finishAssignDestructureExpr

fn finishAssignDestructureExpr(p: *Parse, first_lhs: Node.Index) !Node.Index {
    const scratch_top = p.scratch.items.len;
    defer p.scratch.shrinkRetainingCapacity(scratch_top);

    try p.scratch.append(p.gpa, first_lhs);

    while (p.eatToken(.comma)) |_| {
        const expr = try p.expectExpr();
        try p.scratch.append(p.gpa, expr);
    }

    const equal_token = try p.expectToken(.equal);

    const rhs = try p.expectExpr();

    const lhs_count = p.scratch.items.len - scratch_top;
    assert(lhs_count > 1); // we already had first_lhs, and must have at least one more lvalue

    const extra_start: ExtraIndex = @fromBackingInt(@intCast(p.extra_data.items.len));
    try p.extra_data.ensureUnusedCapacity(p.gpa, lhs_count + 1);
    p.extra_data.appendAssumeCapacity(@intCast(lhs_count));
    p.extra_data.appendSliceAssumeCapacity(@ptrCast(p.scratch.items[scratch_top..]));

    return p.addNode(.{
        .tag = .assign_destructure,
        .main_token = equal_token,
        .data = .{ .extra_and_node = .{
            extra_start,
            rhs,
        } },
    });
}