Function tooltip 

pub fn tooltip(
    world: &dyn IdeWorld,
    document: Option<&PagedDocument>,
    source: &Source,
    cursor: usize,
    side: Side,
) -> Option<Tooltip>

Describe the item under the cursor.

Passing a document (from a previous compilation) is optional, but enhances the tooltips. Label tooltips, for instance, are only generated when the document is available.