Code shared from the Rust Playground

playground.rs

struct TOCSectionEntry {
    sub_entries: Vec<TOCSectionEntry>,
}

fn table_to_insert_to<'a>(
    mut level: Option<&'a mut TOCSectionEntry>,
) -> Option<&'a mut TOCSectionEntry> {
    while let Some(next) = level {
        level = next.sub_entries.last_mut();
    }
    level
}

fn main() {}