Copies the entire contents of a reader into a writer.

This function will continuously read data from reader and then write it into writer in a streaming fashion until reader returns EOF.

On success, the total number of bytes that were copied from reader to writer is returned.

If you’re wanting to copy the contents of one file to another and you’re working with filesystem paths, see the fs::copy function.

This function will return an error immediately if any call to read or write returns an error. All instances of ErrorKind::Interrupted are handled by this function and the underlying operation is retried.