Function cpp_function_declarator_at 

pub fn cpp_function_declarator_at(
    root: Node<'_>,
    start_byte: usize,
) -> Option<Node<'_>>

The callable declarator of the declaration that covers start_byte.

A consumer that holds a declaration’s recorded byte position rather than its syntax node - external header extraction, for instance - uses this to reach the same function_declarator the declaration walk read.