toOwnedSliceSentinel

    pub fn toOwnedSliceSentinel(a: *Allocating, comptime sentinel: u8) Allocator.Error![:sentinel]u8 {
        // This addition can never overflow because `a.writer.buffer` can never occupy the whole address space.
        try ensureTotalCapacityPrecise(a, a.writer.end + 1);
        a.writer.buffer[a.writer.end] = sentinel;
        a.writer.end += 1;
        errdefer a.writer.end -= 1;
        const result = try toOwnedSlice(a);
        return result[0 .. result.len - 1 :sentinel];
    }