I'd like an efficient way to discard data from a Reader.

[canndrew]

The problem.

Say I want to skip the next n bytes of input from a Reader. Currently, I can see two ways of doing this, neither of which are ideal.

• Allocate a Vec<u8> of size n. Call read on it. Discard the Vec<u8>. This has the disadvantage of doing an unnecessary allocation and unnecessary copying. Note that calling read_exact is equivalent to this.
• Call read_u8 n times. This has this the disadvantage of doing n function calls, which may even translate to n syscalls. For any significant n this is going to be worse than the first option.

The solution.

Have Reader expose a method skip_exact that skips the next n bytes. For some readers such as MemReader and BufferedReader this just means advancing the cursor by n. For some other readers this could be implemented using seek. For other readers again this method can be given a default implementation that just calls read_exact.

Note that I can't just call seek(n, SeekCur). Not all readers implement Seek (socket streams for example) but all readers can be advanced from their current position by - in the worst case - just reading and throwing away data.

[aturon]

Closing as a dup of #13989