Trait core::marker::Unpin1.33.0[−][src]

pub auto trait Unpin { }
This is supported on non-crate feature miri-test-libstd or test or doctest only.
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.

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