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    // See the cfg-if crate.
    #[allow(unused_macro_rules)]
    macro_rules! cfg_if {
        // match if/else chains with a final `else`
        ($(
            if #[cfg($($meta:meta),*)] { $($it:item)* }
        ) else * else {
            $($it2:item)*
        }) => {
            cfg_if! {
                @__items
                () ;
                $( ( ($($meta),*) ($($it)*) ), )*
                ( () ($($it2)*) ),
            }
        };

        // match if/else chains lacking a final `else`
        (
            if #[cfg($($i_met:meta),*)] { $($i_it:item)* }
            $(
                else if #[cfg($($e_met:meta),*)] { $($e_it:item)* }
            )*
        ) => {
            cfg_if! {
                @__items
                () ;
                ( ($($i_met),*) ($($i_it)*) ),
                $( ( ($($e_met),*) ($($e_it)*) ), )*
                ( () () ),
            }
        };

        // Internal and recursive macro to emit all the items
        //
        // Collects all the negated cfgs in a list at the beginning and after the
        // semicolon is all the remaining items
        (@__items ($($not:meta,)*) ; ) => {};
        (@__items ($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
            // Emit all items within one block, applying an approprate #[cfg]. The
            // #[cfg] will require all `$m` matchers specified and must also negate
            // all previous matchers.
            cfg_if! { @__apply cfg(all($($m,)* not(any($($not),*)))), $($it)* }

            // Recurse to emit all other items in `$rest`, and when we do so add all
            // our `$m` matchers to the list of `$not` matchers as future emissions
            // will have to negate everything we just matched as well.
            cfg_if! { @__items ($($not,)* $($m,)*) ; $($rest)* }
        };

        // Internal macro to Apply a cfg attribute to a list of items
        (@__apply $m:meta, $($it:item)*) => {
            $(#[$m] $it)*
        };
    }

    // Helper macro for specialization. This also helps avoid parse errors if the
    // default fn syntax for specialization changes in the future.
    #[cfg(feature = "nightly")]
    macro_rules! default_fn {
    	(#[$($a:tt)*] $($tt:tt)*) => {
            #[$($a)*] default $($tt)*
        }
    }
    #[cfg(not(feature = "nightly"))]
    macro_rules! default_fn {
    	($($tt:tt)*) => {
            $($tt)*
        }
    }