Function reduce_impl_span_to_impl_keyword

fn reduce_impl_span_to_impl_keyword(sm: &SourceMap, impl_span: Span) -> Span

Adjust the impl span so that just the impl keyword is taken by removing everything after < ("impl<T> Iterator for A<T> {}" -> "impl") and everything after the first whitespace ("impl Iterator for A" -> "impl").

Attention: the method used is very fragile since it essentially duplicates the work of the parser. If you need to use this function or something similar, please consider updating the source_map functions and this function to something more robust.