Pass None into a function that accepts Option

Question

rust-ini has a function:

pub fn section<\'a, S>(&\'a self, name: Option) -> Option<&\'a Properties>
    where S: Into

        

Answer 1

You could specify the type of the T in the type Option for this None with:

let section = ifo_cfg.section(None::).unwrap();
//                                ^^^^^^^^^^ forces the type to be Option

Alternatively, you could specify the type S of the method section:

let section = ifo_cfg.section::(None).unwrap();
//                           ^^^^^^^^^^ forces S = String

You can also look up E0282's explanation, although it might not really answer your question at this time :)

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The syntax :: is sometimes called the turbofish. Some very generic methods like String::parse() and Iterator::collect() can return almost anything, and type inference does not have enough information to find the actual type. The :: allow the human to tell the compiler what generic parameter should be substituted. From parse()'s reference:

Because parse() is so general, it can cause problems with type inference. As such, parse() is one of the few times you'll see the syntax affectionately known as the 'turbofish': ::<>. This helps the inference algorithm understand specifically which type you're trying to parse into.