When using C getopt_long a long option with optional_argument will have a value only if --flag=value syntax is used. --flag arg is parsed as a flag without an argument, and a free argument (I've verified that on macOS and Linux/glibc).

getopts' optflagopt behaves differently. It always takes the next argument as the value, even if the = syntax is not used.

This is problematic for me, because I can't faithfully port C getopt_long program to Rust. I also think it's a less useful behavior, because it makes it impossible to set a optflagopt flag without a value if it's not the last/only argument.

extern crate getopts;

fn main() {
    let mut o = getopts::Options::new();
    o.optflagopt("", "value", "", "");
    
    let m = o.parse(&["--value", "foo", "bar"]).unwrap();
    println!("with space {:?} + {:?}", m.opt_str("value"), m.free);

    let m = o.parse(&["--value=foo", "bar"]).unwrap();
    println!("with = {:?} + {:?}", m.opt_str("value"), m.free);
}

This prints:

with space Some("foo") + ["bar"]
with = Some("foo") + ["bar"]

I'd prefer it to parse as:

with space None + ["foo", "bar"]
with = Some("foo") + ["bar"]