mirror of
https://github.com/rust-unofficial/patterns
synced 2024-11-04 18:00:27 +00:00
312 lines
22 KiB
HTML
312 lines
22 KiB
HTML
<!DOCTYPE HTML>
|
|
<html lang="en" class="sidebar-visible no-js rust">
|
|
<head>
|
|
<!-- Book generated using mdBook -->
|
|
<meta charset="UTF-8">
|
|
<title>Privacy For Extensibility - Rust Design Patterns</title>
|
|
|
|
|
|
<!-- Custom HTML head -->
|
|
|
|
<meta name="description" content="A catalogue of Rust design patterns, anti-patterns and idioms">
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|
<meta name="theme-color" content="#ffffff" />
|
|
|
|
<link rel="icon" href="../favicon.svg">
|
|
<link rel="shortcut icon" href="../favicon.png">
|
|
<link rel="stylesheet" href="../css/variables.css">
|
|
<link rel="stylesheet" href="../css/general.css">
|
|
<link rel="stylesheet" href="../css/chrome.css">
|
|
<link rel="stylesheet" href="../css/print.css" media="print">
|
|
|
|
<!-- Fonts -->
|
|
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
|
|
<link rel="stylesheet" href="../fonts/fonts.css">
|
|
|
|
<!-- Highlight.js Stylesheets -->
|
|
<link rel="stylesheet" href="../highlight.css">
|
|
<link rel="stylesheet" href="../tomorrow-night.css">
|
|
<link rel="stylesheet" href="../ayu-highlight.css">
|
|
|
|
<!-- Custom theme stylesheets -->
|
|
|
|
</head>
|
|
<body>
|
|
<!-- Provide site root to javascript -->
|
|
<script>
|
|
var path_to_root = "../";
|
|
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "rust";
|
|
</script>
|
|
|
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
|
<script>
|
|
try {
|
|
var theme = localStorage.getItem('mdbook-theme');
|
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
|
|
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
|
}
|
|
|
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
|
}
|
|
} catch (e) { }
|
|
</script>
|
|
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|
<script>
|
|
var theme;
|
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
|
var html = document.querySelector('html');
|
|
html.classList.remove('no-js')
|
|
html.classList.remove('rust')
|
|
html.classList.add(theme);
|
|
html.classList.add('js');
|
|
</script>
|
|
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|
<script>
|
|
var html = document.querySelector('html');
|
|
var sidebar = 'hidden';
|
|
if (document.body.clientWidth >= 1080) {
|
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
|
sidebar = sidebar || 'visible';
|
|
}
|
|
html.classList.remove('sidebar-visible');
|
|
html.classList.add("sidebar-" + sidebar);
|
|
</script>
|
|
|
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
|
<div class="sidebar-scrollbox">
|
|
<ol class="chapter"><li class="chapter-item expanded "><a href="../intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../translations.html"><strong aria-hidden="true">1.1.</strong> Translations</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/index.html"><strong aria-hidden="true">2.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/coercion-arguments.html"><strong aria-hidden="true">2.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../idioms/concat-format.html"><strong aria-hidden="true">2.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../idioms/ctor.html"><strong aria-hidden="true">2.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../idioms/default.html"><strong aria-hidden="true">2.4.</strong> The Default Trait</a></li><li class="chapter-item expanded "><a href="../idioms/deref.html"><strong aria-hidden="true">2.5.</strong> Collections Are Smart Pointers</a></li><li class="chapter-item expanded "><a href="../idioms/dtor-finally.html"><strong aria-hidden="true">2.6.</strong> Finalisation in Destructors</a></li><li class="chapter-item expanded "><a href="../idioms/mem-replace.html"><strong aria-hidden="true">2.7.</strong> mem::{take(_), replace(_)}</a></li><li class="chapter-item expanded "><a href="../idioms/on-stack-dyn-dispatch.html"><strong aria-hidden="true">2.8.</strong> On-Stack Dynamic Dispatch</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/intro.html"><strong aria-hidden="true">2.9.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../idioms/ffi/errors.html"><strong aria-hidden="true">2.9.1.</strong> Idiomatic Errors</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/accepting-strings.html"><strong aria-hidden="true">2.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../idioms/ffi/passing-strings.html"><strong aria-hidden="true">2.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../idioms/option-iter.html"><strong aria-hidden="true">2.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../idioms/pass-var-to-closure.html"><strong aria-hidden="true">2.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../idioms/priv-extend.html" class="active"><strong aria-hidden="true">2.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../idioms/rustdoc-init.html"><strong aria-hidden="true">2.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../idioms/temporary-mutability.html"><strong aria-hidden="true">2.14.</strong> Temporary mutability</a></li><li class="chapter-item expanded "><a href="../idioms/return-consumed-arg-on-error.html"><strong aria-hidden="true">2.15.</strong> Return consumed arg on error</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/index.html"><strong aria-hidden="true">3.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/intro.html"><strong aria-hidden="true">3.1.</strong> Behavioural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/behavioural/command.html"><strong aria-hidden="true">3.1.1.</strong> Command</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/interpreter.html"><strong aria-hidden="true">3.1.2.</strong> Interpreter</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/newtype.html"><strong aria-hidden="true">3.1.3.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/RAII.html"><strong aria-hidden="true">3.1.4.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/strategy.html"><strong aria-hidden="true">3.1.5.</strong> Strategy</a></li><li class="chapter-item expanded "><a href="../patterns/behavioural/visitor.html"><strong aria-hidden="true">3.1.6.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/creational/intro.html"><strong aria-hidden="true">3.2.</strong> Creational</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/creational/builder.html"><strong aria-hidden="true">3.2.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../patterns/creational/fold.html"><strong aria-hidden="true">3.2.2.</strong> Fold</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/structural/intro.html"><strong aria-hidden="true">3.3.</strong> Structural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/structural/compose-structs.html"><strong aria-hidden="true">3.3.1.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../patterns/structural/small-crates.html"><strong aria-hidden="true">3.3.2.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../patterns/structural/unsafe-mods.html"><strong aria-hidden="true">3.3.3.</strong> Contain unsafety in small modules</a></li></ol></li><li class="chapter-item expanded "><a href="../patterns/ffi/intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../patterns/ffi/export.html"><strong aria-hidden="true">3.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../patterns/ffi/wrappers.html"><strong aria-hidden="true">3.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../anti_patterns/index.html"><strong aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../anti_patterns/borrow_clone.html"><strong aria-hidden="true">4.1.</strong> Clone to satisfy the borrow checker</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.2.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../anti_patterns/deref.html"><strong aria-hidden="true">4.3.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../functional/index.html"><strong aria-hidden="true">5.</strong> Functional Programming</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../functional/paradigms.html"><strong aria-hidden="true">5.1.</strong> Programming paradigms</a></li><li class="chapter-item expanded "><a href="../functional/generics-type-classes.html"><strong aria-hidden="true">5.2.</strong> Generics as Type Classes</a></li><li class="chapter-item expanded "><a href="../functional/lenses.html"><strong aria-hidden="true">5.3.</strong> Lenses and Prisms</a></li></ol></li><li class="chapter-item expanded "><a href="../additional_resources/index.html"><strong aria-hidden="true">6.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../additional_resources/design-principles.html"><strong aria-hidden="true">6.1.</strong> Design principles</a></li></ol></li></ol>
|
|
</div>
|
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
|
</nav>
|
|
|
|
<div id="page-wrapper" class="page-wrapper">
|
|
|
|
<div class="page">
|
|
<div id="menu-bar-hover-placeholder"></div>
|
|
<div id="menu-bar" class="menu-bar sticky bordered">
|
|
<div class="left-buttons">
|
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
|
<i class="fa fa-bars"></i>
|
|
</button>
|
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
|
<i class="fa fa-paint-brush"></i>
|
|
</button>
|
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
|
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
|
</ul>
|
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
|
<i class="fa fa-search"></i>
|
|
</button>
|
|
</div>
|
|
|
|
<h1 class="menu-title">Rust Design Patterns</h1>
|
|
|
|
<div class="right-buttons">
|
|
<a href="../print.html" title="Print this book" aria-label="Print this book">
|
|
<i id="print-button" class="fa fa-print"></i>
|
|
</a>
|
|
<a href="https://github.com/rust-unofficial/patterns" title="Git repository" aria-label="Git repository">
|
|
<i id="git-repository-button" class="fa fa-github"></i>
|
|
</a>
|
|
<a href="https://github.com/rust-unofficial/patterns/edit/main/./idioms/priv-extend.md" title="Suggest an edit" aria-label="Suggest an edit">
|
|
<i id="git-edit-button" class="fa fa-edit"></i>
|
|
</a>
|
|
|
|
</div>
|
|
</div>
|
|
|
|
<div id="search-wrapper" class="hidden">
|
|
<form id="searchbar-outer" class="searchbar-outer">
|
|
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
|
</form>
|
|
<div id="searchresults-outer" class="searchresults-outer hidden">
|
|
<div id="searchresults-header" class="searchresults-header"></div>
|
|
<ul id="searchresults">
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
|
|
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
|
<script>
|
|
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
|
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
|
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
|
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
|
});
|
|
</script>
|
|
|
|
<div id="content" class="content">
|
|
<main>
|
|
<h1 id="non_exhaustive-and-private-fields-for-extensibility"><a class="header" href="#non_exhaustive-and-private-fields-for-extensibility"><code>#[non_exhaustive]</code> and private fields for extensibility</a></h1>
|
|
<h2 id="description"><a class="header" href="#description">Description</a></h2>
|
|
<p>A small set of scenarios exist where a library author may want to add public
|
|
fields to a public struct or new variants to an enum without breaking backwards
|
|
compatibility.</p>
|
|
<p>Rust offers two solutions to this problem:</p>
|
|
<ul>
|
|
<li>
|
|
<p>Use <code>#[non_exhaustive]</code> on <code>struct</code>s, <code>enum</code>s, and <code>enum</code> variants.
|
|
For extensive documentation on all the places where <code>#[non_exhaustive]</code> can be
|
|
used, see <a href="https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute">the docs</a>.</p>
|
|
</li>
|
|
<li>
|
|
<p>You may add a private field to a struct to prevent it from being directly
|
|
instantiated or matched against (see Alternative)</p>
|
|
</li>
|
|
</ul>
|
|
<h2 id="example"><a class="header" href="#example">Example</a></h2>
|
|
<pre><pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>mod a {
|
|
// Public struct.
|
|
#[non_exhaustive]
|
|
pub struct S {
|
|
pub foo: i32,
|
|
}
|
|
|
|
#[non_exhaustive]
|
|
pub enum AdmitMoreVariants {
|
|
VariantA,
|
|
VariantB,
|
|
#[non_exhaustive]
|
|
VariantC { a: String }
|
|
}
|
|
}
|
|
|
|
fn print_matched_variants(s: a::S) {
|
|
// Because S is `#[non_exhaustive]`, it cannot be named here and
|
|
// we must use `..` in the pattern.
|
|
let a::S { foo: _, ..} = s;
|
|
|
|
let some_enum = a::AdmitMoreVariants::VariantA;
|
|
match some_enum {
|
|
a::AdmitMoreVariants::VariantA => println!("it's an A"),
|
|
a::AdmitMoreVariants::VariantB => println!("it's a b"),
|
|
|
|
// .. required because this variant is non-exhaustive as well
|
|
a::AdmitMoreVariants::VariantC { a, .. } => println!("it's a c"),
|
|
|
|
// The wildcard match is required because more variants may be
|
|
// added in the future
|
|
_ => println!("it's a new variant")
|
|
}
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<h2 id="alternative-private-fields-for-structs"><a class="header" href="#alternative-private-fields-for-structs">Alternative: <code>Private fields</code> for structs</a></h2>
|
|
<p><code>#[non_exhaustive]</code> only works across crate boundaries.
|
|
Within a crate, the private field method may be used.</p>
|
|
<p>Adding a field to a struct is a mostly backwards compatible change.
|
|
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.
|
|
The client could name some fields and use <code>..</code> in the pattern, in which case adding
|
|
another field is backwards compatible.
|
|
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>
|
|
<p>The downside of this approach is that you might need to add an otherwise unneeded
|
|
field to the struct.
|
|
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>
|
|
<pre><pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>pub struct S {
|
|
pub a: i32,
|
|
// Because `b` is private, you cannot match on `S` without using `..` and `S`
|
|
// cannot be directly instantiated or matched against
|
|
_b: ()
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<h2 id="discussion"><a class="header" href="#discussion">Discussion</a></h2>
|
|
<p>On <code>struct</code>s, <code>#[non_exhaustive]</code> allows adding additional fields in a backwards
|
|
compatible way.
|
|
It will also prevent clients from using the struct constructor, even if all the
|
|
fields are public.
|
|
This may be helpful, but it's worth considering if you <em>want</em> an additional field
|
|
to be found by clients as a compiler error rather than something that may be silently
|
|
undiscovered.</p>
|
|
<p><code>#[non_exhaustive]</code> can be applied to enum variants as well.
|
|
A <code>#[non_exhaustive]</code> variant behaves in the same way as a <code>#[non_exhaustive]</code> struct.</p>
|
|
<p>Use this deliberately and with caution: incrementing the major version when adding
|
|
fields or variants is often a better option.
|
|
<code>#[non_exhaustive]</code> may be appropriate in scenarios where you're modeling an external
|
|
resource that may change out-of-sync with your library, but is not a general purpose
|
|
tool.</p>
|
|
<h3 id="disadvantages"><a class="header" href="#disadvantages">Disadvantages</a></h3>
|
|
<p><code>#[non_exhaustive]</code> can make your code much less ergonomic to use, especially when
|
|
forced to handle unknown enum variants.
|
|
It should only be used when these sorts of evolutions are required <strong>without</strong>
|
|
incrementing the major version.</p>
|
|
<p>When <code>#[non_exhaustive]</code> is applied to <code>enum</code>s, it forces clients to handle a
|
|
wildcard variant.
|
|
If there is no sensible action to take in this case, this may lead to awkward
|
|
code and code paths that are only executed in extremely rare circumstances.
|
|
If a client decides to <code>panic!()</code> in this scenario, it may have been better to
|
|
expose this error at compile time.
|
|
In fact, <code>#[non_exhaustive]</code> forces clients to handle the "Something else" case;
|
|
there is rarely a sensible action to take in this scenario.</p>
|
|
<h2 id="see-also"><a class="header" href="#see-also">See also</a></h2>
|
|
<ul>
|
|
<li><a href="https://github.com/rust-lang/rfcs/blob/master/text/2008-non-exhaustive.md">RFC introducing #[non_exhaustive] attribute for enums and structs</a></li>
|
|
</ul>
|
|
|
|
</main>
|
|
|
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|
<!-- Mobile navigation buttons -->
|
|
<a rel="prev" href="../idioms/pass-var-to-closure.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
<a rel="next" href="../idioms/rustdoc-init.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
|
|
<div style="clear: both"></div>
|
|
</nav>
|
|
</div>
|
|
</div>
|
|
|
|
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
|
<a rel="prev" href="../idioms/pass-var-to-closure.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|
<i class="fa fa-angle-left"></i>
|
|
</a>
|
|
|
|
<a rel="next" href="../idioms/rustdoc-init.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|
<i class="fa fa-angle-right"></i>
|
|
</a>
|
|
</nav>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<script>
|
|
window.playground_copyable = true;
|
|
</script>
|
|
|
|
|
|
<script src="../elasticlunr.min.js"></script>
|
|
<script src="../mark.min.js"></script>
|
|
<script src="../searcher.js"></script>
|
|
|
|
<script src="../clipboard.min.js"></script>
|
|
<script src="../highlight.js"></script>
|
|
<script src="../book.js"></script>
|
|
|
|
<!-- Custom JS scripts -->
|
|
|
|
|
|
</body>
|
|
</html>
|