fnProtoRparen

fn fnProtoRparen(tree: Ast, fn_proto: Ast.full.FnProto, maybe_bang: Ast.TokenIndex) Ast.TokenIndex {
    // These may appear in any order, so we have to check the token_starts array
    // to find out which is first.
    var rparen = if (tree.tokenTag(maybe_bang) == .bang) maybe_bang - 1 else maybe_bang;
    var smallest_start = tree.tokenStart(maybe_bang);
    if (fn_proto.ast.align_expr.unwrap()) |align_expr| {
        const tok = tree.firstToken(align_expr) - 3;
        const start = tree.tokenStart(tok);
        if (start < smallest_start) {
            rparen = tok;
            smallest_start = start;
        }
    }
    if (fn_proto.ast.addrspace_expr.unwrap()) |addrspace_expr| {
        const tok = tree.firstToken(addrspace_expr) - 3;
        const start = tree.tokenStart(tok);
        if (start < smallest_start) {
            rparen = tok;
            smallest_start = start;
        }
    }
    if (fn_proto.ast.section_expr.unwrap()) |section_expr| {
        const tok = tree.firstToken(section_expr) - 3;
        const start = tree.tokenStart(tok);
        if (start < smallest_start) {
            rparen = tok;
            smallest_start = start;
        }
    }
    if (fn_proto.ast.callconv_expr.unwrap()) |callconv_expr| {
        const tok = tree.firstToken(callconv_expr) - 3;
        const start = tree.tokenStart(tok);
        if (start < smallest_start) {
            rparen = tok;
            smallest_start = start;
        }
    }
    assert(tree.tokenTag(rparen) == .r_paren);
    return rparen;
}