Function reparse 

pub fn reparse(
    old_root: &SyntaxNode,
    old_text: &str,
    old_diags: &[ParseDiagnostic],
    edit: &Edit,
) -> Option<Reparsed>

Attempt an incremental reparse of old_root (parsed from old_text, with old_diags) under edit. Returns None when no incremental strategy applies — the caller must then do a full parse. An edit whose range old_text cannot take is one such case, not a panic.