Fix stray spacing of code blocks containing injected main

[dtolnay]

For example see the first code block in https://doc.rust-lang.org/1.47.0/book/ch03-02-data-types.html.

```rust
let guess: u32 = "42".parse().expect("Not a number!");
```

Before:

After:

[ehuss]

Hm, so I'm uncertain about this change. There's a problem that on narrower screens, the buttons will overlap the code (see #1322). I think this could make the problem worse. I would kinda like to have a solution for the overlap problem first before deciding what to do here.

I'm uncertain if it makes sense to have a blank space on every example? Some other docs sites do that (like docusaurus).

Another option is to hide the buttons with a mouseover (like bookdown), but that doesn't work on mobile, and may not be discoverable and too magical.

Another option would be to hang the icons off the right side, but I don't know what that would look like, or if there would be enough space.

I don't really know what would be a good approach.

[dtolnay]

Good call.

I since realized I do not want buttons anyway on my site (​```rust,noplayground & playground = { copyable = false }) and without buttons the blank line does not appear, so I no longer need this -- feel free to close.

[pickfire]

Is it possible to use a css based approach to do some padding on small screens?

Originally, I opened an issue in rust-unofficial/patterns#155. I was searching for issues in this repo and came across this. I believe this would not look so good on desktop or even tablets, maybe very small screens with very long first line would be affected by this, I don't think I have seen any in the wild. Even if I did, it is less annoying than the extra space which leads to me searching issues in this repo.

[Dylan-DPC-zz]

closing this, we can discuss other approaches on the issue or in a new pr

[dtolnay]

I filed #1433 to track in mdBook.