`empty_line_after_outer_attribute` produced a false positive warning when
deriving `Copy` and/or `Clone` for an item.
It looks like the second point in [this comment][that_comment] is related,
as the attribute that causes the false positive has a path of
`rustc_copy_clone_marker`.
Fixes#2475
[that_comment]: https://github.com/rust-lang/rust/issues/35900#issuecomment-245978831
This makes it so that the `empty_line_after_outer_attribute` lint only
checks for newlines between the end of the attribute and the beginning
of the following item.
We need to check for the empty line count being bigger than 2 because
now the snippet of valid code contains only `\n` and splitting it
produces `["", ""]`
Invalid code will contain more than 2 empty strings.
Doc comments are syntactic sugar for #[doc] attributes, so this lint was
catching them, too.
This commit makes it so that doc comments are ignored in this lint.
I think, for normal attributes it makes sense to warn about following empty
lines, for doc comments, less. This way the user has some freedom over
the formatting.
