Function parse_convert_arguments 

pub fn parse_convert_arguments(
    rest: &[OsString],
) -> Result<ConvertArguments, String>

What: Parse the words after convert-worktree-to-subvolume. Why: A refusal names the word it refused.

In TS you’d write (pseudocode):

function parseConvertArguments(rest: string[]): ConvertArguments; // throws the reason