Function decorated_definition_range 

pub fn decorated_definition_range(
    source_file: &SourceFile,
    statement_range: TextRange,
    decorator_list: &[Decorator],
    keyword: &str,
) -> TextRange

The range a decorated definition covers from its def/class keyword on.

The parser hands over a statement that starts at the first decorator, while a definition is located at the keyword that introduces it.