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<h1><a class="header" href="#memreplace-to-keep-owned-values-in-changed-enums" id="memreplace-to-keep-owned-values-in-changed-enums"><code>mem::replace</code> to keep owned values in changed enums</a></h1>
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<h2><a class="header" href="#description" id="description">Description</a></h2>
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<p>Say we have a <code>&mut MyEnum</code> which has (at least) two variants,
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<code>A { name: String, x: u8 }</code> and <code>B { name: String }</code>. Now we want to change
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<code>MyEnum::A</code> to a <code>B</code> if <code>x</code> is zero, while keeping <code>MyEnum::B</code> intact.</p>
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<p>We can do this without cloning the <code>name</code>.</p>
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<h2><a class="header" href="#example" id="example">Example</a></h2>
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<pre><pre class="playground"><code class="language-rust edition2018">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>use std::mem;
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enum MyEnum {
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A { name: String, x: u8 },
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B { name: String }
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}
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fn a_to_b(e: &mut MyEnum) {
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// we mutably borrow `e` here. This precludes us from changing it directly
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// as in `*e = ...`, because the borrow checker won't allow it. Therefore
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// the assignment to `e` must be outside the `if let` clause.
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*e = if let MyEnum::A { ref mut name, x: 0 } = *e {
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// this takes out our `name` and put in an empty String instead
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// (note that empty strings don't allocate).
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// Then, construct the new enum variant (which will
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// be assigned to `*e`, because it is the result of the `if let` expression).
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MyEnum::B { name: mem::replace(name, String::new()) }
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// In all other cases, we return immediately, thus skipping the assignment
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} else { return }
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>This also works with more variants:</p>
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<pre><code class="language-Rust">use std::mem;
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enum MultiVariateEnum {
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A { name: String },
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B { name: String },
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C,
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D
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}
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fn swizzle(e: &mut MultiVariateEnum) {
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use self::MultiVariateEnum::*;
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*e = match *e {
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// Ownership rules do not allow taking `name` by value, but we cannot
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// take the value out of a mutable reference, unless we replace it:
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A { ref mut name } => B { name: mem::replace(name, String::new()) },
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B { ref mut name } => A { name: mem::replace(name, String::new()) },
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C => D,
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D => C
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}
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}
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</code></pre>
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<h2><a class="header" href="#motivation" id="motivation">Motivation</a></h2>
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<p>When working with enums, we may want to change an enum value in place, perhaps
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to another variant. This is usually done in two phases to keep the borrow
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checker happy. In the first phase, we observe the existing value and look at
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its parts to decide what to do next. In the second phase we may conditionally
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change the value (as in the example above).</p>
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<p>The borrow checker won't allow us to take out <code>name</code> of the enum (because
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<em>something</em> must be there. We could of course <code>.clone()</code> name and put the clone
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into our <code>MyEnum::B</code>, but that would be an instance of the [Clone to satisfy
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the borrow checker] antipattern. Anyway, we can avoid the extra allocation by
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changing <code>e</code> with only a mutable borrow.</p>
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<p><code>mem::replace</code> lets us swap out the value, replacing it with something else. In
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this case, we put in an empty <code>String</code>, which does not need to allocate. As a
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result, we get the original <code>name</code> <em>as an owned value</em>. We can then wrap this in
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another enum.</p>
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<p>Note, however, that if we are using an <code>Option</code> and want to replace its
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value with a <code>None</code>, <code>Option</code>’s <code>take()</code> method provides a shorter and
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more idiomatic alternative.</p>
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<h2><a class="header" href="#advantages" id="advantages">Advantages</a></h2>
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<p>Look ma, no allocation! Also you may feel like Indiana Jones while doing it.</p>
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<h2><a class="header" href="#disadvantages" id="disadvantages">Disadvantages</a></h2>
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<p>This gets a bit wordy. Getting it wrong repeatedly will make you hate the
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borrow checker. The compiler may fail to optimize away the double store,
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resulting in reduced performance as opposed to what you'd do in unsafe
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languages.</p>
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<h2><a class="header" href="#discussion" id="discussion">Discussion</a></h2>
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<p>This pattern is only of interest in Rust. In GC'd languages, you'd take the
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reference to the value by default (and the GC would keep track of refs), and in
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other low-level languages like C you'd simply alias the pointer and fix things
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later.</p>
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<p>However, in Rust, we have to do a little more work to do this. An owned value
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may only have one owner, so to take it out, we need to put something back in –
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like Indiana Jones, replacing the artifact with a bag of sand.</p>
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<h2><a class="header" href="#see-also" id="see-also">See also</a></h2>
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<p>This gets rid of the [Clone to satisfy the borrow checker] antipattern in a
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specific case.</p>
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<p>[Clone to satisfy the borrow checker](TODO: Hinges on PR #23)</p>
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