Macro cond

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

Execute a parser only if a condition is met, otherwise return None.

If you are familiar with nom, this is nom’s cond_with_error parser.

• Syntax: cond!(CONDITION, THING)
• Output: Some(THING) if the condition is true, else None

#[macro_use]
extern crate syn;

use syn::{Ident, MacroDelimiter};
use syn::token::{Paren, Bracket, Brace};
use syn::synom::Synom;

/// Parses a macro call with empty input. If the macro is written with
/// parentheses or brackets, a trailing semicolon is required.
///
/// Example: `my_macro!{}` or `my_macro!();` or `my_macro![];`
struct EmptyMacroCall {
    name: Ident,
    bang_token: Token![!],
    empty_body: MacroDelimiter,
    semi_token: Option<Token![;]>,
}

fn requires_semi(delimiter: &MacroDelimiter) -> bool {
    match *delimiter {
        MacroDelimiter::Paren(_) | MacroDelimiter::Bracket(_) => true,
        MacroDelimiter::Brace(_) => false,
    }
}

impl Synom for EmptyMacroCall {
    named!(parse -> Self, do_parse!(
        name: syn!(Ident) >>
        bang_token: punct!(!) >>
        empty_body: alt!(
            parens!(epsilon!()) => { |d| MacroDelimiter::Paren(d.0) }
            |
            brackets!(epsilon!()) => { |d| MacroDelimiter::Bracket(d.0) }
            |
            braces!(epsilon!()) => { |d| MacroDelimiter::Brace(d.0) }
        ) >>
        semi_token: cond!(requires_semi(&empty_body), punct!(;)) >>
        (EmptyMacroCall {
            name,
            bang_token,
            empty_body,
            semi_token,
        })
    ));
}

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