Add a to_std method for tokio's TcpStream

[Daksh14]

Is your feature request related to a problem? Please describe.
Cannot convert from a tokio TcpStream to a std one which allows the conversion to a socket2 socket which allows editing various params for a TcpStream

Describe the solution you'd like
Implement a to_std method for tokio's TcpStream.

Describe alternatives you've considered
None considered

Additional context
None provided

[carllerche]

It should be possible to impl TryFrom to convert between the two. We also could probably provide as_std(&self) -> &TcpStream.

I would be interested to hear what others think. cc @tokio-rs/maintainers

[Darksonn]

Is the conversion fallible?

[carllerche]

Converting will require deregistering, which could fail.

[Darksonn]

I do think that a conversion is valuable. The as_std you proposed would also be neat.

[mariusmuja]

The absence of a way to convert from tokio TcpStream to std TcpStream is preventing me from updating from tokio 0.2 to 0.3.

In my use case, I have a custom TcpStream that also reads RX timestamps from the socket in addition to the payload. I can use AsyncFd, however I need to convert from a tokio::net::TcpStream (received from a tokio::net::TcpListener) to AsyncFd<std::net::TcpStream> and as far as I can tell there is no way of extracting the std::net::TcpStream and de-registering. I could write a custom TcpListener I guess, but I would be great if tokio's TcpStream had an into_std(self) or a TryFrom was available.

[liufuyang]

Maybe I can help on this? Even though I don't have much TcpStream knowledge yet but I'd love to help and learn :)

[carlosb1]

I was checking and I want to be sure... I see that the tcpStream is mapped in the mio library... should it extract the object from there and expose?

[Darksonn]

Yeah, it should take out the mio TcpStream then convert it (possibly through IntoRawFd if necessary).