Use this to match a suggestion to the project's MSRV (rust-version in Cargo.toml). If the MSRV is older
than the version listed, use the "before" column.
Contents
- Stabilisations
- Edition changes that affect traits
- Still unstable (do not suggest for stable code)
- Terminology changes
- MSRV guidance
Stabilisations
| Rust | Feature | Before it |
|---|---|---|
| 1.26 | impl Trait in argument and return position | named generic parameters; Box<dyn Trait> returns |
| 1.27 | dyn Trait syntax | bare Trait as a type (deprecated, an error in edition 2021) |
| 1.34 | TryFrom / TryInto | custom conversion traits or from_str-style constructors |
| 1.41 | relaxed orphan rule: impl ForeignTrait<LocalType> for ForeignType | newtype wrappers |
| 1.51 | const generics (integers, bool, char) | macro-generated impls for array sizes |
| 1.62 | #[derive(Default)] on enums with #[default] on a unit variant | manual impl Default |
| 1.65 | generic associated types (type Item<'a> where Self: 'a) | lending-iterator crates, boxing |
| 1.75 | async fn and return-position impl Trait in traits (RPITIT) | async-trait (boxes every call), associated future types |
| 1.78 | #[diagnostic::on_unimplemented] for custom "trait not implemented" messages | #[rustc_on_unimplemented] (std only) |
| 1.79 | associated type bounds: T: Iterator<Item: Display> | where T::Item: Display |
| 1.81 | core::error::Error (the Error trait usable in no_std) | std-only error types, or crate-specific traits |
| 1.82 | precise capturing impl Trait + use<'a, T> | the Captures<'a> trick |
| 1.85 | edition 2024; async closures and AsyncFn, AsyncFnMut, AsyncFnOnce | Fn(..) -> Fut with owned arguments |
| 1.86 | trait upcasting: &dyn Sub -> &dyn Super coercion | an as_super(&self) -> &dyn Super method on the trait |
Edition changes that affect traits
| Edition | Change |
|---|---|
| 2018 | dyn Trait encouraged (bare trait objects warn); anonymous '_ in impl headers |
| 2021 | bare trait objects are a hard error; TryFrom, TryInto, FromIterator added to the prelude; disjoint closure captures |
| 2024 | -> impl Trait (including in traits) captures all in-scope lifetimes by default; impl_trait_overcaptures migration lint; unsafe extern blocks; IntoIterator for Box<[T]>; the gen keyword is reserved |
Prelude additions matter for trait design: adding a method named try_from or from_iter to your own trait
can become ambiguous once the std trait is in scope everywhere.
Still unstable (do not suggest for stable code)
| Feature | Stable workaround |
|---|---|
Specialization (default fn) | separate traits, or a marker trait plus explicit impls |
Negative impls (impl !Send for T) | a PhantomData<*const ()> field to remove Send/Sync |
Associated type defaults (type Out = (); in the trait) | document the expected type; add it in a major release |
Return type notation (T::method(..): Send) | -> impl Future + Send in the trait, or trait_variant |
impl Trait in associated types / type aliases (TAIT/ATPIT) | name the concrete type, or box it |
Trait aliases (trait Svc = Fn() + Send;) | a subtrait with a blanket impl: trait Svc: Fn() + Send {} impl<T: Fn() + Send> Svc for T {} |
dyn async traits (dyn-compatible async fn) | async-trait, or methods returning Pin<Box<dyn Future + Send + '_>> |
Check the release notes for the project's toolchain before promising any of these; nightly-only features should never appear in advice for a stable MSRV.
Terminology changes
- "Object safety" was renamed dyn compatibility in the Rust reference and compiler messages (E0038 now reads
"the trait
Xis not dyn compatible"; older compilers say "cannot be made into an object"). Both refer to the same rules. - "RPITIT" = return-position
impl Traitin traits; "AFIT" =async fnin traits.
MSRV guidance
- Libraries: support at least the last few stable releases and state
rust-version; CI should build on it (cargo +<msrv> checkorcargo msrv verify). async fnin public traits (1.75) is fine for most libraries now; pair it with aSendstory.- Trait upcasting (1.86) and
get_disjoint_mut(1.86) are recent: offer the pre-1.86 workaround if the MSRV is older. - Edition 2024 requires Rust 1.85; migrating changes RPIT capture behaviour, so run
cargo fix --editionand reviewuse<..>suggestions on public functions.
rust-borrow-checker-error-explainer
Explain and fix Rust borrow-checker, ownership, move and lifetime compiler errors: E0382 use of moved value, E0499 cannot borrow as mutable more than once, E0502 also borrowed as immutable, E0505, E0506, E0503, E0507 cannot move out of index/Rc/reference, E0515 return reference to local, E0597 does not live long enough, E0716 temporary value dropped while borrowed, E0373 closure may outlive (spawn needs move), E0521 borrowed data escapes, E0384, E0596, E0594, E0381, E0713, 'lifetime may not live long enough', RefCell 'already borrowed: BorrowMutError' panics. Ranks fixes from restructuring (split borrows, entry API, split_at_mut, get_disjoint_mut, mem::take, Option::take, indices, thread::scope) down to clone and Rc<RefCell>/Arc<Mutex>; never unsafe. Covers NLL limits (get-or-insert), iterator invalidation, closure captures, self-referential structs. Ships std-only Rust triage tools.
rust-cargo-workspace-designer
Design, review and fix Cargo workspaces: when to split a crate (compile time, boundaries, publishing, proc-macros), crates/* vs apps/ + crates/ layouts, virtual manifest vs root package, members/exclude/default-members, resolver 2 vs 3 (MSRV-aware, edition 2024), [workspace.package] and [workspace.dependencies] inheritance, [workspace.lints], root-only profiles (lto, codegen-units), additive features with dep: and weak ?/ syntax, feature unification pitfalls, internal path + version deps, publish = false, Cargo.lock and MSRV policy, xtask, CI with --locked, nextest, rust-cache, sccache, cargo-hack/deny/machete/udeps/semver-checks, and cargo build --timings. Ships a workspace auditor and crate-graph tool.
rust-error-handling-thiserror-anyhow-designer
Design and review Rust error handling with thiserror 2 and anyhow 1: library vs application split, typed error enums with #[from], #[source], #[error(transparent)] and #[non_exhaustive], ErrorKind patterns, context via .context()/.with_context(), bail!/ensure!, source() chains, {:#} and {:?} reports, RUST_BACKTRACE/RUST_LIB_BACKTRACE, Box<dyn Error + Send + Sync>, main -> anyhow::Result or ExitCode, unwrap/expect/panic policy, Option to Result, collecting iterators of Results, axum IntoResponse mapping, retryable errors, tracing, async Send bounds, testing with matches! and downcast_ref, no_std core::error::Error, and snafu, eyre/color-eyre, miette, error-stack.
rust-lifetime-annotation-advisor
Decide when Rust lifetime annotations are needed and which ones: the three elision rules, structs holding references (struct Parser<'a>), owned vs borrowed (String, &str, Cow<'a, str>, Arc<str>), one vs several lifetimes, 'a: 'b bounds, T: 'static misconceptions, Box<dyn Trait + 'a> default object lifetimes, impl Trait + '_ and edition 2024 capture rules with use<..>, variance and PhantomData, HRTB for<'a> Fn(&'a T), GATs and lending iterators, async fn and tokio::spawn 'static, self-referential structs (ouroboros, self_cell, indices, arenas), Rc/Arc to escape lifetime soup. Ships std-only Rust checkers for redundant, missing, unused and over-restrictive lifetimes.
rust-tokio-async-service-builder
Build and review production async Rust services on Tokio 1.x and axum 0.8: runtime choice (#[tokio::main] multi_thread vs current_thread, Builder), axum routing with /{id} paths, State, extractors, IntoResponse error types, axum::serve with_graceful_shutdown, tower/tower-http middleware (TraceLayer, TimeoutLayer, CorsLayer, ConcurrencyLimitLayer, RequestBodyLimitLayer), Arc<AppState>, std vs tokio Mutex, JoinSet, TaskTracker, CancellationToken, select! cancel safety, bounded mpsc, oneshot, broadcast, watch, actors, Semaphore, timeouts, retries, spawn_blocking, sqlx PgPool, reqwest Client reuse, tracing, tests with start_paused and oneshot, Dockerfile. Fixes 'future cannot be sent between threads safely', E0373, Rc or MutexGuard held across await, 'Cannot start a runtime from within a runtime', 'Path segments must not start with :'. Ships std-only Rust scanners and a compiling axum 0.8 scaffold.
rust-unsafe-block-auditor
Audit unsafe Rust for soundness: inventory every unsafe block, unsafe fn, unsafe impl/trait, extern block and unsafe attribute, check // SAFETY: comments and # Safety docs, and classify each as sound, needs-justification or likely UB, proposing safe alternatives first. Covers the five unsafe superpowers, aliasing (Stacked/Tree Borrows), invalid values, MaybeUninit vs mem::zeroed/uninitialized, transmute, slice::from_raw_parts, Vec::set_len/from_raw_parts, get_unchecked, CString::as_ptr dangling, unsafe impl Send/Sync, Pin projection, FFI (extern "C", repr(C), C-unwind, Box::into_raw/from_raw), strict provenance, static mut and &raw, edition 2024 (unsafe extern, #[unsafe(no_mangle)], unsafe_op_in_unsafe_fn, env::set_var), Miri, sanitizers, cargo-fuzz, loom, cargo-geiger and clippy safety lints.