macros.rs

#[macro_export]
#[doc(hidden)]
macro_rules! define_msg_internal {
    (@as_item $i:item) => ($i);

    (@gen_match $f:ident, $l:ident; $($lang:pat => $format:tt),*; $tail:tt) => (
        match $l {
            $($lang => define_msg_internal!(@gen_arm $f; $format; $tail),)*
        }
    );

    (@gen_arm $f:ident; $format:tt; ($($tail:tt)*)) => (
        write!($f, $format $($tail)*)
    );

    (
        $name:ident
        ($($prefix:tt)*)
        {
            constr: [ $($constr:tt)* ],
            params: [ $($params:tt)* ],
            $($_generics:tt)*
        },
        $({
            $($fname:ident: $ftype:ty),* $(,)*
        })*:
        $($lang:pat => $format:tt),* $(,)*
    ) => (
        // since `$($constr)*` and `$($params)*` have to be expanded lazily,
        // we need to coerce the AST to delay the expansion

        define_msg_internal! { @as_item
            #[derive(Debug)]
            $($prefix)* struct $name<$($constr)*> {
                $($(pub $fname: $ftype,)*)*
            }
        }

        define_msg_internal! { @as_item
            impl<$($constr)*> $crate::Localize for $name<$($params)*> {
                fn fmt_localized(&self, f: &mut ::std::fmt::Formatter,
                                 lang: &str) -> ::std::fmt::Result {
                    // "tt bundling" as in http://stackoverflow.com/a/37754096
                    define_msg_internal!(@gen_match f, lang;
                        $($lang => $format),*;
                        ($($(, $fname = $crate::Localized::new(&self.$fname, lang))*)*))
                }
            }
        }
    );
}

#[macro_export]
macro_rules! define_msg {
    ($(#[$meta:meta])* pub $name:ident $($t:tt)*) => (
        parse_generics_shim! {
            { constr, params, ltimes, tnames },
            then define_msg_internal!($name ($(#[$meta])* pub)),
            $($t)*
        }
    );

    ($(#[$meta:meta])* $name:ident $($t:tt)*) => (
        parse_generics_shim! {
            { constr, params, ltimes, tnames },
            then define_msg_internal!($name ($(#[$meta])*)),
            $($t)*
        }
    );
}

// example usage
define_msg! { pub SomeError<'a> { s: &'a str }:
    "en" => "some error with {s}",
    _    => "(insert a generic message for some error with {s})",
}