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//! 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
//!
//! ```
//! 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() {}
//! ```
#![no_std]
#![deny(missing_docs)]
#![cfg_attr(test, deny(warnings))]
/// The main macro provided by this crate. See crate documentation for more
/// information.
#[macro_export]
macro_rules! cfg_if {
// match if/else chains with a final `else`
($(
if #[cfg($meta:meta)] { $($tokens:tt)* }
) else * else {
$($tokens2:tt)*
}) => {
$crate::cfg_if! {
@__items
() ;
$( ( ($meta) ($($tokens)*) ), )*
( () ($($tokens2)*) ),
}
};
// match if/else chains lacking a final `else`
(
if #[cfg($i_met:meta)] { $($i_tokens:tt)* }
$(
else if #[cfg($e_met:meta)] { $($e_tokens:tt)* }
)*
) => {
$crate::cfg_if! {
@__items
() ;
( ($i_met) ($($i_tokens)*) ),
$( ( ($e_met) ($($e_tokens)*) ), )*
( () () ),
}
};
// 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),*) ($($tokens:tt)*) ), $($rest:tt)*) => {
// Emit all items within one block, applying an appropriate #[cfg]. The
// #[cfg] will require all `$m` matchers specified and must also negate
// all previous matchers.
#[cfg(all($($m,)* not(any($($not),*))))] $crate::cfg_if! { @__identity $($tokens)* }
// 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.
$crate::cfg_if! { @__items ($($not,)* $($m,)*) ; $($rest)* }
};
// Internal macro to make __apply work out right for different match types,
// because of how macros matching/expand stuff.
(@__identity $($tokens:tt)*) => {
$($tokens)*
};
}
#[cfg(test)]
mod tests {
cfg_if! {
if #[cfg(test)] {
use core::option::Option as Option2;
fn works1() -> Option2<u32> { Some(1) }
} else {
fn works1() -> Option<u32> { None }
}
}
cfg_if! {
if #[cfg(foo)] {
fn works2() -> bool { false }
} else if #[cfg(test)] {
fn works2() -> bool { true }
} else {
fn works2() -> bool { false }
}
}
cfg_if! {
if #[cfg(foo)] {
fn works3() -> bool { false }
} else {
fn works3() -> bool { true }
}
}
cfg_if! {
if #[cfg(test)] {
use core::option::Option as Option3;
fn works4() -> Option3<u32> { Some(1) }
}
}
cfg_if! {
if #[cfg(foo)] {
fn works5() -> bool { false }
} else if #[cfg(test)] {
fn works5() -> bool { true }
}
}
#[test]
fn it_works() {
assert!(works1().is_some());
assert!(works2());
assert!(works3());
assert!(works4().is_some());
assert!(works5());
}
#[test]
#[allow(clippy::assertions_on_constants)]
fn test_usage_within_a_function() {
cfg_if! {if #[cfg(debug_assertions)] {
// we want to put more than one thing here to make sure that they
// all get configured properly.
assert!(cfg!(debug_assertions));
assert_eq!(4, 2+2);
} else {
assert!(works1().is_some());
assert_eq!(10, 5+5);
}}
}
trait Trait {
fn blah(&self);
}
#[allow(dead_code)]
struct Struct;
impl Trait for Struct {
cfg_if! {
if #[cfg(feature = "blah")] {
fn blah(&self) {
unimplemented!();
}
} else {
fn blah(&self) {
unimplemented!();
}
}
}
}
}