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</pre><div class="example-wrap"><pre class="rust ">
<span class="doccomment">//! `Cell` variant for (scoped) existential lifetimes.</span>
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">Cell</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ops</span>::{<span class="ident">Deref</span>, <span class="ident">DerefMut</span>};
<span class="doccomment">/// Type lambda application, with a lifetime.</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Out</span>;
}
<span class="doccomment">/// Type lambda taking a lifetime, i.e., `Lifetime -> Type`.</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">LambdaL</span>: <span class="kw">for</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="kw">for</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">>></span> <span class="ident">LambdaL</span> <span class="kw">for</span> <span class="ident">T</span> {}
<span class="comment">// HACK(eddyb) work around projection limitations with a newtype</span>
<span class="comment">// FIXME(#52812) replace with `&'a mut <T as ApplyL<'b>>::Out`</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">RefMutL</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="lifetime">'b</span>, <span class="ident">T</span>: <span class="ident">LambdaL</span><span class="op">></span>(<span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="op"><</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'b</span><span class="op">>></span>::<span class="ident">Out</span>);
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="lifetime">'b</span>, <span class="ident">T</span>: <span class="ident">LambdaL</span><span class="op">></span> <span class="ident">Deref</span> <span class="kw">for</span> <span class="ident">RefMutL</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="lifetime">'b</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Target</span> <span class="op">=</span> <span class="op"><</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'b</span><span class="op">>></span>::<span class="ident">Out</span>;
<span class="kw">fn</span> <span class="ident">deref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="self">Self</span>::<span class="ident">Target</span> {
<span class="self">self</span>.<span class="number">0</span>
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="lifetime">'b</span>, <span class="ident">T</span>: <span class="ident">LambdaL</span><span class="op">></span> <span class="ident">DerefMut</span> <span class="kw">for</span> <span class="ident">RefMutL</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="lifetime">'b</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">deref_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">Self</span>::<span class="ident">Target</span> {
<span class="self">self</span>.<span class="number">0</span>
}
}
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">ScopedCell</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">LambdaL</span><span class="op">></span>(<span class="ident">Cell</span><span class="op"><<</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">>></span>::<span class="ident">Out</span><span class="op">></span>);
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">LambdaL</span><span class="op">></span> <span class="ident">ScopedCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">pub</span> <span class="kw">const</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">value</span>: <span class="op"><</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">>></span>::<span class="ident">Out</span>) <span class="op">-></span> <span class="self">Self</span> {
<span class="ident">ScopedCell</span>(<span class="ident">Cell</span>::<span class="ident">new</span>(<span class="ident">value</span>))
}
<span class="doccomment">/// Sets the value in `self` to `replacement` while</span>
<span class="doccomment">/// running `f`, which gets the old value, mutably.</span>
<span class="doccomment">/// The old value will be restored after `f` exits, even</span>
<span class="doccomment">/// by panic, including modifications made to it by `f`.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">replace</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">R</span><span class="op">></span>(
<span class="kw-2">&</span><span class="self">self</span>,
<span class="ident">replacement</span>: <span class="op"><</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">>></span>::<span class="ident">Out</span>,
<span class="ident">f</span>: <span class="kw">impl</span> <span class="kw">for</span><span class="op"><</span><span class="lifetime">'b</span>, <span class="lifetime">'c</span><span class="op">></span> <span class="ident">FnOnce</span>(<span class="ident">RefMutL</span><span class="op"><</span><span class="lifetime">'b</span>, <span class="lifetime">'c</span>, <span class="ident">T</span><span class="op">></span>) <span class="op">-></span> <span class="ident">R</span>,
) <span class="op">-></span> <span class="ident">R</span> {
<span class="doccomment">/// Wrapper that ensures that the cell always gets filled</span>
<span class="doccomment">/// (with the original state, optionally changed by `f`),</span>
<span class="doccomment">/// even if `f` had panicked.</span>
<span class="kw">struct</span> <span class="ident">PutBackOnDrop</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="ident">LambdaL</span><span class="op">></span> {
<span class="ident">cell</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">ScopedCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
<span class="ident">value</span>: <span class="prelude-ty">Option</span><span class="op"><<</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'static</span><span class="op">>></span>::<span class="ident">Out</span><span class="op">></span>,
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="ident">LambdaL</span><span class="op">></span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">PutBackOnDrop</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) {
<span class="self">self</span>.<span class="ident">cell</span>.<span class="number">0</span>.<span class="ident">set</span>(<span class="self">self</span>.<span class="ident">value</span>.<span class="ident">take</span>().<span class="ident">unwrap</span>());
}
}
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">put_back_on_drop</span> <span class="op">=</span> <span class="ident">PutBackOnDrop</span> {
<span class="ident">cell</span>: <span class="self">self</span>,
<span class="ident">value</span>: <span class="prelude-val">Some</span>(<span class="self">self</span>.<span class="number">0</span>.<span class="ident">replace</span>(<span class="kw">unsafe</span> {
<span class="kw">let</span> <span class="ident">erased</span> <span class="op">=</span> <span class="ident">mem</span>::<span class="ident">transmute_copy</span>(<span class="kw-2">&</span><span class="ident">replacement</span>);
<span class="ident">mem</span>::<span class="ident">forget</span>(<span class="ident">replacement</span>);
<span class="ident">erased</span>
})),
};
<span class="ident">f</span>(<span class="ident">RefMutL</span>(<span class="ident">put_back_on_drop</span>.<span class="ident">value</span>.<span class="ident">as_mut</span>().<span class="ident">unwrap</span>()))
}
<span class="doccomment">/// Sets the value in `self` to `value` while running `f`.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">set</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">R</span><span class="op">></span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">value</span>: <span class="op"><</span><span class="ident">T</span> <span class="kw">as</span> <span class="ident">ApplyL</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">>></span>::<span class="ident">Out</span>, <span class="ident">f</span>: <span class="kw">impl</span> <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">R</span>) <span class="op">-></span> <span class="ident">R</span> {
<span class="self">self</span>.<span class="ident">replace</span>(<span class="ident">value</span>, <span class="op">|</span><span class="kw">_</span><span class="op">|</span> <span class="ident">f</span>())
}
}
</pre></div>
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