sgx_unwind/macros.rs

// 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)*
    };
}