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Rollup merge of #111065 - est31:send_mut_ref, r=m-ou-se

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Explicitly document how Send and Sync relate to references

Some of these relations were already mentioned in the text, but that Send is implemented for &mut impl Send was not mentioned, neither did the docs list when &T is Sync. Inspired by the discussion in #110961.

[Proof](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ed77bfc3c77ba664400ebc2734f500e6) based on `@lukas-code` 's [example](https://github.com/rust-lang/rust/pull/110961#discussion_r1181220662).
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Matthias Krüger 2023-05-01 17:10:25 +02:00 committed by GitHub
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7
library/core/src/marker.rs
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@ -24,7 +24,7 @@ use crate::hash::Hasher;
/// operations. Its cousin [`sync::Arc`][arc] does use atomic operations (incurring
/// some overhead) and thus is `Send`.
///
/// See [the Nomicon](../../nomicon/send-and-sync.html) for more details.
/// See [the Nomicon](../../nomicon/send-and-sync.html) and the [`Sync`] trait for more details.
///
/// [`Rc`]: ../../std/rc/struct.Rc.html
/// [arc]: ../../std/sync/struct.Arc.html
@ -426,6 +426,11 @@ pub macro Copy($item:item) {
/// becomes read-only, as if it were a `& &T`. Hence there is no risk
/// of a data race.
///
/// A shorter overview of how [`Sync`] and [`Send`] relate to referencing:
/// * `&T` is [`Send`] if and only if `T` is [`Sync`]
/// * `&mut T` is [`Send`] if and only if `T` is [`Send`]
/// * `&T` and `&mut T` are [`Sync`] if and only if `T` is [`Sync`]
///
/// Types that are not `Sync` are those that have "interior
/// mutability" in a non-thread-safe form, such as [`Cell`][cell]
/// and [`RefCell`][refcell]. These types allow for mutation of