`len_without_is_empty` requires `allow` in the wrong place.

[sgrif]

When a struct fails the len_without_is_empty lint, the error span shows the definition of the len method. However, adding #[allow(len_without_is_empty)] on the method does nothing. Clippy wants it on the struct itself. I'd expect that the place to add #[allow] is always on the span shown in the error.

[theotherphil]

I'm new to clippy, but I'd like to give this a go.

[mcarton]

@theotherphil Great! Don't hesitate to open a PR even if it's not finished or to ask questions if you have any problem!