Common copy logic shared between install and cp in uucore?

[DaringCuteSeal]

Upon investigating an issue with install (#5080) after my package build with Arch Linux's makepkg failed, I discovered that the latter bug also affects cp:

$ echo text | /bin/uu-cp /dev/stdin file
cp: '/proc/6793/fd/pipe:[467812]' -> 'file': No such file or directory (os error 2)
$ echo text | /bin/cp /dev/stdin file
$ # the latter works fine with GNU coreutils

Looking at uutils' install source code, it seems that it uses the copy functionality from fs::copy directly. Since cp has some other optimizations (like sparse copying), this makes install less performant in some cases. I took a look at GNU's install and they seem to have sparse_mode set to automatic by default to enable copying optimizations. I think that it would be a good idea to place a copy subroutine (that also handles special files) in uucore, as suggested by a comment under the issue I mentioned earlier.

Now, if this is relevant, I have some questions in regards to the code migration. How much of cp should be moved to uucore? Is it the whole thing—like with GNU's, so we get the whole cp as API? Or is it just one or a few functions that does copy optimizations directly without other logic handling? The problem with the latter is that both cp (the app) and the copy under uucore would have so much duplicates since they are quite coupled (but I need to make the uucore copy code functional on its own). Besides, uucore only has thiserror while cp uses quick-error and not unifying them would be quite unclean I think.