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</pre><div class="example-wrap"><pre class="rust ">
<span class="doccomment">//! A macro for defining `#[cfg]` if-else statements.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The macro provided by this crate, `cfg_if`, is similar to the `if/elif` C</span>
<span class="doccomment">//! preprocessor macro by allowing definition of a cascade of `#[cfg]` cases,</span>
<span class="doccomment">//! emitting the implementation which matches first.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This allows you to conveniently provide a long list `#[cfg]`'d blocks of code</span>
<span class="doccomment">//! without having to rewrite each clause multiple times.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # Example</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! cfg_if::cfg_if! {</span>
<span class="doccomment">//! if #[cfg(unix)] {</span>
<span class="doccomment">//! fn foo() { /* unix specific functionality */ }</span>
<span class="doccomment">//! } else if #[cfg(target_pointer_width = "32")] {</span>
<span class="doccomment">//! fn foo() { /* non-unix, 32-bit functionality */ }</span>
<span class="doccomment">//! } else {</span>
<span class="doccomment">//! fn foo() { /* fallback implementation */ }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # fn main() {}</span>
<span class="doccomment">//! ```</span>
<span class="attribute">#![<span class="ident">no_std</span>]</span>
<span class="attribute">#![<span class="ident">doc</span>(<span class="ident">html_root_url</span> <span class="op">=</span> <span class="string">"https://docs.rs/cfg-if"</span>)]</span>
<span class="attribute">#![<span class="ident">deny</span>(<span class="ident">missing_docs</span>)]</span>
<span class="attribute">#![<span class="ident">cfg_attr</span>(<span class="ident">test</span>, <span class="ident">deny</span>(<span class="ident">warnings</span>))]</span>
<span class="doccomment">/// The main macro provided by this crate. See crate documentation for more</span>
<span class="doccomment">/// information.</span>
<span class="attribute">#[<span class="ident">macro_export</span>]</span>
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">cfg_if</span> {
<span class="comment">// match if/else chains with a final `else`</span>
($(
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meta</span>:<span class="ident">meta</span>)]</span> { $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens</span>:<span class="ident">tt</span>)<span class="op">*</span> }
) <span class="kw">else</span> <span class="op">*</span> <span class="kw">else</span> {
$(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens2</span>:<span class="ident">tt</span>)<span class="op">*</span>
}) <span class="op">=</span><span class="op">></span> {
<span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro">cfg_if</span><span class="macro">!</span> {
@<span class="macro-nonterminal">__items</span>
() ;
$( ( (<span class="macro-nonterminal">$</span><span class="macro-nonterminal">meta</span>) ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens</span>)<span class="kw-2">*</span>) ), )<span class="op">*</span>
( () ($(<span class="macro-nonterminal">$</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="attribute">#[<span class="ident">cfg</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i_met</span>:<span class="ident">meta</span>)]</span> { $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i_tokens</span>:<span class="ident">tt</span>)<span class="op">*</span> }
$(
<span class="kw">else</span> <span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e_met</span>:<span class="ident">meta</span>)]</span> { $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e_tokens</span>:<span class="ident">tt</span>)<span class="op">*</span> }
)<span class="op">*</span>
) <span class="op">=</span><span class="op">></span> {
<span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro">cfg_if</span><span class="macro">!</span> {
@<span class="macro-nonterminal">__items</span>
() ;
( (<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i_met</span>) ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">i_tokens</span>)<span class="kw-2">*</span>) ),
$( ( (<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e_met</span>) ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">e_tokens</span>)<span class="kw-2">*</span>) ), )<span class="op">*</span>
( () () ),
}
};
<span class="comment">// Internal and recursive macro to emit all the items</span>
<span class="comment">//</span>
<span class="comment">// Collects all the negated cfgs in a list at the beginning and after the</span>
<span class="comment">// semicolon is all the remaining items</span>
(@<span class="ident">__items</span> ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>:<span class="ident">meta</span>,)<span class="kw-2">*</span>) ; ) <span class="op">=</span><span class="op">></span> {};
(@<span class="ident">__items</span> ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>:<span class="ident">meta</span>,)<span class="kw-2">*</span>) ; ( ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>:<span class="ident">meta</span>),<span class="kw-2">*</span>) ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) ), $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=</span><span class="op">></span> {
<span class="comment">// Emit all items within one block, applying an appropriate #[cfg]. The</span>
<span class="comment">// #[cfg] will require all `$m` matchers specified and must also negate</span>
<span class="comment">// all previous matchers.</span>
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">all</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>,)<span class="op">*</span> <span class="ident">not</span>(<span class="ident">any</span>($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>),<span class="kw-2">*</span>))))]</span> <span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro">cfg_if</span><span class="macro">!</span> { @<span class="macro-nonterminal">__identity</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens</span>)<span class="op">*</span> }
<span class="comment">// Recurse to emit all other items in `$rest`, and when we do so add all</span>
<span class="comment">// our `$m` matchers to the list of `$not` matchers as future emissions</span>
<span class="comment">// will have to negate everything we just matched as well.</span>
<span class="macro-nonterminal">$</span><span class="kw">crate</span>::<span class="macro">cfg_if</span><span class="macro">!</span> { @<span class="macro-nonterminal">__items</span> ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">not</span>,)<span class="op">*</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">m</span>,)<span class="kw-2">*</span>) ; $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">rest</span>)<span class="op">*</span> }
};
<span class="comment">// Internal macro to make __apply work out right for different match types,</span>
<span class="comment">// because of how macros matching/expand stuff.</span>
(@<span class="ident">__identity</span> $(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=</span><span class="op">></span> {
$(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tokens</span>)<span class="op">*</span>
};
}
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span>
<span class="kw">mod</span> <span class="ident">tests</span> {
<span class="macro">cfg_if</span><span class="macro">!</span> {
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span> {
<span class="kw">use</span> <span class="ident">core</span>::<span class="ident">option</span>::<span class="prelude-ty">Option</span> <span class="kw">as</span> <span class="ident">Option2</span>;
<span class="kw">fn</span> <span class="ident">works1</span>() <span class="op">-</span><span class="op">></span> <span class="ident">Option2</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span> { <span class="prelude-val">Some</span>(<span class="number">1</span>) }
} <span class="kw">else</span> {
<span class="kw">fn</span> <span class="ident">works1</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span> { <span class="prelude-val">None</span> }
}
}
<span class="macro">cfg_if</span><span class="macro">!</span> {
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">foo</span>)]</span> {
<span class="kw">fn</span> <span class="ident">works2</span>() <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> { <span class="bool-val">false</span> }
} <span class="kw">else</span> <span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span> {
<span class="kw">fn</span> <span class="ident">works2</span>() <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> { <span class="bool-val">true</span> }
} <span class="kw">else</span> {
<span class="kw">fn</span> <span class="ident">works2</span>() <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> { <span class="bool-val">false</span> }
}
}
<span class="macro">cfg_if</span><span class="macro">!</span> {
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">foo</span>)]</span> {
<span class="kw">fn</span> <span class="ident">works3</span>() <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> { <span class="bool-val">false</span> }
} <span class="kw">else</span> {
<span class="kw">fn</span> <span class="ident">works3</span>() <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> { <span class="bool-val">true</span> }
}
}
<span class="macro">cfg_if</span><span class="macro">!</span> {
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span> {
<span class="kw">use</span> <span class="ident">core</span>::<span class="ident">option</span>::<span class="prelude-ty">Option</span> <span class="kw">as</span> <span class="ident">Option3</span>;
<span class="kw">fn</span> <span class="ident">works4</span>() <span class="op">-</span><span class="op">></span> <span class="ident">Option3</span><span class="op"><</span><span class="ident">u32</span><span class="op">></span> { <span class="prelude-val">Some</span>(<span class="number">1</span>) }
}
}
<span class="macro">cfg_if</span><span class="macro">!</span> {
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">foo</span>)]</span> {
<span class="kw">fn</span> <span class="ident">works5</span>() <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> { <span class="bool-val">false</span> }
} <span class="kw">else</span> <span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span> {
<span class="kw">fn</span> <span class="ident">works5</span>() <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> { <span class="bool-val">true</span> }
}
}
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">it_works</span>() {
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">works1</span>().<span class="ident">is_some</span>());
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">works2</span>());
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">works3</span>());
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">works4</span>().<span class="ident">is_some</span>());
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">works5</span>());
}
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="attribute">#[<span class="ident">allow</span>(<span class="ident">clippy</span>::<span class="ident">assertions_on_constants</span>)]</span>
<span class="kw">fn</span> <span class="ident">test_usage_within_a_function</span>() {
<span class="macro">cfg_if</span><span class="macro">!</span> {<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">debug_assertions</span>)]</span> {
<span class="comment">// we want to put more than one thing here to make sure that they</span>
<span class="comment">// all get configured properly.</span>
<span class="macro">assert</span><span class="macro">!</span>(<span class="macro">cfg</span><span class="macro">!</span>(<span class="ident">debug_assertions</span>));
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">4</span>, <span class="number">2</span><span class="op">+</span><span class="number">2</span>);
} <span class="kw">else</span> {
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">works1</span>().<span class="ident">is_some</span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="number">10</span>, <span class="number">5</span><span class="op">+</span><span class="number">5</span>);
}}
}
<span class="kw">trait</span> <span class="ident">Trait</span> {
<span class="kw">fn</span> <span class="ident">blah</span>(<span class="kw-2">&</span><span class="self">self</span>);
}
<span class="attribute">#[<span class="ident">allow</span>(<span class="ident">dead_code</span>)]</span>
<span class="kw">struct</span> <span class="ident">Struct</span>;
<span class="kw">impl</span> <span class="ident">Trait</span> <span class="kw">for</span> <span class="ident">Struct</span> {
<span class="macro">cfg_if</span><span class="macro">!</span> {
<span class="kw">if</span> <span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"blah"</span>)]</span> {
<span class="kw">fn</span> <span class="ident">blah</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="macro">unimplemented</span><span class="macro">!</span>();
}
} <span class="kw">else</span> {
<span class="kw">fn</span> <span class="ident">blah</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="macro">unimplemented</span><span class="macro">!</span>();
}
}
}
}
}
</pre></div>
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