No way to extract an owned RawFd from ChildStdout

[satakuma]

Is your feature request related to a problem? Please describe.
I am trying to implement support for named Unix pipes (#5318) and I am considering adding conversion from types in tokio::process which hold a file descriptor to a pipe end. However, I've noticed that currently there is no way to turn ChildStdout, ChildStderr and ChildStdin types into an owned RawFd, even though they implement AsRawFd.

In this particular case, I can probably hack my way through internally to get the fd, but I think it would be useful to expose such conversion to the public API.

Describe the solution you'd like
Add a new method to the ChildStdout, analogous to the from_std method:

fn into_std(self) -> Result<std::process::ChildStdout>

which would perform conversion from an asynchronous ChildStdout to a synchronous one, which in turn implements IntoRawFd. Also add similar methods to ChildStderr and ChildStdin types.

Describe alternatives you've considered
Implement IntoRawFd to those types directly, but from what I can see tokio does not implement this trait for most types in favor of into_std-like methods.

[Darksonn]

Generally, the reason that we don't implement IntoRawFd is that the operation is fallible. I wouldn't mind adding an into_fd method that returns a result.

I would have to think about it when it comes to an into_std method. I recall there being some reason for not having that method, but I don't remember what it is.

[ipetkov]

@Darksonn pretty much the fallible unwrap IIRC

[satakuma]

The into_std method was mentioned in #4403, which I failed to notice while opening this issue.
ChildStd{in,err,out} are lacking FromRawFd impls, so it is impossible to create them from tokio types. New IO safety traits for them also haven't landed yet, see rust-lang/rust#98704.

As I said, I can live without this conversion for now, but the topic of into_std methods may appear again once it becomes possible to create std types from raw file descriptors. Feel free to close this issue.