Macro easy_parse

macro_rules! easy_parse {
    (
        $(#[$meta:meta])*
        $vis:vis struct $name:ident;
    ) => { ... };
    (
        $(#[$meta:meta])*
        $vis:vis struct $name:ident ();
    ) => { ... };
    (
        $(#[$meta:meta])*
        $vis:vis struct $name:ident {}
    ) => { ... };
    (
        $(#[$meta:meta])*
        $vis:vis struct $name:ident {
            $(#[$hmeta:meta])* $hvis:vis $hname:ident : $htype:ty
            $(, $(#[$fmeta:meta])* $fvis:vis $fname:ident : $ftype:ty)*
            $(,)?
        }
    ) => { ... };
    (
        $(#[$meta:meta])*
        $vis:vis struct $name:ident (
            $(#[$hmeta:meta])* $hvis:vis $htype:ty
            $(, $(#[$fmeta:meta])* $fvis:vis $ftype:ty)*
            $(,)?
        );
    ) => { ... };
    (
        $(#[$meta:meta])*
        $vis:vis enum $name:ident {
            $(#[$vdmeta:meta])* ! $vdname:ident $( ( $($vdt:tt)* ) )? $( { $($vdr:tt)* } )?,
            $($(#[$vmeta:meta])* $( ? $vsname:ident )? $( $vname:ident )? $( ( $($vt:tt)* ) )? $( { $($vr:tt)* } )?, )*
        }
    ) => { ... };
    (
        $(#[$meta:meta])*
        $vis:vis enum $name:ident {
            $($(#[$vmeta:meta])* $( ? $vsname:ident )? $( $vname:ident )? $( ( $($vt:tt)* ) )? $( { $($vr:tt)* } )?, )*
        }
    ) => { ... };
}

Defines structure or enum and implements Parse for it.

Implements EasyPeek for structs if first field implements EasyPeek.

For enums, if first variant is prefixed by ! it becomes a default variant that is parsed if no other variants peeking succeeds. Variants prefixed with ? are skipped during parsing and peeking.

For enums EasyPeek is implemented if enum does not have a default variant. First field in all non-default non-skipped variants must implement EasyPeek.

Therefore unit and empty tuple and struct-like variants may be present but must be marked as default or skipped.