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    // implements the unary operator "op &T"
    // based on "op T" where T is expected to be `Copy`able
    macro_rules! forward_ref_unop {
        (impl $imp:ident, $method:ident for $t:ty) => {
            forward_ref_unop!(impl $imp, $method for $t,
                    #[stable(feature = "rust1", since = "1.0.0")]);
        };
        (impl $imp:ident, $method:ident for $t:ty, #[$attr:meta]) => {
            #[$attr]
            impl $imp for &$t {
                type Output = <$t as $imp>::Output;

                #[inline]
                fn $method(self) -> <$t as $imp>::Output {
                    $imp::$method(*self)
                }
            }
        }
    }

    // implements binary operators "&T op U", "T op &U", "&T op &U"
    // based on "T op U" where T and U are expected to be `Copy`able
    macro_rules! forward_ref_binop {
        (impl $imp:ident, $method:ident for $t:ty, $u:ty) => {
            forward_ref_binop!(impl $imp, $method for $t, $u,
                    #[stable(feature = "rust1", since = "1.0.0")]);
        };
        (impl $imp:ident, $method:ident for $t:ty, $u:ty, #[$attr:meta]) => {
            #[$attr]
            impl<'a> $imp<$u> for &'a $t {
                type Output = <$t as $imp<$u>>::Output;

                #[inline]
                fn $method(self, other: $u) -> <$t as $imp<$u>>::Output {
                    $imp::$method(*self, other)
                }
            }

            #[$attr]
            impl $imp<&$u> for $t {
                type Output = <$t as $imp<$u>>::Output;

                #[inline]
                fn $method(self, other: &$u) -> <$t as $imp<$u>>::Output {
                    $imp::$method(self, *other)
                }
            }

            #[$attr]
            impl $imp<&$u> for &$t {
                type Output = <$t as $imp<$u>>::Output;

                #[inline]
                fn $method(self, other: &$u) -> <$t as $imp<$u>>::Output {
                    $imp::$method(*self, *other)
                }
            }
        }
    }

    // implements "T op= &U", based on "T op= U"
    // where U is expected to be `Copy`able
    macro_rules! forward_ref_op_assign {
        (impl $imp:ident, $method:ident for $t:ty, $u:ty) => {
            forward_ref_op_assign!(impl $imp, $method for $t, $u,
                    #[stable(feature = "op_assign_builtins_by_ref", since = "1.22.0")]);
        };
        (impl $imp:ident, $method:ident for $t:ty, $u:ty, #[$attr:meta]) => {
            #[$attr]
            impl $imp<&$u> for $t {
                #[inline]
                fn $method(&mut self, other: &$u) {
                    $imp::$method(self, *other);
                }
            }
        }
    }

    /// Create a zero-size type similar to a closure type, but named.
    #[unstable(feature = "std_internals", issue = "0")]
    macro_rules! impl_fn_for_zst {
        ($(
            $( #[$attr: meta] )*
            // FIXME: when libcore is in the 2018 edition, use `?` repetition in
            // $( <$( $li : lifetime ),+> )?
            struct $Name: ident impl$( <$( $lifetime : lifetime ),+> )* Fn =
                |$( $arg: ident: $ArgTy: ty ),*| -> $ReturnTy: ty
                $body: block;
        )+) => {
            $(
                $( #[$attr] )*
                struct $Name;

                impl $( <$( $lifetime ),+> )* Fn<($( $ArgTy, )*)> for $Name {
                    #[inline]
                    extern "rust-call" fn call(&self, ($( $arg, )*): ($( $ArgTy, )*)) -> $ReturnTy {
                        $body
                    }
                }

                impl $( <$( $lifetime ),+> )* FnMut<($( $ArgTy, )*)> for $Name {
                    #[inline]
                    extern "rust-call" fn call_mut(
                        &mut self,
                        ($( $arg, )*): ($( $ArgTy, )*)
                    ) -> $ReturnTy {
                        Fn::call(&*self, ($( $arg, )*))
                    }
                }

                impl $( <$( $lifetime ),+> )* FnOnce<($( $ArgTy, )*)> for $Name {
                    type Output = $ReturnTy;

                    #[inline]
                    extern "rust-call" fn call_once(self, ($( $arg, )*): ($( $ArgTy, )*)) -> $ReturnTy {
                        Fn::call(&self, ($( $arg, )*))
                    }
                }
            )+
        }
    }

    /// A macro for defining `#[cfg]` if-else statements.
    ///
    /// The macro provided by this crate, `cfg_if`, is similar to the `if/elif` C
    /// preprocessor macro by allowing definition of a cascade of `#[cfg]` cases,
    /// emitting the implementation which matches first.
    ///
    /// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
    /// without having to rewrite each clause multiple times.
    ///
    /// # Example
    ///
    /// ```
    /// #[macro_use]
    /// extern crate cfg_if;
    ///
    /// cfg_if! {
    ///     if #[cfg(unix)] {
    ///         fn foo() { /* unix specific functionality */ }
    ///     } else if #[cfg(target_pointer_width = "32")] {
    ///         fn foo() { /* non-unix, 32-bit functionality */ }
    ///     } else {
    ///         fn foo() { /* fallback implementation */ }
    ///     }
    /// }
    ///
    /// # fn main() {}
    /// ```
    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)*
        };
    }

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