Rust Knowledge Patch
Covers Rust 1.84–1.94 (2025-01-09 through 2026-03-05). Claude Opus 4.6 knows Rust through 1.83 and the 2021 Edition. It is unaware of the Rust 2024 Edition and any of the features below.
Index
| Topic | Reference | Key features |
|---|
| Rust 2024 Edition | references/rust-2024-edition.md | Edition migration, breaking changes, let chains |
| Language features | references/language-features.md | Async closures, trait upcasting, naked functions, cfg booleans |
| Collections & iterators | references/collections.md | extract_if, as_chunks, array_windows, slice splits |
| Numeric methods | references/numerics.md | isqrt, midpoint, strict_*, sub_signed, const floats |
| Memory & unsafe | references/memory.md | Provenance APIs, NonNull, MaybeUninit, smart ptr alloc |
| Std library additions | references/std-additions.md | Pipes, file locking, sync, paths, Duration, fmt::from_fn |
| Cargo & toolchain | references/cargo.md | Resolver v3, publish --workspace, LLD linker, TOML 1.1 |
| Lints & diagnostics | references/lints.md | New default warnings, never-type lints, diagnostic hints |
Rust 2024 Edition — Breaking Changes (inline)
Enable with edition = "2024" in Cargo.toml. Migrate with cargo fix --edition.
| Change | Before (2021) | After (2024) |
|---|
extern blocks | extern "C" {fn foo();} | unsafe extern "C" {fn foo();} |
| Link attributes | #[no_mangle] | #[unsafe(no_mangle)] |
unsafe fn bodies | implicit unsafe inside | require explicit unsafe {} |
static mut refs | warned | hard error → use &raw const/&raw mut |
set_var/remove_var | safe | now unsafe |
gen keyword | valid identifier | reserved |
impl Trait lifetime capture | opt-in | captures all in-scope lifetimes by default |
Future/IntoFuture in prelude | not in prelude | added (may cause name conflicts) |
// 2021 → 2024 migration examples
// extern blocks
unsafe extern "C" {
fn foo();
safe fn bar(); // opt-in safe: callable without unsafe
}
// attributes on linked items
#[unsafe(no_mangle)]
pub extern "C" fn my_fn() {}
// unsafe fn bodies
unsafe fn helper() {
unsafe { some_unsafe_op(); } // now required
}
// static mut: use raw refs instead
static mut GLOBAL: u32 = 0;
let r = &raw const GLOBAL; // safe in 2024, was safe since 1.84
// impl Trait lifetime restriction
fn foo<'a>(x: &'a str) -> impl Display + use<'a> { x } // restrict captures
Let Chains — 1.88 (Rust 2024 Edition only)
Chain let bindings with && in if/while. Earlier bindings are available in later conditions.
if let Channel::Stable(v) = release_info()
&& let Semver { major, minor, .. } = v
&& major == 1
&& minor == 88
{
println!("let chains stabilized here");
}
while let Some(x) = iter.next() && x < 10 {
process(x);
}
Async Closures — 1.85
async || {} can borrow captures across .await. Unlike || async {}, the inner future holds a borrow into the closure's environment. New traits: AsyncFn, AsyncFnMut, AsyncFnOnce.
let mut vec: Vec<String> = vec![];
let closure = async || {
vec.push(ready(String::from("")).await); // borrows vec across await point
};
// Higher-ranked async bounds — not expressible with Fn + Future:
async fn call_it(_: impl for<'a> AsyncFn(&'a u8)) {}
Trait Upcasting — 1.86
Coerce &dyn Trait to &dyn Supertrait (or any pointer wrapper). Previously required manual as_supertrait() workarounds.
trait Trait: Supertrait {}
trait Supertrait {}
fn upcast(x: &dyn Trait) -> &dyn Supertrait { x }
// Also: Arc<dyn Trait> -> Arc<dyn Supertrait>
// Downcasting without external crates:
use std::any::Any;
trait MyAny: Any {}
impl dyn MyAny {
fn downcast_ref<T: 'static>(&self) -> Option<&T> {
(self as &dyn Any).downcast_ref()
}
}
Quick Method Reference
New in 1.84
| Method | Description |
|---|
n.isqrt() / n.checked_isqrt() | Integer square root (floor); checked returns None for negative |
ptr::dangling::<T>() | Non-null, well-aligned dangling pointer |
ptr.addr() / .with_addr(a) / .map_addr(f) | Pointer provenance-preserving address ops |
ptr.expose_provenance() / ptr::with_exposed_provenance(a) | Round-trip through integer with provenance |
ptr::without_provenance(a) | Pointer with no provenance (sentinel values) |
&raw const *p | Now safe (was unsafe) |
New in 1.85–1.86
| Method | Description |
|---|
a.midpoint(b) | Overflow-safe (a+b)/2 for floats and unsigned ints |
| `v.pop_if(\ | x\ | ...)` | Pop last element if predicate holds |
v.get_disjoint_mut([i, j]) | Multiple &mut into slice/HashMap simultaneously |
lock.wait() | Block until OnceLock/Once is initialized |
New in 1.87–1.88
| Method | Description |
|---|
std::io::pipe() | Returns (PipeReader, PipeWriter) |
| `v.extract_if(.., \ | x\ | ...)` | Drain matching elements (lazy iterator) |
s.split_off(n) / split_off_first() / split_off_last() | Split slice, return tuple |
os_str.display() | Lossily display OsStr / OsString |
n.unbounded_shl(k) / unbounded_shr(k) | Shift returning 0 instead of panic when k ≥ bits |
s.as_chunks::<N>() / as_rchunks::<N>() | Fixed-size array chunks with remainder |
| `map.extract_if(\ | k,v\ | ...)` | HashMap/HashSet drain by predicate |
| `cell.update(\ | x\ | ...)` | Update Cell<T> in place, return new value |
New in 1.89–1.91
| Method | Description |
|---|
f.lock() / f.try_lock() / f.unlock() | Advisory file locking (no fs2 needed) |
r.flatten() | Result<Result<T,E>,E> → Result<T,E> |
NonNull::from_ref(&x) / from_mut(&mut x) | Safe NonNull from references |
x.checked_sub_signed(n) etc. | Subtract signed from unsigned (checked_, wrapping_, saturating_, overflowing_) |
n.strict_add(m) etc. | Panic on overflow in debug AND release |
s.ceil_char_boundary(i) / floor_char_boundary(i) | Nearest valid UTF-8 boundary |
Path::file_prefix() | Stem with ALL extensions stripped |
p.add_extension("gz") | Append extension (unlike set_extension which replaces) |
Duration::from_mins(n) / from_hours(n) | Convenience constructors |
Path == "/some/str" | PartialEq<str> / PartialEq<String> now implemented |
New in 1.92–1.94
| Method | Description |
|---|
RwLockWriteGuard::downgrade(guard) | Atomically write→read lock downgrade |
Box::new_zeroed() / new_zeroed_slice(n) | Zero-initialized allocation (also Rc, Arc) |
s.into_raw_parts() / v.into_raw_parts() | Decompose String/Vec to (ptr, len, cap) |
Duration::from_nanos_u128(n) | Like from_nanos but accepts u128 |
s.as_chunks::<N>() | (also as_array::<N>() in 1.93: slice → fixed-size array ref) |
| `fmt::from_fn(\ | f\ | ...)` | Display value from closure, no new type needed |
| `dq.pop_front_if(\ | x\ | ...) / pop_back_if(...)` | Conditional pop from VecDeque |
cell.get() on LazyCell/LazyLock | Option<&T> without forcing init |
LazyCell::force_mut(&mut cell) | Force init, return &mut T |
| `iter.next_if_map(\ | x\ | ...)` | Peek + transform; advance only if Some |
v.element_offset(&v[i]) | Index of element by reference |
f64::consts::EULER_GAMMA / GOLDEN_RATIO | New float constants |
