Trait core::marker::Unpin

1.33.0 · source · [−]
pub auto trait Unpin { }
Expand description

Types that can be safely moved after being pinned.

Rust itself has no notion of immovable types, and considers moves (e.g., through assignment or mem::replace) to always be safe.

The Pin type is used instead to prevent moves through the type system. Pointers P<T> wrapped in the Pin<P<T>> wrapper can’t be moved out of. See the pin module documentation for more information on pinning.

Implementing the Unpin trait for T lifts the restrictions of pinning off the type, which then allows moving T out of Pin<P<T>> with functions such as mem::replace.

Unpin has no consequence at all for non-pinned data. In particular, mem::replace happily moves !Unpin data (it works for any &mut T, not just when T: Unpin). However, you cannot use mem::replace on data wrapped inside a Pin<P<T>> because you cannot get the &mut T you need for that, and that is what makes this system work.

So this, for example, can only be done on types implementing Unpin:

use std::mem;
use std::pin::Pin;

let mut string = "this".to_string();
let mut pinned_string = Pin::new(&mut string);

// We need a mutable reference to call `mem::replace`.
// We can obtain such a reference by (implicitly) invoking `Pin::deref_mut`,
// but that is only possible because `String` implements `Unpin`.
mem::replace(&mut *pinned_string, "other".to_string());
Run

This trait is automatically implemented for almost every type.

Implementors

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