Function mk_ident 

fn mk_ident<'i>(
    span: Span,
    change_case: Option<ChangeCase>,
    pieces: impl Iterator<Item = (&'i str, Option<Span>)> + Clone,
    atoms: Vec<AtomForReport>,
) -> Pasted

Make a leaf identifier out of pieces

Actually pastes together the pieces.

Any surrounding path elements, generics, etc., of a nontrivial expansion, are handled by assemble, not here.