<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cfg-if-1.0.0/src/lib.rs`."><title>lib.rs - source</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../static.files/rustdoc-42caa33d.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="cfg_if" data-themes="" data-resource-suffix="" data-rustdoc-version="1.84.0 (9fc6b4312 2025-01-07)" data-channel="1.84.0" data-search-js="search-92e6798f.js" data-settings-js="settings-0f613d39.js" ><script src="../../static.files/storage-59e33391.js"></script><script defer src="../../static.files/src-script-56102188.js"></script><script defer src="../../src-files.js"></script><script defer src="../../static.files/main-5f194d8c.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-893ab5e7.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-6580c154.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-044be391.svg"></head><body class="rustdoc src"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="src-sidebar-title"><h2>Files</h2></div></nav><div class="sidebar-resizer"></div><main><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1><div class="sub-heading">cfg_if/</div>lib.rs</h1><rustdoc-toolbar></rustdoc-toolbar></div><div class="example-wrap"><div data-nosnippet><pre class="src-line-numbers">
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//!
//! 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() {}
//! ```

</span><span class="attr">#![no_std]
#![doc(html_root_url = <span class="string">"https://docs.rs/cfg-if"</span>)]
#![deny(missing_docs)]
#![cfg_attr(test, deny(warnings))]

</span><span class="doccomment">/// The main macro provided by this crate. See crate documentation for more
/// information.
</span><span class="attr">#[macro_export]
</span><span class="macro">macro_rules!</span> cfg_if {
    <span class="comment">// match if/else chains with a final `else`
    </span>($(
        <span class="kw">if </span><span class="attr">#[cfg(<span class="macro-nonterminal">$meta</span>:meta)] </span>{ $(<span class="macro-nonterminal">$tokens</span>:tt)* }
    ) <span class="kw">else </span>* <span class="kw">else </span>{
        $(<span class="macro-nonterminal">$tokens2</span>:tt)*
    }) =&gt; {
        <span class="macro-nonterminal">$</span><span class="macro">crate::cfg_if!</span> {
            @<span class="macro-nonterminal">__items
            </span>() ;
            $( ( (<span class="macro-nonterminal">$meta</span>) ($(<span class="macro-nonterminal">$tokens</span>)<span class="kw-2">*</span>) ), )*
            ( () ($(<span class="macro-nonterminal">$tokens2</span>)<span class="kw-2">*</span>) ),
        }
    };

    <span class="comment">// match if/else chains lacking a final `else`
    </span>(
        <span class="kw">if </span><span class="attr">#[cfg(<span class="macro-nonterminal">$i_met</span>:meta)] </span>{ $(<span class="macro-nonterminal">$i_tokens</span>:tt)* }
        $(
            <span class="kw">else if </span><span class="attr">#[cfg(<span class="macro-nonterminal">$e_met</span>:meta)] </span>{ $(<span class="macro-nonterminal">$e_tokens</span>:tt)* }
        )*
    ) =&gt; {
        <span class="macro-nonterminal">$</span><span class="macro">crate::cfg_if!</span> {
            @<span class="macro-nonterminal">__items
            </span>() ;
            ( (<span class="macro-nonterminal">$i_met</span>) ($(<span class="macro-nonterminal">$i_tokens</span>)<span class="kw-2">*</span>) ),
            $( ( (<span class="macro-nonterminal">$e_met</span>) ($(<span class="macro-nonterminal">$e_tokens</span>)<span class="kw-2">*</span>) ), )*
            ( () () ),
        }
    };

    <span class="comment">// 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
    </span>(@__items ($(<span class="macro-nonterminal">$not</span>:meta,)<span class="kw-2">*</span>) ; ) =&gt; {};
    (@__items ($(<span class="macro-nonterminal">$not</span>:meta,)<span class="kw-2">*</span>) ; ( ($(<span class="macro-nonterminal">$m</span>:meta),<span class="kw-2">*</span>) ($(<span class="macro-nonterminal">$tokens</span>:tt)<span class="kw-2">*</span>) ), $(<span class="macro-nonterminal">$rest</span>:tt)<span class="kw-2">*</span>) =&gt; {
        <span class="comment">// Emit all items within one block, applying an appropriate #[cfg]. The
        // #[cfg] will require all `$m` matchers specified and must also negate
        // all previous matchers.
        </span><span class="attr">#[cfg(all($(<span class="macro-nonterminal">$m</span>,)* not(any($(<span class="macro-nonterminal">$not</span>),<span class="kw-2">*</span>))))] </span><span class="macro-nonterminal">$</span><span class="macro">crate::cfg_if!</span> { @<span class="macro-nonterminal">__identity </span>$(<span class="macro-nonterminal">$tokens</span>)* }

        <span class="comment">// Recurse to emit all other items in `$rest`, and when we do so add all
        // our `$m` matchers to the list of `$not` matchers as future emissions
        // will have to negate everything we just matched as well.
        </span><span class="macro-nonterminal">$</span><span class="macro">crate::cfg_if!</span> { @<span class="macro-nonterminal">__items </span>($(<span class="macro-nonterminal">$not</span>,)* $(<span class="macro-nonterminal">$m</span>,)<span class="kw-2">*</span>) ; $(<span class="macro-nonterminal">$rest</span>)* }
    };

    <span class="comment">// Internal macro to make __apply work out right for different match types,
    // because of how macros matching/expand stuff.
    </span>(@__identity $(<span class="macro-nonterminal">$tokens</span>:tt)<span class="kw-2">*</span>) =&gt; {
        $(<span class="macro-nonterminal">$tokens</span>)*
    };
}

<span class="attr">#[cfg(test)]
</span><span class="kw">mod </span>tests {
    <span class="macro">cfg_if!</span> {
        <span class="kw">if </span><span class="attr">#[cfg(test)] </span>{
            <span class="kw">use </span>core::option::Option <span class="kw">as </span>Option2;
            <span class="kw">fn </span>works1() -&gt; Option2&lt;u32&gt; { <span class="prelude-val">Some</span>(<span class="number">1</span>) }
        } <span class="kw">else </span>{
            <span class="kw">fn </span>works1() -&gt; <span class="prelude-ty">Option</span>&lt;u32&gt; { <span class="prelude-val">None </span>}
        }
    }

    <span class="macro">cfg_if!</span> {
        <span class="kw">if </span><span class="attr">#[cfg(foo)] </span>{
            <span class="kw">fn </span>works2() -&gt; bool { <span class="bool-val">false </span>}
        } <span class="kw">else if </span><span class="attr">#[cfg(test)] </span>{
            <span class="kw">fn </span>works2() -&gt; bool { <span class="bool-val">true </span>}
        } <span class="kw">else </span>{
            <span class="kw">fn </span>works2() -&gt; bool { <span class="bool-val">false </span>}
        }
    }

    <span class="macro">cfg_if!</span> {
        <span class="kw">if </span><span class="attr">#[cfg(foo)] </span>{
            <span class="kw">fn </span>works3() -&gt; bool { <span class="bool-val">false </span>}
        } <span class="kw">else </span>{
            <span class="kw">fn </span>works3() -&gt; bool { <span class="bool-val">true </span>}
        }
    }

    <span class="macro">cfg_if!</span> {
        <span class="kw">if </span><span class="attr">#[cfg(test)] </span>{
            <span class="kw">use </span>core::option::Option <span class="kw">as </span>Option3;
            <span class="kw">fn </span>works4() -&gt; Option3&lt;u32&gt; { <span class="prelude-val">Some</span>(<span class="number">1</span>) }
        }
    }

    <span class="macro">cfg_if!</span> {
        <span class="kw">if </span><span class="attr">#[cfg(foo)] </span>{
            <span class="kw">fn </span>works5() -&gt; bool { <span class="bool-val">false </span>}
        } <span class="kw">else if </span><span class="attr">#[cfg(test)] </span>{
            <span class="kw">fn </span>works5() -&gt; bool { <span class="bool-val">true </span>}
        }
    }

    <span class="attr">#[test]
    </span><span class="kw">fn </span>it_works() {
        <span class="macro">assert!</span>(works1().is_some());
        <span class="macro">assert!</span>(works2());
        <span class="macro">assert!</span>(works3());
        <span class="macro">assert!</span>(works4().is_some());
        <span class="macro">assert!</span>(works5());
    }

    <span class="attr">#[test]
    #[allow(clippy::assertions_on_constants)]
    </span><span class="kw">fn </span>test_usage_within_a_function() {
        <span class="macro">cfg_if!</span> {<span class="kw">if </span><span class="attr">#[cfg(debug_assertions)] </span>{
            <span class="comment">// we want to put more than one thing here to make sure that they
            // all get configured properly.
            </span><span class="macro">assert!</span>(<span class="macro">cfg!</span>(debug_assertions));
            <span class="macro">assert_eq!</span>(<span class="number">4</span>, <span class="number">2</span>+<span class="number">2</span>);
        } <span class="kw">else </span>{
            <span class="macro">assert!</span>(works1().is_some());
            <span class="macro">assert_eq!</span>(<span class="number">10</span>, <span class="number">5</span>+<span class="number">5</span>);
        }}
    }

    <span class="kw">trait </span>Trait {
        <span class="kw">fn </span>blah(<span class="kw-2">&amp;</span><span class="self">self</span>);
    }

    <span class="attr">#[allow(dead_code)]
    </span><span class="kw">struct </span>Struct;

    <span class="kw">impl </span>Trait <span class="kw">for </span>Struct {
        <span class="macro">cfg_if!</span> {
            <span class="kw">if </span><span class="attr">#[cfg(feature = <span class="string">"blah"</span>)] </span>{
                <span class="kw">fn </span>blah(<span class="kw-2">&amp;</span><span class="self">self</span>) {
                    <span class="macro">unimplemented!</span>();
                }
            } <span class="kw">else </span>{
                <span class="kw">fn </span>blah(<span class="kw-2">&amp;</span><span class="self">self</span>) {
                    <span class="macro">unimplemented!</span>();
                }
            }
        }
    }
}
</code></pre></div></section></main></body></html>