tee: dedup code by static dispatch macro

[oech3]

No description provided.

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/pipe-f is now passing!

[xtqqczze]

This may be a bit more idiomatic than the macro approach:

impl Writer {
    fn as_write(&mut self) -> &mut dyn Write {
        match self {
            Writer::File(f) => f,
            Writer::Stdout(s) => s,
        }
    }
}

[oech3]

dyn is not acceptible.

[oech3]

I would just move previous code to uucore.

[xtqqczze]

impl Writer {
    fn with_write<T>(&mut self, f: impl FnOnce(&mut dyn Write) -> T) -> T {
        match self {
            Writer::File(w) => f(w),
            Writer::Stdout(w) => f(w),
        }
    }
}

Could quality is equivalent: https://rust.godbolt.org/z/f5bMhYfTY

[oech3]

Is the optimization guaranteed for all Rust version and targets?

[xtqqczze]

Can't use enum_dispatch crate.

[oech3]

Hesitating to add new dep.

[xtqqczze]

I would just move previous code to uucore.

Yeah, I think the duplication would be fine there