Macro option

macro_rules! option {
    ($i:expr, $submac:ident!( $($args:tt)* )) => { ... };
    ($i:expr, $f:expr) => { ... };
}

Turn a failed parse into None and a successful parse into Some.

A failed parse consumes none of the input.

• Syntax: option!(THING)
• Output: Option<THING>

#[macro_use]
extern crate syn;

use syn::{Label, Block};
use syn::synom::Synom;

/// Parses a Rust loop. Equivalent to syn::ExprLoop.
///
/// Examples:
///     loop { println!("y"); }
///     'x: loop { break 'x; }
struct ExprLoop {
    label: Option<Label>,
    loop_token: Token![loop],
    body: Block,
}

impl Synom for ExprLoop {
    named!(parse -> Self, do_parse!(
        // Loop may or may not have a label.
        label: option!(syn!(Label)) >>
        loop_token: keyword!(loop) >>
        body: syn!(Block) >>
        (ExprLoop {
            label,
            loop_token,
            body,
        })
    ));
}

This macro is available if Syn is built with the "parsing" feature.