Add an assertion to NodeRange::new.

Author: nnethercote

Diff for: compiler/rustc_parse/src/parser/mod.rs

@@ -238,6 +238,7 @@ impl NodeRange {
     // is the position of the function's start token. This gives
     // `NodeRange(10..15)`.
     fn new(ParserRange(parser_range): ParserRange, start_pos: u32) -> NodeRange {
+        assert!(parser_range.start >= start_pos && parser_range.end >= start_pos);
         NodeRange((parser_range.start - start_pos)..(parser_range.end - start_pos))
     }
 }