lib::quick_error!

macro_rules! quick_error {
    (// public with docstring
        $(#[$meta:meta])*
        pub enum $name:ident {
           $(
               #[doc=$doc:expr]
               $item:ident $( ( $($var:ident : $typ:ty),* ) )* { $($funcs:tt)* }
           )*
        }
    ) => {
        $(#[$meta])*
        pub enum $name {
           $(
               #[doc=$doc]
               $item $( ( $($typ),* ) )*,
           )*
        }
        quick_error!(IMPLEMENTATIONS $name { $(
           $item $( ( $($var: $typ),* ) )* { $($funcs)* }
           )* });
        $(
            quick_error!(ERROR_CHECK $($funcs)*);
        )*
    };
    (// private with docstring
        $(#[$meta:meta])*
        enum $name:ident {
           $(
               #[doc=$doc:expr]
               $item:ident $( ( $($var:ident : $typ:ty),* ) )* { $($funcs:tt)* }
           )*
        }
    ) => {
        $(#[$meta])*
         enum $name {
            $(
                #[doc=$doc]
                $item $( ( $($typ),* ) )*,
            )*
         }
         quick_error!(IMPLEMENTATIONS $name { $(
            $item $( ( $($var: $typ),* ) )* { $($funcs)* }
            )* });
        $(
            quick_error!(ERROR_CHECK  $($funcs)*);
        )*
    };
    (// public no meta
        $(#[$meta:meta])*
        pub enum $name:ident {
           $(
               $item:ident $( ( $($var:ident : $typ:ty),* ) )* { $($funcs:tt)* }
           )*
        }
    ) => {
        $(#[$meta])*
        pub enum $name {
           $( $item $( ( $($typ),* ) )*, )*
        }
        quick_error!(IMPLEMENTATIONS $name { $(
           $item $( ( $($var: $typ),* ) )* { $($funcs)* }
           )* });
        $(
            quick_error!(ERROR_CHECK $($funcs)*);
        )*
    };
    (// private no meta
        $(#[$meta:meta])*
        enum $name:ident {
           $(
               $item:ident $( ( $($var:ident : $typ:ty),* ) )* { $($funcs:tt)* }
           )*
        }
    ) => {
        $(#[$meta])*
         enum $name {
            $( $item $( ( $($typ),* ) )*, )*
         }
         quick_error!(IMPLEMENTATIONS $name { $(
            $item $( ( $($var: $typ),* ) )* { $($funcs)* }
            )* });
        $(
            quick_error!(ERROR_CHECK  $($funcs)*);
        )*
    };
    (IMPLEMENTATIONS
        $name:ident {
            $(
                $item:ident $( ( $($var:ident : $typ:ty),* ) )* { $($funcs:tt)* }
            )*
        }
    ) => {
        #[allow(unused)]
        impl ::std::fmt::Display for $name {
            fn fmt(&self, fmt: &mut ::std::fmt::Formatter)
                -> ::std::fmt::Result
            {
                match self {
                    $(
                        &$name::$item $( ( $(ref $var),* ) )* => {
                            quick_error!(FIND_DISPLAY_IMPL
                                $item self fmt [ $( ( $($var)* ) )* ]
                                { $($funcs)* })
                        }
                    )*
                }
            }
        }
        #[allow(unused)]
        impl ::std::error::Error for $name {
            fn description(&self) -> &str {
                match self {
                    $(
                        &$name::$item $( ( $(ref $var),* ) )* => {
                            quick_error!(FIND_DESCRIPTION_IMPL
                                $item self fmt [ $( ( $($var)* ) )* ]
                                { $($funcs)* })
                        }
                    )*
                }
            }
            fn cause(&self) -> Option<&::std::error::Error> {
                match self {
                    $(
                        &$name::$item $( ( $(ref $var),* ) )* => {
                            quick_error!(FIND_CAUSE_IMPL
                                $item [ $( ( $($var)* ) )* ]
                                { $($funcs)* })
                        }
                    )*
                }
            }
        }
        $(
            quick_error!(FIND_FROM_IMPL
                $name $item [ $( ( $($var:$typ)* ) )* ]
                { $($funcs)* });
        )*
    };
    (FIND_DISPLAY_IMPL $item:ident $me:ident $fmt:ident
        [ $( ( $($var:ident)* ) )* ]
        { display($($exprs:expr),*) $($tail:tt)* }
    ) => {
        write!($fmt, $($exprs),*)
    };
    (FIND_DISPLAY_IMPL $item:ident $me:ident $fmt:ident
        [ $( ( $($var:ident)* ) )* ]
        { $t:tt $($tail:tt)* }
    ) => {
        quick_error!(FIND_DISPLAY_IMPL
            $item $me $fmt [ $( ( $($var)* ) )* ]
            { $($tail)* })
    };
    (FIND_DISPLAY_IMPL $item:ident $me:ident $fmt:ident
        [ $( ( $($var:ident)* ) )* ]
        { }
    ) => {
        write!($fmt, "{}", ::std::error::Error::description($me))
    };
    (FIND_DESCRIPTION_IMPL $item:ident $me:ident $fmt:ident
        [ $( ( $($var:ident)* ) )* ]
        { description($expr:expr) $($tail:tt)* }
    ) => {
        $expr
    };
    (FIND_DESCRIPTION_IMPL $item:ident $me:ident $fmt:ident
        [ $( ( $($var:ident)* ) )* ]
        { $t:tt $($tail:tt)* }
    ) => {
        quick_error!(FIND_DESCRIPTION_IMPL
            $item $me $fmt [ $( ( $($var)* ) )* ]
            { $($tail)* })
    };
    (FIND_DESCRIPTION_IMPL $item:ident $me:ident $fmt:ident
        [ $( ( $($var:ident)* ) )* ]
        { }
    ) => {
        stringify!($item)
    };
    (FIND_CAUSE_IMPL $item:ident
        [ $( ( $($var:ident)* ) )* ]
        { cause($expr:expr) $($tail:tt)* }
    ) => {
        Some($expr)
    };
    (FIND_CAUSE_IMPL $item:ident
        [ $( ( $($var:ident)* ) )* ]
        { $t:tt $($tail:tt)* }
    ) => {
        quick_error!(FIND_CAUSE_IMPL
            $item [ $( ( $($var)* ) )* ]
            { $($tail)* })
    };
    (FIND_CAUSE_IMPL $item:ident
        [ $( ( $($var:ident)* ) )* ]
        { }
    ) => {
        None
    };
    (FIND_FROM_IMPL $name:ident $item:ident
        [ $( ( $($var:ident : $typ:ty)* ) )* ]
        { from() $($tail:tt)* }
    ) => {
        $( $(
            impl From<$typ> for $name {
                fn from($var: $typ) -> $name {
                    $name::$item($var)
                }
            }
        )* )*
        quick_error!(FIND_FROM_IMPL
            $name $item [ $( ( $($var:$typ)* ) )* ]
            { $($tail)* });
    };
    (FIND_FROM_IMPL $name:ident $item:ident
        [ ]
        { from($ftyp:ty) $($tail:tt)* }
    ) => {
        impl From<$ftyp> for $name {
            fn from(_discarded_error: $ftyp) -> $name {
                $name::$item
            }
        }
        quick_error!(FIND_FROM_IMPL
            $name $item [  ]
            { $($tail)* });
    };
    (FIND_FROM_IMPL $name:ident $item:ident
        [ $( ( $($var:ident : $typ:ty)* ) )* ]
        { from($fvar:ident : $ftyp:ty) -> ($($expr:expr),*) $($tail:tt)* }
    ) => {
        impl From<$ftyp> for $name {
            fn from($fvar: $ftyp) -> $name {
                $name::$item($($expr),*)
            }
        }
        quick_error!(FIND_FROM_IMPL
            $name $item [ $( ( $($var:$typ)* ) )* ]
            { $($tail)* });
    };
    (FIND_FROM_IMPL $name:ident $item:ident
        [ $( ( $($var:ident : $typ:ty)* ) )* ]
        { $t:tt $($tail:tt)* }
    ) => {
        quick_error!(FIND_FROM_IMPL
            $name $item[ $( ( $($var:$typ)* ) )* ]
            { $($tail)* });
    };
    (FIND_FROM_IMPL $name:ident $item:ident
        [ $( ( $($var:ident : $typ:ty)* ) )* ]
        { }
    ) => {
    };
    // This one should match all allowed sequences in "funcs" but not match
    // anything else.
    // This is to contrast FIND_* clauses which just find stuff they need and
    // skip everything else completely
    (ERROR_CHECK display($($exprs:expr),*) $($tail:tt)*)
    => { quick_error!(ERROR_CHECK $($tail)*); };
    (ERROR_CHECK description($expr:expr) $($tail:tt)*)
    => { quick_error!(ERROR_CHECK $($tail)*); };
    (ERROR_CHECK cause($expr:expr) $($tail:tt)*)
    => { quick_error!(ERROR_CHECK $($tail)*); };
    (ERROR_CHECK from() $($tail:tt)*)
    => { quick_error!(ERROR_CHECK $($tail)*); };
    (ERROR_CHECK from($ftyp:ty) $($tail:tt)*)
    => { quick_error!(ERROR_CHECK $($tail)*); };
    (ERROR_CHECK from($fvar:ident : $ftyp:ty) -> ($($e:expr),*) $($tail:tt)*)
    => { quick_error!(ERROR_CHECK $($tail)*); };
    (ERROR_CHECK) => {};
}

Main macro that does all the work