//! `Send` and `Sync`
//!
//!   - **Date:** January 31, 2018
//!   - **Subject:** The “marker traits” that Rust uses for safe concurrency.
//!   - [**Audio**][mp3]
//!
//! [mp3]: https://www.podtrac.com/pts/redirect.mp3/cdn.newrustacean.com/file/newrustacean/e022.mp3
//!
//! <audio style="width: 100%" title="e022: Send and Sync" controls preload=metadata><source src="https://www.podtrac.com/pts/redirect.mp3/cdn.newrustacean.com/file/newrustacean/e022.mp3"></audio>
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use std::thread;

#[doc = include_str!("../docs/e022-script.md")]
pub struct Script;

/// How `Send` and `Sync` work
///
/// Here we have a function which spawns a thread to run `println!()`. This is
/// silly, of course, but it works because (and only because!) `String`
/// implements the `Send` trait.
///
/// ```
/// # use std::thread;
/// pub fn demo_send() {
///     let foo = String::from("Hallo!");
///     thread::spawn(move || {
///         println!("{}", foo);
///     });
/// }
/// ```
///
/// If we had a *non-`Send` type here – e.g. an `Rc<String>` instead of just a
/// `String` – we would get a compiler error:
///
/// ```compile_fail
/// # use std::thread;
/// # use std::rc::Rc;
/// pub fn will_fail() {
///     let foo = Rc::new(String::from("Hallo!"));
///     thread::spawn(|| {
///         println!("{:?}", foo);
///     });
/// }
/// ```
///
/// Instead, you'll get a compiler error:
///
/// ```plain
/// error[E0277]: the trait bound `std::rc::Rc<std::string::String>: std::marker::Sync` is not satisfied
///    --> src/e022.rs:214:5    |
/// 214 |     thread::spawn(|| {
///     |     ^^^^^^^^^^^^^ `std::rc::Rc<std::string::String>` cannot be shared between threads safely
///     |
///     = help: the trait `std::marker::Sync` is not implemented for `std::rc::Rc<std::string::String>`
///     = note: required because of the requirements on the impl of `std::marker::Send` for `&std::rc::Rc<std::string::String>`
///     = note: required because it appears within the type `[closure@src/e022.rs:214:19: 216:6 foo:&std::rc::Rc<std::string::String>]`
///     = note: required by `std::thread::spawn`
///```
///
/// Notice that it's common to see those errors together, and notice moreover
/// that the error here includes *both* `Send` and `Sync`. The compiler tries to
/// see if `foo` can be taken via reference, but the `Rc` type is not `Sync`,
/// i.e. you cannot share references to it across threads, and that's so because
/// it is not `Send`. (Strictly speaking, all `Sync` types are `Send`, but not
/// all `Send` types must be `Sync`, though off the top of my head I can't think
/// of a scenario where a type *would* be `Send` but *not* also `Sync`.)
///
/// What's somewhat curious is that there really isn't a lot more than this to
/// demo here! To get into anything more interesting in terms of
/// *implementation* related to these traits, we'd have to be off in `unsafe`
/// land, and we haven't talked about `unsafe` at *all* yet, so we're not going
/// to do that today. However, if you want to see a good example of what makes
/// for code that *can* be safely shared across threads, take a look at the
/// implementation of [`Vec`] and the [`RawVec`] type it's built on! Reading the
/// standard library is a really good way to learn things about Rust, and it's
/// surprisingly straightforward most of the time.
///
/// [`Vec`]: https://github.com/rust-lang/rust/blob/master/src/liballoc/vec.rs
/// [`RawVec`]: https://github.com/rust-lang/rust/blob/master/src/liballoc/raw_vec.rs
pub fn demo_send_and_sync() {
    let some_value = 12;
    thread::spawn(move || {
        println!("{}", some_value);
    });
}