| <!DOCTYPE HTML> |
| <html lang="en" class="light sidebar-visible" dir="ltr"> |
| <head> |
| <!-- Book generated using mdBook --> |
| <meta charset="UTF-8"> |
| <title>Data Schemas - Cargo Contributor Guide</title> |
| |
| |
| <!-- Custom HTML head --> |
| |
| <meta name="description" content=""> |
| <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 --> |
| |
| |
| <!-- Provide site root to javascript --> |
| <script> |
| var path_to_root = "../"; |
| var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light"; |
| </script> |
| <!-- Start loading toc.js asap --> |
| <script src="../toc.js"></script> |
| </head> |
| <body> |
| <div id="body-container"> |
| <!-- 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; } |
| const html = document.documentElement; |
| html.classList.remove('light') |
| html.classList.add(theme); |
| html.classList.add("js"); |
| </script> |
| |
| <input type="checkbox" id="sidebar-toggle-anchor" class="hidden"> |
| |
| <!-- Hide / unhide sidebar before it is displayed --> |
| <script> |
| var sidebar = null; |
| var sidebar_toggle = document.getElementById("sidebar-toggle-anchor"); |
| if (document.body.clientWidth >= 1080) { |
| try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { } |
| sidebar = sidebar || 'visible'; |
| } else { |
| sidebar = 'hidden'; |
| } |
| sidebar_toggle.checked = sidebar === 'visible'; |
| html.classList.remove('sidebar-visible'); |
| html.classList.add("sidebar-" + sidebar); |
| </script> |
| |
| <nav id="sidebar" class="sidebar" aria-label="Table of contents"> |
| <!-- populated by js --> |
| <mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox> |
| <noscript> |
| <iframe class="sidebar-iframe-outer" src="../toc.html"></iframe> |
| </noscript> |
| <div id="sidebar-resize-handle" class="sidebar-resize-handle"> |
| <div class="sidebar-resize-indicator"></div> |
| </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"> |
| <div class="left-buttons"> |
| <label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar"> |
| <i class="fa fa-bars"></i> |
| </label> |
| <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">Cargo Contributor Guide</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-lang/cargo/tree/master/src/doc/contrib/src" title="Git repository" aria-label="Git repository"> |
| <i id="git-repository-button" class="fa fa-github"></i> |
| </a> |
| <a href="https://github.com/rust-lang/cargo/edit/master/src/doc/contrib/src/implementation/schemas.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="data-schemas"><a class="header" href="#data-schemas">Data Schemas</a></h1> |
| <p>Cargo reads and writes user and machine facing data formats, like</p> |
| <ul> |
| <li><code>Cargo.toml</code>, read and written on <code>cargo package</code></li> |
| <li><code>Cargo.lock</code>, read and written</li> |
| <li><code>.cargo/config.toml</code>, read-only</li> |
| <li><code>cargo metadata</code> output</li> |
| <li><code>cargo build --message-format</code> output</li> |
| </ul> |
| <h2 id="schema-design"><a class="header" href="#schema-design">Schema Design</a></h2> |
| <p>Generally,</p> |
| <ul> |
| <li>Fields should be kebab case |
| <ul> |
| <li><code>#[serde(rename_all = "kebab-case")]</code> should be applied defensively</li> |
| </ul> |
| </li> |
| <li>Fields should only be present when needed, saving space and parse time |
| <ul> |
| <li>Also, we can always switch to always outputting the fields but its harder to stop outputting them</li> |
| <li><code>#[serde(skip_serializing_if = "Default::default")]</code> should be applied liberally</li> |
| </ul> |
| </li> |
| <li>For output, prefer <a href="https://jsonlines.org/">jsonlines</a> as it allows streaming output and flexibility to mix content (e.g. adding diagnostics to output that didn’t previously have it</li> |
| <li><code>#[serde(deny_unknown_fields)]</code> should not be used to allow evolution of formats, including feature gating</li> |
| </ul> |
| <h2 id="schema-evolution-strategies"><a class="header" href="#schema-evolution-strategies">Schema Evolution Strategies</a></h2> |
| <p>When changing a schema for data that is read, some options include:</p> |
| <ul> |
| <li>Adding new fields is relatively safe |
| <ul> |
| <li>If the field must not be ignored when present, |
| have a transition period where it is invalid to use on stable Cargo before stabilizing it or |
| error if its used before supported within the schema version |
| (e.g. <code>edition</code> requires a minimum <code>package.rust-version</code>, if present)</li> |
| </ul> |
| </li> |
| <li>Adding new values to a field is relatively safe |
| <ul> |
| <li>Unstable values should fail on stable Cargo</li> |
| </ul> |
| </li> |
| <li>Version the structure and interpretation of the data (e.g. the <code>edition</code> field or <code>package.resolver</code> which has an <code>edition</code> fallback)</li> |
| </ul> |
| <p>Note: some formats that are read are also written back out |
| (e.g. <code>cargo package</code> generating a <code>Cargo.toml</code> file) |
| and those strategies need to be considered as well.</p> |
| <p>When changing a schema for data that is written, some options include:</p> |
| <ul> |
| <li>Add new fields if the presence can be ignored</li> |
| <li>Infer permission from the users use of the new schema (e.g. a new alias for an <code>enum</code> variant)</li> |
| <li>Version the structure and interpretation of the format |
| <ul> |
| <li>Defaulting to the latest version with a warning that behavior may change (e.g. <code>cargo metadata --format-version</code>, <code>edition</code> in cargo script)</li> |
| <li>Defaulting to the first version, eventually warning the user of the implicit stale behavior (e.g. <code>package.edition</code> in <code>Cargo.toml</code>)</li> |
| <li>Without a default (e.g. <code>package.rust-version</code>, or a command-line flag like <code>--format-version</code>)</li> |
| </ul> |
| </li> |
| </ul> |
| <p>Note: While <code>serde</code> makes it easy to support data formats that add new fields, |
| new data types or supported values for a field are more difficult to future-proof |
| against.</p> |
| |
| </main> |
| |
| <nav class="nav-wrapper" aria-label="Page navigation"> |
| <!-- Mobile navigation buttons --> |
| <a rel="prev" href="../implementation/subcommands.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 prefetch" href="../implementation/console.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="../implementation/subcommands.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 prefetch" href="../implementation/console.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 --> |
| |
| |
| </div> |
| </body> |
| </html> |
