Consider implementing AsRawFd for Stdout, Stdin and Stderr

[malbarbo]

[nagisa]

This cannot happen, because Stdout and Stderr may be replaced by arbitrary instance of Write internally.

[nagisa]

Although if all the platforms Rust runs on support something along the lines of memfd (not true, as we support linux 2.6.xx, whereas memfd is 3.17+), then it may be feasible.

[malbarbo]

@nagisa

This cannot happen, because Stdout and Stderr may be replaced by arbitrary instance of Write internally.

How can Stdout be replaced by an arbitrary instance?

Note that the trait AsRawFd is an extension trait defined in some platforms, like unix, in platforms that does not define AsRawFd trait or does not have a reasonable implementation of AsRawFd for Stdout, the implementation would be missing.

[nagisa]

It’s private API to libstd.

[codyps]

Due to the Send bound, it seems like one only needs threads + pipes to implement AsRawFd for these types.

[alexcrichton]

Done in #43459!