Parser::slice (and Parser::slice_from) in src/tokenizer.rs calls self.input.get_unchecked(range.start.0..range.end.0) without verifying range.start.0 <= range.end.0 or range.end.0 <= self.input.len():

#[inline]
pub fn slice(&self, range: Range<SourcePosition>) -> &'a str {
    debug_assert!(self.input.is_char_boundary(range.start.0));
    debug_assert!(self.input.is_char_boundary(range.end.0));
    unsafe { self.input.get_unchecked(range.start.0..range.end.0) }
}

Safe callers can trigger undefined behavior without unsafe:

1. Passing an inverted range p1..p0 where p1 > p0 (both is_char_boundary assertions still pass in debug builds, and end - start underflows in get_unchecked).
2. Calling parser.reset(&saved_state) to rewind the parser position and then calling parser.slice_from(later_pos).
3. Passing a SourcePosition obtained from another Parser with a longer input string in release builds where debug_assert! is disabled.

Replacing unsafe { self.input.get_unchecked(range.start.0..range.end.0) } with &self.input[range.start.0..range.end.0] avoids the soundness issue.