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aria-hidden="true">3.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../idioms/ffi-passing-strings.html"><strong aria-hidden="true">3.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/option-iter.html"><strong aria-hidden="true">3.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../idioms/pass-var-to-closure.html"><strong aria-hidden="true">3.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../idioms/priv-extend.html" class="active"><strong aria-hidden="true">3.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../idioms/rustdoc-init.html"><strong aria-hidden="true">3.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../idioms/temporary-mutability.html"><strong aria-hidden="true">3.14.</strong> Temporary mutability</a></li></ol></li><li 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Consolidation into Wrappers</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/fold.html"><strong aria-hidden="true">4.5.</strong> Fold</a></li><li class="chapter-item expanded "><a href="../patterns/newtype.html"><strong aria-hidden="true">4.6.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/RAII.html"><strong aria-hidden="true">4.7.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/small-crates.html"><strong aria-hidden="true">4.8.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/unsafe-mods.html"><strong aria-hidden="true">4.9.</strong> Contain unsafety in small modules</a></li><li class="chapter-item expanded "><a href="../patterns/visitor.html"><strong aria-hidden="true">4.10.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">5.</strong> 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<h1><a class="header" href="#privacy-for-extensibility" id="privacy-for-extensibility">Privacy for extensibility</a></h1>
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<h2><a class="header" href="#description" id="description">Description</a></h2>
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<p>Use a private field to ensure that a struct is extensible without breaking
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stability guarantees.</p>
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<h2><a class="header" href="#example" id="example">Example</a></h2>
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<pre><code class="language-rust ignore">mod a {
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// Public struct.
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pub struct S {
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pub foo: i32,
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// Private field.
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bar: i32,
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}
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}
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fn main(s: a::S) {
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// Because S::bar is private, it cannot be named here and we must use `..`
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// in the pattern.
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let a::S { foo: _, ..} = s;
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}
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</code></pre>
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<h2><a class="header" href="#discussion" id="discussion">Discussion</a></h2>
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<p>Adding a field to a struct is a mostly backwards compatible change.
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However, if a client uses a pattern to deconstruct a struct instance, they might name all the fields in the struct and adding a new one would break that pattern.
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The client could name some of the fields and use <code>..</code> in the pattern, in which case adding another field is backwards compatible.
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Making at least one of the struct's fields private forces clients to use the latter form of patterns, ensuring that the struct is future-proof.</p>
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<p>The downside of this approach is that you might need to add an otherwise unneeded field to the struct.
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You can use the <code>()</code> type so that there is no runtime overhead and prepend <code>_</code> to the field name to avoid the unused field warning.</p>
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<p>If Rust allowed private variants of enums, we could use the same trick to make adding a variant to an enum backwards compatible.
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The problem there is exhaustive match expressions.
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A private variant would force clients to have a <code>_</code> wildcard pattern.
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A common way to implement this instead is using the <a href="https://doc.rust-lang.org/reference/attributes/type_system.html">#[non_exhaustive]</a> attribute.</p>
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