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Remove reference to NoSend in concurrency chapter of the book #23180

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Remove reference to NoSend in concurrency chapter of the book #23180

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11 changes: 2 additions & 9 deletions src/doc/trpl/concurrency.md
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Expand Up @@ -223,15 +223,8 @@ method which has this signature:
fn lock(&self) -> LockResult<MutexGuard<T>>
```

If we [look at the code for MutexGuard](https://github.com/rust-lang/rust/blob/ca4b9674c26c1de07a2042cb68e6a062d7184cef/src/libstd/sync/mutex.rs#L172), we'll see
this:

```ignore
__marker: marker::NoSend,
```

Because our guard is `NoSend`, it's not `Send`. Which means we can't actually
transfer the guard across thread boundaries, which gives us our error.
Because `Send` is not implemented for `MutexGuard<T>`, we can't transfer the
guard across thread boundaries, which gives us our error.

We can use `Arc<T>` to fix this. Here's the working version:

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