Auto merge of #28265 - boblehest:master, r=bluss

In the last code snippet on the following page there is a bug in the
implementation of Vec::drain().

https://doc.rust-lang.org/nightly/nomicon/vec-drain.html

```rust
pub fn drain(&mut self) -> Drain<T> {
    // Oops, setting it to 0 while we still need the old value!
    self.len = 0;

    unsafe {
        Drain {
            // len is used to create a &[T] from &self here,
            // so we end up always creating an empty slice.
            iter: RawValIter::new(&self),
            vec: PhantomData,
        }
    }
}
```

A simple test to verify that Drain is broken can be found here:
https://play.rust-lang.org/?gist=30f579565e4bbf4836ce&version=nightly

And here's one with a fixed implementation:
https://play.rust-lang.org/?gist=2ec0c1a6dcf5defd7a53&version=nightly

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