Source code
Revision control
Copy as Markdown
Other Tools
// 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 appropriate #[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)*
}
}