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</pre><pre class="rust"><code><span class="kw">use </span>alloc::boxed::Box;
<span class="kw">use </span>core::fmt;
<span class="kw">use </span>core::marker::PhantomData;
<span class="kw">use </span>core::mem::{<span class="self">self</span>, MaybeUninit};
<span class="kw">use </span>core::ptr;
<span class="doccomment">/// Number of words a piece of `Data` can hold.
///
/// Three words should be enough for the majority of cases. For example, you can fit inside it the
/// function pointer together with a fat pointer representing an object that needs to be destroyed.
</span><span class="kw">const </span>DATA_WORDS: usize = <span class="number">3</span>;
<span class="doccomment">/// Some space to keep a `FnOnce()` object on the stack.
</span><span class="kw">type </span>Data = [usize; DATA_WORDS];
<span class="doccomment">/// A `FnOnce()` that is stored inline if small, or otherwise boxed on the heap.
///
/// This is a handy way of keeping an unsized `FnOnce()` within a sized structure.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Deferred {
call: <span class="kw">unsafe fn</span>(<span class="kw-2">*mut </span>u8),
data: MaybeUninit&lt;Data&gt;,
_marker: PhantomData&lt;<span class="kw-2">*mut </span>()&gt;, <span class="comment">// !Send + !Sync
</span>}
<span class="kw">impl </span>fmt::Debug <span class="kw">for </span>Deferred {
<span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; <span class="prelude-ty">Result</span>&lt;(), fmt::Error&gt; {
f.pad(<span class="string">&quot;Deferred { .. }&quot;</span>)
}
}
<span class="kw">impl </span>Deferred {
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">const </span>NO_OP: <span class="self">Self </span>= {
<span class="kw">fn </span>no_op_call(_raw: <span class="kw-2">*mut </span>u8) {}
<span class="self">Self </span>{
call: no_op_call,
data: MaybeUninit::uninit(),
_marker: PhantomData,
}
};
<span class="doccomment">/// Constructs a new `Deferred` from a `FnOnce()`.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new&lt;F: FnOnce()&gt;(f: F) -&gt; <span class="self">Self </span>{
<span class="kw">let </span>size = mem::size_of::&lt;F&gt;();
<span class="kw">let </span>align = mem::align_of::&lt;F&gt;();
<span class="kw">unsafe </span>{
<span class="kw">if </span>size &lt;= mem::size_of::&lt;Data&gt;() &amp;&amp; align &lt;= mem::align_of::&lt;Data&gt;() {
<span class="kw">let </span><span class="kw-2">mut </span>data = MaybeUninit::&lt;Data&gt;::uninit();
ptr::write(data.as_mut_ptr().cast::&lt;F&gt;(), f);
<span class="kw">unsafe fn </span>call&lt;F: FnOnce()&gt;(raw: <span class="kw-2">*mut </span>u8) {
<span class="kw">let </span>f: F = ptr::read(raw.cast::&lt;F&gt;());
f();
}
Deferred {
call: call::&lt;F&gt;,
data,
_marker: PhantomData,
}
} <span class="kw">else </span>{
<span class="kw">let </span>b: Box&lt;F&gt; = Box::new(f);
<span class="kw">let </span><span class="kw-2">mut </span>data = MaybeUninit::&lt;Data&gt;::uninit();
ptr::write(data.as_mut_ptr().cast::&lt;Box&lt;F&gt;&gt;(), b);
<span class="kw">unsafe fn </span>call&lt;F: FnOnce()&gt;(raw: <span class="kw-2">*mut </span>u8) {
<span class="comment">// It&#39;s safe to cast `raw` from `*mut u8` to `*mut Box&lt;F&gt;`, because `raw` is
// originally derived from `*mut Box&lt;F&gt;`.
</span><span class="kw">let </span>b: Box&lt;F&gt; = ptr::read(raw.cast::&lt;Box&lt;F&gt;&gt;());
(<span class="kw-2">*</span>b)();
}
Deferred {
call: call::&lt;F&gt;,
data,
_marker: PhantomData,
}
}
}
}
<span class="doccomment">/// Calls the function.
</span><span class="attribute">#[inline]
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>call(<span class="kw-2">mut </span><span class="self">self</span>) {
<span class="kw">let </span>call = <span class="self">self</span>.call;
<span class="kw">unsafe </span>{ call(<span class="self">self</span>.data.as_mut_ptr().cast::&lt;u8&gt;()) };
}
}
<span class="attribute">#[cfg(all(test, not(crossbeam_loom)))]
</span><span class="kw">mod </span>tests {
<span class="attribute">#![allow(clippy::drop_copy)]
</span><span class="kw">use </span><span class="kw">super</span>::Deferred;
<span class="kw">use </span>std::cell::Cell;
<span class="attribute">#[test]
</span><span class="kw">fn </span>on_stack() {
<span class="kw">let </span>fired = <span class="kw-2">&amp;</span>Cell::new(<span class="bool-val">false</span>);
<span class="kw">let </span>a = [<span class="number">0usize</span>; <span class="number">1</span>];
<span class="kw">let </span>d = Deferred::new(<span class="kw">move </span>|| {
drop(a);
fired.set(<span class="bool-val">true</span>);
});
<span class="macro">assert!</span>(!fired.get());
d.call();
<span class="macro">assert!</span>(fired.get());
}
<span class="attribute">#[test]
</span><span class="kw">fn </span>on_heap() {
<span class="kw">let </span>fired = <span class="kw-2">&amp;</span>Cell::new(<span class="bool-val">false</span>);
<span class="kw">let </span>a = [<span class="number">0usize</span>; <span class="number">10</span>];
<span class="kw">let </span>d = Deferred::new(<span class="kw">move </span>|| {
drop(a);
fired.set(<span class="bool-val">true</span>);
});
<span class="macro">assert!</span>(!fired.get());
d.call();
<span class="macro">assert!</span>(fired.get());
}
<span class="attribute">#[test]
</span><span class="kw">fn </span>string() {
<span class="kw">let </span>a = <span class="string">&quot;hello&quot;</span>.to_string();
<span class="kw">let </span>d = Deferred::new(<span class="kw">move </span>|| <span class="macro">assert_eq!</span>(a, <span class="string">&quot;hello&quot;</span>));
d.call();
}
<span class="attribute">#[test]
</span><span class="kw">fn </span>boxed_slice_i32() {
<span class="kw">let </span>a: Box&lt;[i32]&gt; = <span class="macro">vec!</span>[<span class="number">2</span>, <span class="number">3</span>, <span class="number">5</span>, <span class="number">7</span>].into_boxed_slice();
<span class="kw">let </span>d = Deferred::new(<span class="kw">move </span>|| <span class="macro">assert_eq!</span>(<span class="kw-2">*</span>a, [<span class="number">2</span>, <span class="number">3</span>, <span class="number">5</span>, <span class="number">7</span>]));
d.call();
}
<span class="attribute">#[test]
</span><span class="kw">fn </span>long_slice_usize() {
<span class="kw">let </span>a: [usize; <span class="number">5</span>] = [<span class="number">2</span>, <span class="number">3</span>, <span class="number">5</span>, <span class="number">7</span>, <span class="number">11</span>];
<span class="kw">let </span>d = Deferred::new(<span class="kw">move </span>|| <span class="macro">assert_eq!</span>(a, [<span class="number">2</span>, <span class="number">3</span>, <span class="number">5</span>, <span class="number">7</span>, <span class="number">11</span>]));
d.call();
}
}
</code></pre></div>
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