From the clippy side it's difficult to detect empty lines between
an attributes and the following item because empty lines and comments
are not part of the AST. The parsing currently works for basic cases
but is not perfect and can cause false positives.
Maybe libsyntax 2.0 will fix some of the problems around attributes but
comments will probably be never part of the AST so we would still have
to do some manual parsing.
Before, when you had a block comment between an attribute and the
following item like this:
```rust
\#[crate_type = "lib"]
/*
*/
pub struct Rust;
```
It would cause a false positive on the lint, because there is an empty
line inside the block comment.
This makes sure that basic block comments are detected and removed from
the snippet that was created before.
`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.
This fixes an incorrect suggestion from the `useless_attribute` lint
when using `cfg_attr`.
Additionally, it will not show a suggestion anymore, if the attribute
begins on a previous line, because it is much harder to construct the
span of multi-line `cfg_attr` attributes as they don't appear in the AST.
To fix it completely, one would have to parse upwards into the file,
and find the beginning of the `cfg_attr` attribute.
