Rust: add a warning on attributes that can be confused with global ones

[tanriol]

I was asked by a person new to Rust why #[allow(unused_variables)] does not work in the following code.

#[allow(dead_code)]
#[allow(unused_variables)]

fn trip() {
}

fn main() {
 let a =1;
// println!("{}",a);
}

It would be nice to have a warning here that these attributes are for the next item, not for the whole file as one could think from their position in file. Probably a warning when there's a blank line between item body and its attributes would work.

[phansch]

I think Clippy's empty_line_after_outer_attr is what you're looking for? https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_outer_attr

[tanriol]

Looks right, yes. I'd argue that it's a good idea to have it in rustc because this looks like a mistake a new user can accidentally do and they likely would not know about clippy yet. However, the note about possible false positives is not exactly optimistic.

[tanriol]

And even in Clippy it's allow-by-default, so you cannot really use it when you're a newcomer and have no idea about this specific problem.