syntax_data: {"ast_node_type":"function","tokens":[{"end":59,"kind":"keyword","start":0,"text":"pub"}]}
source_snippet:     pub fn span_until_whitespace(&self, sp: Span) -> Span {
context_before:     /// If `sp` points to `"let mut x"`, then a span pointing at `"let"` will be returned.
context_after:         self.span_take_while(sp, |c| !c.is_whitespace())