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</pre><pre class="rust"><code><span class="kw">use</span> <span class="ident">std::sync::atomic</span>::{<span class="ident">AtomicUsize</span>, <span class="ident">Ordering</span>};
<span class="kw">use</span> <span class="ident">std::sync::Once</span>;
<span class="kw">static</span> <span class="ident">WORKS</span>: <span class="ident">AtomicUsize</span> <span class="op">=</span> <span class="ident">AtomicUsize::new</span>(<span class="number">0</span>);
<span class="kw">static</span> <span class="ident">INIT</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">Once::new</span>();
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">inside_proc_macro</span>() -&gt; <span class="ident">bool</span> {
<span class="kw">match</span> <span class="ident">WORKS</span>.<span class="ident">load</span>(<span class="ident">Ordering::Relaxed</span>) {
<span class="number">1</span> =&gt; <span class="kw">return</span> <span class="bool-val">false</span>,
<span class="number">2</span> =&gt; <span class="kw">return</span> <span class="bool-val">true</span>,
<span class="kw">_</span> =&gt; {}
}
<span class="ident">INIT</span>.<span class="ident">call_once</span>(<span class="ident">initialize</span>);
<span class="ident">inside_proc_macro</span>()
}
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">force_fallback</span>() {
<span class="ident">WORKS</span>.<span class="ident">store</span>(<span class="number">1</span>, <span class="ident">Ordering::Relaxed</span>);
}
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn</span> <span class="ident">unforce_fallback</span>() {
<span class="ident">initialize</span>();
}
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">not</span>(<span class="ident">no_is_available</span>))]</span>
<span class="kw">fn</span> <span class="ident">initialize</span>() {
<span class="kw">let</span> <span class="ident">available</span> <span class="op">=</span> <span class="ident">proc_macro::is_available</span>();
<span class="ident">WORKS</span>.<span class="ident">store</span>(<span class="ident">available</span> <span class="kw">as</span> <span class="ident">usize</span> <span class="op">+</span> <span class="number">1</span>, <span class="ident">Ordering::Relaxed</span>);
}
<span class="comment">// Swap in a null panic hook to avoid printing &quot;thread panicked&quot; to stderr,</span>
<span class="comment">// then use catch_unwind to determine whether the compiler&#39;s proc_macro is</span>
<span class="comment">// working. When proc-macro2 is used from outside of a procedural macro all</span>
<span class="comment">// of the proc_macro crate&#39;s APIs currently panic.</span>
<span class="comment">//</span>
<span class="comment">// The Once is to prevent the possibility of this ordering:</span>
<span class="comment">//</span>
<span class="comment">// thread 1 calls take_hook, gets the user&#39;s original hook</span>
<span class="comment">// thread 1 calls set_hook with the null hook</span>
<span class="comment">// thread 2 calls take_hook, thinks null hook is the original hook</span>
<span class="comment">// thread 2 calls set_hook with the null hook</span>
<span class="comment">// thread 1 calls set_hook with the actual original hook</span>
<span class="comment">// thread 2 calls set_hook with what it thinks is the original hook</span>
<span class="comment">//</span>
<span class="comment">// in which the user&#39;s hook has been lost.</span>
<span class="comment">//</span>
<span class="comment">// There is still a race condition where a panic in a different thread can</span>
<span class="comment">// happen during the interval that the user&#39;s original panic hook is</span>
<span class="comment">// unregistered such that their hook is incorrectly not called. This is</span>
<span class="comment">// sufficiently unlikely and less bad than printing panic messages to stderr</span>
<span class="comment">// on correct use of this crate. Maybe there is a libstd feature request</span>
<span class="comment">// here. For now, if a user needs to guarantee that this failure mode does</span>
<span class="comment">// not occur, they need to call e.g. `proc_macro2::Span::call_site()` from</span>
<span class="comment">// the main thread before launching any other threads.</span>
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">no_is_available</span>)]</span>
<span class="kw">fn</span> <span class="ident">initialize</span>() {
<span class="kw">use</span> <span class="ident">std::panic</span>::{<span class="self">self</span>, <span class="ident">PanicInfo</span>};
<span class="kw">type</span> <span class="ident">PanicHook</span> <span class="op">=</span> <span class="kw">dyn</span> <span class="ident">Fn</span>(<span class="kw-2">&amp;</span><span class="ident">PanicInfo</span>) <span class="op">+</span> <span class="ident">Sync</span> <span class="op">+</span> <span class="ident">Send</span> <span class="op">+</span> <span class="lifetime">&#39;static</span>;
<span class="kw">let</span> <span class="ident">null_hook</span>: <span class="ident">Box</span><span class="op">&lt;</span><span class="ident">PanicHook</span><span class="op">&gt;</span> <span class="op">=</span> <span class="ident">Box::new</span>(<span class="op">|</span><span class="ident">_panic_info</span><span class="op">|</span> { <span class="comment">/* ignore */</span> });
<span class="kw">let</span> <span class="ident">sanity_check</span> <span class="op">=</span> <span class="kw-2">&amp;</span><span class="kw-2">*</span><span class="ident">null_hook</span> <span class="kw">as</span> <span class="kw-2">*const</span> <span class="ident">PanicHook</span>;
<span class="kw">let</span> <span class="ident">original_hook</span> <span class="op">=</span> <span class="ident">panic::take_hook</span>();
<span class="ident">panic::set_hook</span>(<span class="ident">null_hook</span>);
<span class="kw">let</span> <span class="ident">works</span> <span class="op">=</span> <span class="ident">panic::catch_unwind</span>(<span class="ident">proc_macro::Span::call_site</span>).<span class="ident">is_ok</span>();
<span class="ident">WORKS</span>.<span class="ident">store</span>(<span class="ident">works</span> <span class="kw">as</span> <span class="ident">usize</span> <span class="op">+</span> <span class="number">1</span>, <span class="ident">Ordering::Relaxed</span>);
<span class="kw">let</span> <span class="ident">hopefully_null_hook</span> <span class="op">=</span> <span class="ident">panic::take_hook</span>();
<span class="ident">panic::set_hook</span>(<span class="ident">original_hook</span>);
<span class="kw">if</span> <span class="ident">sanity_check</span> <span class="op">!</span><span class="op">=</span> <span class="kw-2">&amp;</span><span class="kw-2">*</span><span class="ident">hopefully_null_hook</span> {
<span class="macro">panic!</span>(<span class="string">&quot;observed race condition in proc_macro2::inside_proc_macro&quot;</span>);
}
}
</code></pre></div>
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