write_fmt return number of bytes written

[MabezDev]

The current write_fmt implementation has a return type of Result<()> is there any technical reason it couldn't return the number of bytes written? Similar to sprintf in C.

An example use case in Smoltcp:

socket.send(|mut buffer| {
    let bytes_written = write!(buffer, "Http Error {:?}", e).unwrap();
    (bytes_written, ())
}).unwrap();

The return type of the closure requires indicates the number of bytes used such that smoltcp can flush those bytes to the wire.

[jonas-schievink]

I don't think there's a technical reason for it, but changing it would be a breaking change, so it's not going to happen.

[MabezDev]

Fair enough. Is there anyway to use the formatter and get the number of bytes it will create? I really want to avoid hand rolling formatting if possible.

[jonas-schievink]

The only way I can think of is wrapping the formatter and keeping your own count

[MabezDev]

It's a bit awkward, but it works. Thanks!