Rename ``file_descriptor`` to ``file_description_ref``

[tiif]

After #3712 renamed previous FileDescriptor to FileDescriptionRef, several variables still use the old name file_descriptor and should be updated to file_description_ref to avoid confusion. This is a simple cleanup task, but I am currently focused on other work, so I am just opening this issue for anyone who might be interested in helping.

[RalfJung]

That's a very long variable name, so we can also consider something like fd. That doesn't disambiguate between "file descriptor" and "file description", but we have types to help make that distinction, so maybe we don't have to repeat this in the variable name as well.

[DeSevilla]

I'm willing to take this on - seems pretty straightforward. From a quick look, there's one variable named exactly file_descriptor which is a FileDescriptionRef (in fs.rs), but several named file_description which are FileDescriptionRef (in fs.rs and fd.rs). Should both cases be renamed, or only file_descriptor?

[tiif]

I think there is one in fd.rs

miri/src/shims/unix/fd.rs

Line 414 in 335c91c

let Some(file_descriptor) = this.machine.fds.get(fd) else {

I personally don't find the naming of file_description confusing, but I am happy to see it being changed if it would be clearer. Maybe fd_ref would work if we were to change it? But I am not really good in naming variables, so cc @RalfJung :)

[DeSevilla]

Sorry, yes, I meant fd.rs for that first one, just mixed up the file names. fd_ref would match up with some other parts of the code, like these two:

miri/src/shims/unix/fd.rs

Line 335 in 335c91c

let fd_ref = self.new_ref(fd);

miri/src/shims/unix/linux/epoll.rs

Line 277 in 335c91c

let Some(fd_ref) = this.machine.fds.get(fd) else {

We could also just change file_descriptor to file_description if we're keeping that name.

[RalfJung]

I did a bunch more renaming in #3867 to hopefully make things more consistent wrt file description vs file descriptor: the latter (of type i32) are now called fd_num. We should only use them in the outer layers of the shims anyway, internally in Miri everything should be file descriptions.