| // Copyright 2017 The Rust Project Developers. See the COPYRIGHT |
| // file at the top-level directory of this distribution and at |
| // http://rust-lang.org/COPYRIGHT. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| /// A macro for defining `#[cfg]` if-else statements. |
| /// |
| /// This 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. |
| |
| #[allow(unused_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)* |
| }; |
| } |
| |