).poll(cx);` + // - If the `pin!(value)` expression is part of a scrutinee: + // ```rust + // match pin!(fut) { pinned_fut => { + // pinned_fut.as_mut().poll(...); + // pinned_fut.as_mut().poll(...); + // }} // <- `fut` is dropped here. + // ``` + // Alas, it doesn't work for the more straight-forward use-case: `let` bindings. + // ```rust + // let pinned_fut = pin!(fut); // <- temporary value is freed at the end of this statement + // pinned_fut.poll(...) // error[E0716]: temporary value dropped while borrowed + // // note: consider using a `let` binding to create a longer lived value + // ``` + // - Issues such as this one are the ones motivating https://github.com/rust-lang/rfcs/pull/66 + // + // This makes such a macro incredibly unergonomic in practice, and the reason most macros + // out there had to take the path of being a statement/binding macro (_e.g._, `pin!(future);`) + // instead of featuring the more intuitive ergonomics of an expression macro. + // + // Luckily, there is a way to avoid the problem. Indeed, the problem stems from the fact that a + // temporary is dropped at the end of its enclosing statement when it is part of the parameters + // given to function call, which has precisely been the case with our `Pin::new_unchecked()`! + // For instance, + // ```rust + // let p = Pin::new_unchecked(&mut <temporary>); + // ``` + // becomes: + // ```rust + // let p = { let mut anon = <temporary>; &mut anon }; + // ``` + // + // However, when using a literal braced struct to construct the value, references to temporaries + // can then be taken. This makes Rust change the lifespan of such temporaries so that they are, + // instead, dropped _at the end of the enscoping block_. + // For instance, + // ```rust + // let p = Pin { pointer: &mut <temporary> }; + // ``` + // becomes: + // ```rust + // let mut anon = <temporary>; + // let p = Pin { pointer: &mut anon }; + // ``` + // which is *exactly* what we want. + // + // See https://doc.rust-lang.org/1.58.1/reference/destructors.html#temporary-lifetime-extension + // for more info. + $crate::pin::Pin::<&mut _> { pointer: &mut { $value } } +} |