diff options
| -rw-r--r-- | library/core/src/pin.rs | 83 |
1 files changed, 44 insertions, 39 deletions
diff --git a/library/core/src/pin.rs b/library/core/src/pin.rs index 2a8a127b6ca..52c1c1975cc 100644 --- a/library/core/src/pin.rs +++ b/library/core/src/pin.rs @@ -14,12 +14,12 @@ //! 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 `&mut T` allow replacing and +//! 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 `&mut T`. However, <code>[Pin]\<P></code> does -//! not let clients actually obtain a <code>[Box]\<T></code> or `&mut T` to pinned data, which implies that you +//! 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`]: //! //! ``` @@ -35,12 +35,12 @@ //! 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 `&mut T` on them +//! 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 `P: Deref` should be considered -//! as a "`P`-style pointer" to a pinned `P::Target` -- so, a <code>[Pin]<[Box]\<T>></code> is +//! [`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 @@ -53,11 +53,11 @@ //! 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 `T: Unpin`, -//! <code>[Pin]<[Box]\<T>></code> and <code>[Box]\<T></code> function identically, as do <code>[Pin]<&mut T></code> and -//! `&mut T`. +//! 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 `P::Target`, not the pointer +//! 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). @@ -65,7 +65,7 @@ //! # Example: self-referential struct //! //! Before we go into more details to explain the guarantees and choices -//! associated with `Pin<T>`, we discuss some examples for how it might be used. +//! 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 @@ -165,18 +165,18 @@ //! # `Drop` implementation //! //! If your type uses pinning (such as the two examples above), you have to be careful -//! when implementing [`Drop`]. The [`drop`] function takes `&mut self`, but this +//! when implementing [`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`] +//! <code>[Pin]<[&]Self></code> or <code>[Pin]<[&mut] Self></code>) has consequences for your [`Drop`] //! implementation as well: if an element of your type could have been pinned, -//! you must treat [`Drop`] as implicitly taking <code>[Pin]<&mut Self></code>. +//! you must treat [`Drop`] as implicitly taking <code>[Pin]<[&mut] Self></code>. //! -//! For example, you could implement `Drop` as follows: +//! For example, you could implement [`Drop`] as follows: //! //! ```rust,no_run //! # use std::pin::Pin; @@ -204,10 +204,10 @@ //! # 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>. +//! 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 `&mut Field`? +//! 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 @@ -215,7 +215,7 @@ //! //! 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 `&mut Field`. There are some +//! <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, @@ -230,12 +230,12 @@ //! ## 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, +//! 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 `&mut Field`: +//! <code>[Pin]<[&mut] Struct></code> into <code>[&mut] Field</code>: //! //! ```rust,no_run //! # use std::pin::Pin; @@ -249,16 +249,16 @@ //! } //! ``` //! -//! You may also `impl Unpin for Struct` *even if* the type of `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. +//! 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 +//! This allows writing a projection that creates a <code>[Pin]<[&mut] Field></code>, thus //! witnessing that the field is pinned: //! //! ```rust,no_run @@ -278,12 +278,12 @@ //! 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 -//! `impl<T> Unpin for Struct<T>`. (Notice that adding a projection operation +//! <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`.) +//! 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 -//! `&mut self`, but the struct (and hence its fields) might have been pinned before. +//! 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`] implementation. //! In particular, as explained previously, this means that your struct must *not* //! be `#[repr(packed)]`. @@ -299,13 +299,13 @@ //! 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`-like operation with type -//! `fn(Pin<&mut Struct<T>>) -> Option<T>`, -//! that operation can be used to move a `T` out of a pinned `Struct<T>` -- which means +//! <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 `fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T>`. +//! 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>>) { @@ -316,7 +316,7 @@ //! } //! ``` //! This is catastrophic, it means we can first pin the content of the <code>[RefCell]\<T></code> -//! (using `RefCell::get_pin_mut`) and then move that content using the mutable +//! (using <code>[RefCell]::get_pin_mut</code>) and then move that content using the mutable //! reference we got later. //! //! ## Examples @@ -328,15 +328,15 @@ //! 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 `impl<T> Unpin for Vec<T>`, because 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 `Box<T>: Unpin` holds for all `T`. -//! It makes sense to do this for pointer types, because moving the `Box<T>` -//! 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 +//! 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 @@ -346,10 +346,12 @@ //! 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 +//! mutable reference even when you just have <code>[Pin]<[&mut] Self></code> (such as in your own //! [`poll`] implementation). //! +//! [Deref]: crate::ops::Deref //! [`Deref`]: crate::ops::Deref +//! [Target]: crate::ops::Deref::Target //! [`DerefMut`]: crate::ops::DerefMut //! [`mem::swap`]: crate::mem::swap //! [`mem::forget`]: crate::mem::forget @@ -367,6 +369,9 @@ //! [drop-impl]: #drop-implementation //! [drop-guarantee]: #drop-guarantee //! [`poll`]: crate::future::Future::poll +//! [&]: ../../std/primitive.reference.html +//! [&mut]: ../../std/primitive.reference.html +//! [`unsafe`]: ../../std/keyword.unsafe.html #![stable(feature = "pin", since = "1.33.0")] |