>That even sounds dangerous, since numeric variables can contain filenames with spaces.

Or filenames that contain the number zero :D

    #!/bin/sh
    #
    # Usage : popc_unchecked BINARY_STRING
    #
    #   Count number of 1s in BINARY_STRING.  Made to demonstrate a use of IFS that
    #   can bite you if you do not quote all the variables you don't want to split.
    
    len="${#1}"
    count() { printf '%s\n' "$((len + 1 - $#))"; }
    saved="${IFS}"
    IFS=0
    count 1${1}1
    IFS="${saved}"
    
    # PS: we do not run the code in a subshell because popcount needs to be highly
    # performant (≖ ᴗ ≖ )