Rust Knowledge Patch

Rust Knowledge Patch

Use this patch when changing Rust source, Cargo configuration, edition settings, unsafe or FFI code, platform targets, documentation tests, or compiler policy.

Reference index

Apply this patch

1. Identify the crate edition, declared rust-version, compilation targets, panic strategy, and whether the build invokes Cargo or rustc directly.
2. Check the declared MSRV before adopting versioned syntax, APIs, Cargo keys, or TOML forms.
3. Check the breaking-change sections below before adopting newer APIs, then read the topic reference that covers the affected subsystem.
4. For unsafe, FFI, linker, or custom-target work, validate the target-specific requirements instead of relying on host behavior.

Breaking changes and migrations

Treat the 2024 edition as a semantic migration

• Run cargo fix --edition, then review its edits. It does not update doctests or uninvoked exported macro bodies, and some rewrites preserve old behavior rather than choosing the desired new behavior.
• Write unsafe extern blocks; mark explicitly safe foreign items safe. Wrap no_mangle, export_name, and link_section in unsafe(...), and put unsafe operations inside explicit blocks even within an unsafe fn.
• Audit calls to std::env::set_var, std::env::remove_var, and deprecated CommandExt::before_exec; they are unsafe in this edition and have no general safe replacement.
• Replace references to static mut with raw references or synchronization; implicit references such as formatting or method calls are denied too.
• Audit return-position impl Trait capture, never-type fallback, boxed-slice .into_iter(), match ergonomics, if let scrutinee temporaries, and tail-expression destruction order.
• Review macro match order because expr accepts const blocks and _; retain expr_2021 only where the narrower grammar is intentional.
• Rename gen identifiers or use r#gen, and separate guarded-string # tokens with whitespace.
• Resolver v3 is the default for edition 2024.
• Read edition-2024.md before changing a crate's edition.

Compiler acceptance and observable behavior changed

• Coherence uses the next-generation trait solver from 1.84.0, so some impl pairs previously accepted can become conflicting.
• Never-type fallback lints are deny-by-default on every edition from 1.92.0 when the affected crate is built directly.
• Pattern capture, binding, and destruction behavior changed. Add focused tests when partial moves or visible destructor effects matter.
• Trailing semicolons in macro expansions, missing fragment specifiers, malformed attributes, unsupported ABI strings, invalid export/link attributes, and several formerly tolerated syntax forms now warn or fail.
• Custom JSON target specifications require nightly options from 1.95.0.
• WebAssembly unresolved symbols fail linking from 1.96.0; either declare the import explicitly or intentionally pass the linker opt-out flag.
• Read language-and-macros.md and targets-and-toolchains.md during upgrade triage.

ABI, linking, and symbol expectations changed

• wasm32-unknown-unknown adopted the standards-compliant C ABI in 1.89.0; rebuild both sides and do not mix objects made under the old convention.
• x86_64-unknown-linux-gnu uses LLD by default from 1.90.0. Disable the lld linker feature only for confirmed BFD-specific requirements.
• Linux panic=abort builds retain unwind tables from 1.92.0 unless -Cforce-unwind-tables=no is set.
• Stable rustc emits v0 Rust symbols by default from 1.97.0. Update debuggers, profilers, demanglers, and backtrace expectations.
• Enum layouts without explicit repr are not stable contracts; observable layout changes occurred again in 1.97.0.
• Read safety-ffi-and-low-level.md before altering FFI or unsafe code.

Pinning and pointer assumptions need review

• pin!(&mut_value) no longer deref-coerces in 1.97.0; make the intended pointee explicit with a reborrow or Pin::new/Pin::as_mut.
• Forming a raw reference through a raw pointer or to a union field is safe, but dereferencing the result still needs valid unsafe reasoning.
• Debug null-pointer checks disappear when debug assertions are disabled and are never a soundness guarantee.
• Prefer exposed- or strict-provenance APIs over integer transmutation when reconstructing or tagging pointers.

Cargo workflows and policy changed

• Resolver v3 can prefer dependency versions compatible with declared rust-version; use the configuration override for latest-dependency CI.
• Automatic cache collection can remove stale downloads. Disable it when Cargo versions predating access-time tracking share the cache.
• cargo publish --workspace publishes in dependency order but is not atomic.
• cargo publish does not reliably leave a local .crate; run cargo package when the archive is required.
• Published packages always include Cargo.lock; cargo package --exclude-lockfile skips lockfile verification when creating a local package.
• build.build-dir, resolver.lockfile-path, configuration include, target-specific rustdocflags, and build.warnings address separate build layout and policy needs.
• Prefer prompts, stdin, environment variables, or registry credential providers over command-line tokens.
• Read cargo.md before changing CI, packaging, registry, or workspace automation.

High-value language features

Async closures, traits, and opaque types

• async || {} and the AsyncFn* traits support futures borrowing closure captures and higher-ranked async callbacks (1.85.0).
• Trait-object upcasting to a supertrait is stable for references, smart pointers, and raw pointers (1.86.0); raw trait-object pointers still require a valid vtable.
• Trait return-position impl Trait supports precise use<...> capture (1.87.0). Every in-scope type and const parameter, including Self, must be named; lifetimes may be omitted from the capture set.

trait Super {}
trait Sub: Super {}

fn upcast(value: &dyn Sub) -> &dyn Super { value }

trait Value {
    fn value<'a>(&'a self) -> impl Sized + use<Self>;
}

Conditions, configuration, and patterns

• Edition 2024 supports let chains in if and while from 1.88.0.
• Match arms support if let guards from 1.95.0; guard patterns do not contribute to exhaustiveness.
• cfg_select! selects the first matching item or expression arm from 1.95.0.
• assert_matches! and debug_assert_matches! are stable from 1.96.0, print the failed value with Debug, and require an explicit import.

match value {
    Some(x) if let Ok(y) = compute(x) => use_pair(x, y),
    _ => {}
}

Ranges and slices

• array_windows yields overlapping &[T; N] windows from 1.94.0.
• core::range provides copyable range values with separate iterator types; range syntax still constructs legacy core::ops ranges.
• Prefer RangeBounds in public APIs that should accept both range families.

use core::range::Range;

#[derive(Clone, Copy)]
struct Span(Range<usize>);

High-value library features

Multi-borrowing, extraction, and locking

• Slices and HashMap support checked and unchecked disjoint mutable access from 1.86.0.
• Vec, LinkedList, HashMap, HashSet, BTreeMap, and BTreeSet have distinct extract_if signatures; check whether the type accepts a range.
• File provides advisory shared/exclusive locking from 1.89.0.
• RwLockWriteGuard::downgrade atomically produces a read guard from 1.92.0.

Process, allocation, and formatting primitives

• std::io::pipe() integrates anonymous pipes with Command and Stdio from 1.87.0. Drop/move every writer and drain output before waiting to avoid a full-pipe deadlock.
• Box<MaybeUninit<T>>::write, zeroed smart-pointer allocation, slice-wide MaybeUninit operations, and raw-parts decomposition cover staged initialization and ownership transfer; callers of assume_init must assert the zeroed representation is valid.
• std::fmt::from_fn creates a Display and Debug value backed by a formatting callback from 1.93.0.

Target and documentation checks

• Query the host with rustc --print host-tuple, or use Cargo's portable host-tuple target where supported.
• Check target tiers before assuming rustup artifacts, host tools, or test-suite guarantees.
• Cross-target doctests can run through the configured runner; tests previously skipped may now fail.
• Rustdoc target ignores, standalone doctests, external runners, emitted-path remapping, and formatting changes are detailed in docs-tests-and-formatting.md.