That function is returning a Vec<Token>, and so it knows the .collect() call needs to return a Vec<Token>, and so therefore the .map() function needs to return a Token. Therefore each match arm needs to return a Token too, so therefore the compiler selects the implementation of .parse() that returns Token.

I admit when I started rust, seeing calls to .parse() was one of the more confusing things I saw in rust code, because of how much it leans on type inference to be readable. In places like these, it's a bit more readable:

    let ip: IpAddr = ip_str.parse()?;

But when you see the .parse buried several levels deep and you have no idea what type it's trying to produce, it's a pain in the ass to read. This is why it's nice to use the turbo-fish syntax:

    let ip = ip_str.parse::<IpAddr>()?;

Since you can drop .parse::<IpAddr>()? anywhere to make the type explicit, especially when buried in type-inferred blocks like the code in TFA.