Function contains_rejected_control_chars 

pub fn contains_rejected_control_chars(s: &str) -> bool

Input-boundary predicate: returns true when s carries a C0 control byte that mdwright treats as evidence the input is not well-formed Markdown.

Allowed bytes inside 0x00..=0x1f: TAB (0x09), LF (0x0a), FF (0x0c), CR (0x0d). Everything else in C0 is rejected. DEL (0x7f) is not rejected; CommonMark accepts it verbatim and real documents occasionally carry it.