How to determine where Vim will be writing special files?

I'm trying to write a plugin that will set the 'hidden' attribute in
Windows for:
swap files
undo files
backup files
viminfo file

Undo files have a nice undofile() function, and swap files have the
:swapname command (although that isn't as nice). Is there a similar way
to get the file name that will be used for the backup file, other than
parsing the 'backupdir' option? How about the .viminfo file? The help
pages list how the default directory is determined, but the user can
still use the -i option to override it.

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