| //! 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() {} |
| //! ``` |
| |
| #![no_std] |
| |
| #![doc(html_root_url = "https://docs.rs/cfg-if")] |
| #![deny(missing_docs)] |
| #![cfg_attr(test, deny(warnings))] |
| |
| /// The main macro provided by this crate. See crate documentation for more |
| /// information. |
| #[macro_export(local_inner_macros)] |
| 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)* |
| }; |
| } |
| |
| #[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()); |
| } |
| } |