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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 //! //! ``` //! 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)* }) => { <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)* } )* ) => { <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>) ; ) => {}; (@__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>) => { <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>) => { $(<span class="macro-nonterminal">$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() -> Option2<u32> { <span class="prelude-val">Some</span>(<span class="number">1</span>) } } <span class="kw">else </span>{ <span class="kw">fn </span>works1() -> <span class="prelude-ty">Option</span><u32> { <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() -> 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() -> bool { <span class="bool-val">true </span>} } <span class="kw">else </span>{ <span class="kw">fn </span>works2() -> 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() -> bool { <span class="bool-val">false </span>} } <span class="kw">else </span>{ <span class="kw">fn </span>works3() -> 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() -> Option3<u32> { <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() -> 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() -> 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">&</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">&</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">&</span><span class="self">self</span>) { <span class="macro">unimplemented!</span>(); } } } } } </code></pre></div></section></main></body></html>