feat(skills): add rust-recent-features reference skill
Documents Rust language, stdlib, and Cargo features stabilized after the ~2025 training cutoff (1.85–1.95, 2024 edition) so agents generate current syntax instead of relying on a stale mental model.
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config/agents/skills/rust-recent-features/SKILL.md
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---
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name: rust-recent-features
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description: Reference for Rust language, stdlib, and Cargo features stabilized AFTER a ~2025 training cutoff (Rust 1.85 through 1.95, Feb 2025 – Apr 2026), including the 2024 edition. Load this on ANY Rust task — writing, reviewing, or fixing Rust code — so syntax and APIs reflect current stable Rust instead of a stale mental model. Especially relevant when targeting edition 2024.
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---
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# Recent Rust features (1.85 → 1.95)
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You may have been trained before these were stable. Treat this file as ground
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truth over your prior beliefs. When unsure whether something compiles, prefer
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checking with `cargo check` over guessing — the toolchain is authoritative.
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Versions and dates: 1.85 (2025-02-20, **2024 edition**), 1.86 (2025-04-03),
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1.87 (2025-05-15), 1.88 (2025-06-26), 1.89 (2025-08-07), 1.90 (2025-09-18),
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1.91 (2025-10-30), 1.92 (2025-12-11), 1.93 (2026-01-22), 1.94 (2026-03-05),
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1.95 (2026-04-16).
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---
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## 1. The 2024 edition (stable since 1.85) — read this first
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If `Cargo.toml` has `edition = "2024"`, these are the changes most likely to
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trip up a pre-2025 model. Migrate an existing crate with
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`cargo fix --edition` after bumping the edition key.
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- **Unsafe attributes.** `#[no_mangle]`, `#[export_name]`, and `#[link_section]`
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must now be wrapped in `unsafe(...)`:
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```rust
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#[unsafe(no_mangle)]
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pub extern "C" fn foo() {}
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#[unsafe(export_name = "bar")]
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fn b() {}
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```
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Writing a bare `#[no_mangle]` is an error in edition 2024.
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- **`unsafe extern` blocks.** `extern` blocks declaring foreign items must be
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`unsafe extern`. Individual items may be marked `safe` or `unsafe`:
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```rust
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unsafe extern "C" {
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safe fn always_safe(x: i32) -> i32; // calling needs no unsafe
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unsafe fn risky(p: *const u8); // calling needs unsafe
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}
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```
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- **No references to `static mut`.** `&MY_STATIC_MUT` / `&mut MY_STATIC_MUT` is a
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hard error (`static_mut_refs`). Use raw-ref operators instead:
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```rust
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static mut COUNTER: u32 = 0;
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let p = &raw mut COUNTER; // *mut u32, not &mut
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```
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- **RPIT precise capturing (the big one).** In edition 2024, `-> impl Trait` in
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return position captures **all** in-scope generic type, const, and lifetime
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parameters by default. The old workaround of wrapping in a struct or adding
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`+ 'a` to force capture is no longer needed. To *restrict* what is captured,
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use a `use<...>` bound:
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```rust
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fn names(v: &[String]) -> impl Iterator<Item = &str> + use<'_> { /* ... */ }
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fn no_capture<T>() -> impl Sized + use<> {} // capture nothing
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```
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Do NOT reach for the old lifetime-wrapper patterns; reach for `use<>`.
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- **`if let` temporary scope.** Temporaries in the scrutinee of an
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`if let … { } else { }` are dropped before the `else` branch runs (previously
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they lived to the end of the whole construct). This fixes a class of deadlocks
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where a lock guard was held across the `else`.
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- **Tail-expression temporary scope.** In a block, temporaries in the final
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tail expression are dropped *before* the block's local variables, changing
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`Drop` ordering in some code.
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- **Never-type fallback.** The `!` type now falls back to `!` (not `()`) in
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certain diverging contexts; this can surface new inference results or
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`never_type_fallback_flowing_into_unsafe` lints.
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- **`gen` is a reserved keyword.** Use `r#gen` if you need it as an identifier.
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- **Macro `expr` fragment** now also matches `const { … }` blocks and `_`.
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---
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## 2. New language features (by area)
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### Control flow, patterns, bindings
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- **Let chains** (1.88, **edition 2024 only**): chain `let` with `&&` in `if`/`while`:
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```rust
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if let Some(x) = a && let Ok(y) = parse(x) && y > 0 { /* ... */ }
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```
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This is stable now — do not claim it requires nightly.
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- **`if let` guards** (1.95): a `let` pattern in a match-arm guard:
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```rust
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match val {
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v if let Some(y) = lookup(v) => use_it(y),
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_ => {}
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}
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```
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- **Open-start ranges after unary ops** (1.87): `..EXPR` now parses after `!`, `-`, `*`.
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### Async
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- **Async closures** (1.85): `async || { … }` is stable, with the `AsyncFn` /
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`AsyncFnMut` / `AsyncFnOnce` traits. No boxing/crate needed:
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```rust
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async fn run<F: AsyncFn(u32) -> u32>(f: F) -> u32 { f(1).await }
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run(async |x| x + 1).await;
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```
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### Traits & generics
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- **Trait upcasting** (1.86): coerce `dyn Sub` to `dyn Super` where `Sub: Super`:
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```rust
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let up: &dyn Super = sub_ref; // now stable
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```
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- **`use<>` precise capturing in trait RPIT** (1.87): `use<...>` bounds allowed on
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return-position `impl Trait` in trait definitions.
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- **Inferred const args** (1.89, `generic_arg_infer`): `_` may stand in for a
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const generic argument, e.g. `let a: [_; _] = make();` style inference.
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- **`#[repr(u128)]` / `#[repr(i128)]`** enums (1.89).
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### unsafe / FFI / asm
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- **Naked functions** (1.88): `#[unsafe(naked)]` with a `naked_asm!` body, stable.
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- **C-variadic *declarations*** for more ABIs: `sysv64`/`win64`/`efiapi`/`aapcs`
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(1.91) and `system` (1.93) `extern` fns may declare `...` variadics.
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- **`asm_goto`** (1.87): `asm!` may jump to label blocks.
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- **`asm_cfg`** (1.93): `#[cfg(...)]` allowed on individual `asm!` operands/lines.
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- Many target features stabilized: **AVX-512** family (1.89), AES key-locker
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(`kl`/`widekl`), `sha512`/`sm3`/`sm4`, `sse4a`/`tbm` (1.91), LoongArch & RISC-V
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profiles, etc.
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### Attributes, lints, macros
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- **`#[diagnostic::do_not_recommend]`** (1.85): hide an impl from "consider
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implementing" diagnostics.
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- **`#[target_feature]` on safe fns** (1.86): safe functions may carry it.
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- **`cfg_boolean_literals`** (1.88): `#[cfg(true)]` / `#[cfg(false)]`.
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- **`cfg_select!`** (1.95): a `match`-like macro selecting a token tree by cfg.
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---
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## 3. Notable stdlib stabilizations (high-signal)
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Slices / arrays:
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- `<[T]>::as_array::<N>()` / `as_mut_array` → `Option<&[T; N]>` (1.93). Prefer
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this over `try_into()` gymnastics for fixed-size views.
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- `<[T]>::array_windows::<N>()` (1.94); `<[T]>::element_offset` (1.94).
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Vec / collections:
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- `Vec::push_mut`, `Vec::insert_mut` return `&mut T` to the new element (1.95);
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same family on `VecDeque`/`LinkedList`.
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- `Vec::into_raw_parts`, `String::into_raw_parts` (1.93).
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- `VecDeque::pop_front_if` / `pop_back_if` (1.93).
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- `Peekable::next_if_map` / `next_if_map_mut` (1.94).
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- `btree_map::Entry::insert_entry` / `VacantEntry::insert_entry` (1.92).
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Integers / math:
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- **Strict arithmetic**: `{int}::strict_add/sub/mul/div/rem/neg/shl/shr/pow` etc.
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(1.91) — like the `checked_*` family but panic on overflow instead of `None`.
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- `<iN>::unchecked_neg/shl/shr`, `<uN>::unchecked_shl/shr` (1.93).
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- `NonZero<u{N}>::div_ceil` (1.92).
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- `f32`/`f64` consts `EULER_GAMMA`, `GOLDEN_RATIO`; `f32/f64::mul_add` const (1.94).
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Sync / atomics:
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- `Atomic{Bool,Ptr,Isize,Usize,…}::update` / `try_update` (1.95).
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- `AtomicPtr::fetch_ptr_add/sub`, `fetch_byte_add/sub`, `fetch_or/and/xor` (1.91).
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- `RwLockWriteGuard::downgrade` to a read guard (1.92).
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Memory / smart pointers:
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- `Box/Rc/Arc::new_zeroed` and `new_zeroed_slice` (1.92).
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- `core::range` module with a `Copy`, cleanly-iterable `RangeInclusive` (1.95).
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- `MaybeUninit<[T;N]>` ⇄ `[MaybeUninit<T>;N]` conversions; slice
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`assume_init_ref/mut/drop`, `write_copy_of_slice` (1.93/1.95).
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Misc:
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- `bool: TryFrom<{integer}>` (1.95); `char::MAX_LEN_UTF8/UTF16` (1.93).
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- `Duration::from_nanos_u128` (1.93); `Path::file_prefix` (1.91).
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- A large number of previously-stable APIs are now usable in `const` contexts.
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---
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## 4. Cargo
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- **2024 edition** is the default for new crates via `cargo new`.
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- **Automatic cache garbage collection** (1.88): the global download/source cache
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is pruned automatically; no manual cleanup script needed.
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- **`build.build-dir`** stabilized (1.91): put intermediate artifacts in a
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separate directory from the final `target/` outputs.
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- **Multi-package publishing** (1.90): publish an entire workspace in one
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`cargo publish` invocation, in dependency order.
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- `cargo clean --workspace` (1.93); `config` top-level **`include`** key (1.94)
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for composing config files; `CARGO_BIN_EXE_<crate>` available at runtime (1.94).
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- Cargo parses TOML v1.1 manifests (1.94).
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---
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## 5. Cheat sheet — corrections a pre-2025 model commonly needs
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| If you were about to write… | In current Rust (edition 2024) write… |
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|---|---|
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| `#[no_mangle]` | `#[unsafe(no_mangle)]` |
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| `extern "C" { … }` (declaration) | `unsafe extern "C" { … }` |
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| `&mut MY_STATIC_MUT` | `&raw mut MY_STATIC_MUT` |
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| wrapper struct / `+ 'a` to constrain RPIT | `-> impl Trait + use<'a, T>` |
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| "let chains need nightly" | stable in edition 2024 (since 1.88) |
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| box/crate an async closure | `async || { … }` is stable (1.85) |
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| `slice.try_into::<[T; N]>()` for a view | `slice.as_array::<N>()` |
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| hand-rolled overflow-panicking add | `x.strict_add(y)` |
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| "trait upcasting is unstable" | stable since 1.86 |
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When in doubt, set the toolchain to a known version (`rustup show`) and let
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`cargo check` / `cargo clippy` confirm. Edition-specific behavior depends on the
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`edition` field in `Cargo.toml`, not the compiler version alone.
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