Add rustX check to codeblock attributes lint

[GuillaumeGomez]

We discovered this issue here.

I assume that the issue will be present in other places outside of the compiler so it's worth adding a check for it.

First commit is just a small cleanup about variables creation which was a bit strange (at least more than necessary).

r? @notriddle

[GuillaumeGomez]

Thanks for the suggestion @Urgau!

[GuillaumeGomez]

Fixed the error.

[notriddle]

What happens if you write two bad attributes in a row?

```rust2018 shouldpanic
panic!();
```

[GuillaumeGomez]

This:

warning: unknown attribute `rust2018`
 --> foo.rs:3:1
  |
3 | / /**
4 | | ```rust2018 shouldpanic
5 | | panic!();
6 | | ```
7 | | */
  | |__^
  |
  = help: there is an attribute with a similar name: `edition2018`
  = note: `#[warn(rustdoc::invalid_codeblock_attributes)]` on by default

warning: unknown attribute `shouldpanic`
 --> foo.rs:3:1
  |
3 | / /**
4 | | ```rust2018 shouldpanic
5 | | panic!();
6 | | ```
7 | | */
  | |__^
  |
  = help: there is an attribute with a similar name: `should_panic`
  = help: the code block will either not be tested if not marked as a rust one or won't fail if it doesn't panic when running

warning: 2 warnings emitted

[notriddle]

Other than the above comment about combining multiple mistakes, this seems like a great diagnostic improvement. Thanks!

Add a test case for combining them, then r=me

[GuillaumeGomez]

@bors r=notriddle rollup

[bors]

📌 Commit 58327c1 has been approved by notriddle

It is now in the queue for this repository.