Inverse `?` operator in Rust (aka return if Some/Ok)

Question

I am building a macro parser with syn and need to check if a ParseStream is one of several keywords. The code currently looks similar to this:

mod kw {
    syn::custom_keyword!(a);
    syn::custom_keyword!(b);
    syn::custom_keyword!(c);
}

enum MyKeywordEnum {
    A(kw::a),
    B(kw::b),
    C(kw::c),
}

impl syn::parse::Parse for MyKeywordEnum {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        Ok(if let Ok(ret) = input.parse::<kw::a>() {
            Self::A(ret)
        } else if let Ok(ret) = input.parse::<kw::b>() {
            Self::B(ret)
        } else if let Ok(ret) = input.parse::<kw::c>() {
            Self::C(ret)
        } else {
            abort!(input.span(), "Couldn't parse primitive type"); // Via #[proc_macro_error]
        })
    }
}

Is there a built-in operator or macro to return immediately if an expression is Option::Some or Result::Ok?
Is there a better way to organize these checks?
Because ParseStream::parse statically compiles to the specific impl Parse type, I can't use match, right?

You can use [`lookahead1()`](https://docs.rs/syn/latest/syn/parse/struct.ParseBuffer.html#method.lookahead1). — Chayim Friedman, Dec 07 '22 at 22:40
@ChayimFriedman Good point, I ended up using a combination of this approach and the accepted answer — Gabriel Pizarro, Dec 08 '22 at 17:55

score 2 · Accepted Answer · answered Dec 07 '22 at 18:40

As far as I know, there is not. You could create a macro to do this, or you could use the Result combinator functions to make things a little prettier:

impl syn::parse::Parse for MyKeywordEnum {
    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
        input.parse::<kw::a>().map(Self::A)
            .or_else(|_| input.parse::<kw::b>().map(Self::B))
            .or_else(|_| input.parse::<kw::c>().map(Self::C))
            .or_else(|_| abort!(input.span(), "Couldn't parse primitive type"))
    }
}

Thanks, this seems like the best option for now – Gabriel Pizarro Dec 08 '22 at 17:54 — Gabriel Pizarro, Dec 08 '22 at 17:54

Inverse `?` operator in Rust (aka return if Some/Ok)

1 Answers1