--- title: Protecting the Rust standard library from accidental... description: The Rust standard library now runs cargo-semver-checks in CI to catch accidental breakage of stable APIs, following a history of incidents where unstable... canonical: https://daily.dev/posts/protecting-the-rust-standard-library-from-accidental-breakage-6185skjif twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Protecting the Rust standard library from accidental breakage | daily.dev og:description: The Rust standard library now runs cargo-semver-checks in CI to catch accidental breakage of stable APIs, following a history of incidents where unstable... og:url: https://daily.dev/posts/protecting-the-rust-standard-library-from-accidental-breakage-6185skjif og:image: https://api.daily.dev/og/posts/6185skjIf.png og:image:alt: Protecting the Rust standard library from accidental breakage og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Protecting the Rust standard library from accidental breakage **[Predrag Gruevski](https://daily.dev/sources/predrag)** · 10 min read · 35 upvotes · 2 comments ## Summary The Rust standard library now runs cargo-semver-checks in CI to catch accidental breakage of stable APIs, following a history of incidents where unstable methods or soundness fixes accidentally broke stable code (async-std in 2020, BuildHasher's dyn-safety in 2021, ChunksMut's Send/Sync in 2022, and a tokio Windows breakage in Rust 1.94 that required a 1.94.1 point release). The work required teaching cargo-semver-checks to understand Rust's stability attributes (#[stable], #[unstable], const-stability, default-body stability) and exposing that data through new rustdoc JSON fields, reusing existing #[doc(hidden)] handling infrastructure so hundreds of existing lints work correctly without modification. The effort grew out of hallway conversations at RustWeek 2026 and the Rust All Hands, took months and 15,000+ lines of code, and remaining work includes better UX for reporting stability breakage and covering more platforms. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why did tokio's test suite fail to compile on Windows in Rust 1.94? A standard library trait gained new unstable methods that inadvertently broke tokio's test suite compilation on Windows in Rust 1.94. The breakage was significant enough that a fix shipped in the Rust 1.94.1 point release rather than waiting for the next stable release cycle. _Rust upgraders tracking point-release fixes like this can follow stdlib stability work on daily.dev._ ### How does cargo-semver-checks handle unstable Rust standard library APIs without flagging their changes as breaking? Items marked #[unstable] are treated as non-public API, exactly like #[doc(hidden)] items, so their changes never trigger breakage warnings. The same logic extends to const-unstable items (treated as non-const) and unstable trait method defaults (treated as not provided), letting hundreds of existing lints work correctly without rewrites. _Developers weighing semver tooling for their own crates can dig into these stability-modeling tricks on daily.dev._ ### What kinds of stability metadata did rustdoc JSON need to expose to support semver checking of the Rust standard library? Rustdoc JSON needed three new fields: an Item::stability field for whether an item is stable or unstable, an Item::const_stability field for whether const usage is stable, and default_unstable fields on Function, AssocConst, and AssocType to capture whether a trait's default implementation is stable. Together these let tooling distinguish genuine public API breakage from changes to unstable surface area. _Anyone building on rustdoc JSON internals can track this kind of stability metadata work via daily.dev._ ## Community discussion Top comments from developers on daily.dev. **@agustinbarrientos** · 2 upvotes > Will Windows join the CI matrix after that tokio regression? **@trevorsuna** · 0 upvotes > Encoding Rust’s stability attributes into the semver check turns a subtle social contract into a CI invariant. The impressive part is reusing hundreds of existing lints once rustdoc exposed the right metadata; that should make protection much broader without duplicating rules. ## Similar posts on daily.dev - [Announcing Rust 1.97.0](https://daily.dev/posts/announcing-rust-1-97-0-fqxhq3pfj) · Rust · 22 upvotes · 0 comments - [Recent Rust Changes](https://daily.dev/posts/recent-rust-changes-iaytt6tvw) · Nick Cameron · 9 upvotes · 0 comments - [Supply chain nightmare: How Rust will be attacked and what we can do to mitigate the inevitable](https://daily.dev/posts/supply-chain-nightmare-how-rust-will-be-attacked-and-what-we-can-do-to-mitigate-the-inevitable-xdchrtlnw) · Hacker News · 0 upvotes · 1 comments --- Tags: [#rust](https://daily.dev/tags/rust), [#semantic-versioning](https://daily.dev/tags/semantic-versioning) [View this post on daily.dev](https://daily.dev/posts/protecting-the-rust-standard-library-from-accidental-breakage-6185skjif) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Protecting the Rust standard library from accidental breakage","url":"https://daily.dev/posts/protecting-the-rust-standard-library-from-accidental-breakage-6185skjif","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/protecting-the-rust-standard-library-from-accidental-breakage-6185skjif"},"datePublished":"2026-08-16T13:49:21.053Z","dateModified":"2026-10-15T02:22:42.243Z","description":"The Rust standard library now runs cargo-semver-checks in CI to catch accidental breakage of stable APIs, following a history of incidents where unstable...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/ed73b274deb303f22aaa100461f0ab4c?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/ed73b274deb303f22aaa100461f0ab4c?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Predrag Gruevski","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Predrag Gruevski","logo":"https://media.daily.dev/image/upload/s--zyVDLYwG--/f_auto,q_auto/v1780213710/logos/predrag?_a=BAMAMiWQ0","url":"https://daily.dev/sources/predrag"},"commentCount":2,"discussionUrl":"https://daily.dev/posts/protecting-the-rust-standard-library-from-accidental-breakage-6185skjif","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":35},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":2}],"keywords":"rust,semantic-versioning","timeRequired":"PT10M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Predrag Gruevski","item":"https://daily.dev/sources/predrag"},{"@type":"ListItem","position":3,"name":"Protecting the Rust standard library from accidental breakage"}]} {"@context":"https://schema.org","@type":"WebPage","@id":"https://daily.dev/posts/protecting-the-rust-standard-library-from-accidental-breakage-6185skjif","comment":[{"@type":"Comment","text":"Will Windows join the CI matrix after that tokio regression?","datePublished":"2026-08-16T19:46:09.622Z","url":"https://daily.dev/posts/6185skjIf#c-VYFYxPLOp","author":{"@type":"Person","name":"Agustin Barrientos","url":"https://daily.dev/agustinbarrientos","image":"https://media.daily.dev/image/upload/s--5ayxQnqn--/f_auto/v1788281802/avatars/avatar_wQYYVe5Tbj0NJ7C7qPoa8?_a=BAMAMicg0"},"interactionStatistic":{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":2}},{"@type":"Comment","text":"Encoding Rust’s stability attributes into the semver check turns a subtle social contract into a CI invariant. 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The breakage was significant enough that a fix shipped in the Rust 1.94.1 point release rather than waiting for the next stable release cycle. Rust upgraders tracking point-release fixes like this can follow stdlib stability work on daily.dev."}},{"@type":"Question","name":"How does cargo-semver-checks handle unstable Rust standard library APIs without flagging their changes as breaking?","acceptedAnswer":{"@type":"Answer","text":"Items marked #[unstable] are treated as non-public API, exactly like #[doc(hidden)] items, so their changes never trigger breakage warnings. The same logic extends to const-unstable items (treated as non-const) and unstable trait method defaults (treated as not provided), letting hundreds of existing lints work correctly without rewrites. 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