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Use when the user migrates a crate from edition 2021 to 2024, writes new edition-2024 code, hits the never-type fallback semantic change, encounters `unsafe extern` requirement, or wonders why RPIT lifetime capture default changed. Prevents writing pre-2024 idioms in 2024 code, missing the `cargo fix --edition` migration step, ignoring the `unsafe_op_in_unsafe_fn` lint becoming warn-by-default, or running into the `tail_expr_drop_order` change silently. Covers: all 13 edition-2024 changes shipped in Rust 1.85 (RPIT lifetime capture, never-type fallback, `unsafe extern`, `unsafe(no_mangle)` and `unsafe(link_section)`, `expr` fragment widening, prelude additions, `unsafe_op_in_unsafe_fn` warn-by-default, `if let` chains scope, `tail_expr_drop_order`, gen blocks status, `rust_2024_compatibility` lint group, reserved syntax), migration workflow. Keywords: edition 2024, "cargo fix --edition", "rust 2024", "1.85", "edition migration", "never type fallback", "! to ()", "unsafe extern", "unsafe(no_mangle)", "RPIT capture", "use<>", "unsafe_op_in_unsafe_fn", "if let chain", "tail_expr_drop_order", "expr fragment", "gen block", prelude 2024, "rust_2024_compatibility", "edition guide", "what changed in 2024", "should I migrate".
license
MIT
compatibility
Designed for Claude Code. Requires Rust 1.85+, edition 2024.
metadata
{"author":"OpenAEC-Foundation","version":"1.0"}
rust-syntax-edition-2024
Edition 2024 stabilized in Rust 1.85.0 (2025-02-20). It is the largest edition the Rust project has shipped: 13 language changes, library prelude additions, Cargo default shifts (cargo new defaults to edition = "2024" and resolver "3"), and Rustfmt rule updates. This skill is the authoritative reference every other syntax/impl skill in this package depends on. ALWAYS treat edition 2024 as the baseline; NEVER ship code in this package using pre-2024 idioms unless explicitly demonstrating a migration delta.
Quick Reference
Core rules (deterministic)
ALWAYS migrate via cargo fix --edition on a clean working tree. NEVER hand-edit edition idioms before running the automated migration; you will miss lints that only the rustc driver emits.
NEVER write extern "C" { ... } in edition 2024 code. ALWAYS write unsafe extern "C" { ... }; items inside are unsafe by default and may be opted into safe per item.
NEVER use bare #[no_mangle], #[link_section = "..."], or #[export_name = "..."] in edition 2024. ALWAYS wrap them as #[unsafe(no_mangle)], #[unsafe(link_section = "...")], #[unsafe(export_name = "...")].
ALWAYS wrap every unsafe operation inside an unsafe fn body in its own unsafe { ... } block. The 2024 default is #[warn(unsafe_op_in_unsafe_fn)]; pre-2024 code that calls unsafe ops bare inside an unsafe fn body breaks the lint.
NEVER assume RPIT (-> impl Trait) does NOT capture an input lifetime in edition 2024. The 2024 default captures all in-scope generics and lifetimes. Opt out with + use<> (or list specific params with + use<'a, T>).
NEVER rely on ! falling back to () in 2024. The fallback is ! itself; type-inference outcomes for ?-and-unwrap-only paths change. Annotate the turbofish (f::<()>()) at the call site when migration breaks.
NEVER take a shared or mutable reference to a static mut. The static_mut_refs lint is deny-by-default in 2024. ALWAYS migrate to AtomicX, Mutex<T>, , , or /.
OnceLock<T>
LazyLock<T>
&raw const
&raw mut
NEVER use gen as an identifier in edition 2024 code. ALWAYS rename, or escape as r#gen if you cannot. gen blocks themselves remain unstable.
ALWAYS bind the result of a RefCell::borrow() / Mutex::lock() to a let before the tail expression dot-chains it. The tail_expr_drop_order change drops temporaries before locals in 2024.
ALWAYS rewrite if let Some(x) = read_lock() { ... } else { write_lock() ... } patterns. In 2024 the scrutinee temporary drops BEFORE entering else, which fixes deadlocks but changes drop order observably.
Decision tree: "Should I migrate this crate to 2024?"
Does the crate compile cleanly on Rust 1.85 or newer with edition 2021?
NO -> Fix 1.85 compile errors first, THEN migrate edition.
YES -> Run `cargo +stable fix --edition` on clean working tree.
Did `cargo fix --edition` apply changes without error?
YES -> Bump `edition = "2024"` in Cargo.toml. Build + test.
Tests pass?
YES -> Commit. Run `cargo clippy` for new lints. Done.
NO -> Inspect `never_type_fallback`, `tail_expr_drop_order`,
`if_let_rescope` lints in build log. Apply targeted fixes.
NO -> Read failure output. Most common cause is references to
`static mut` (lint `static_mut_refs`) which has NO autofix.
Refactor to atomics / Mutex / OnceLock, then re-run.
Migration checklist (mandatory order)
git status must be clean. cargo build must be green on edition 2021 with rustc 1.85+.
cargo fix --edition (NOT cargo fix --edition --allow-dirty unless you committed your cargo fix output deliberately).
Review diff. The driver runs every lint in the rust_2024_compatibility lint group.
Edit Cargo.toml: edition = "2024". Optionally set resolver = "3" if you have a workspace root.
Manually inspect any tail_expr_drop_order warnings (no autofix); migrate or accept.
Manually inspect any static_mut_refs errors (no autofix); refactor away from static mut.
Commit migration as a single dedicated commit.
The 13 language changes (cheat-sheet table)
#
Change
Autofix?
Lint name
1
RPIT captures all in-scope lifetimes
YES
impl_trait_overcaptures
2
Never-type fallback () -> !
partial
never_type_fallback_flowing_into_unsafe
3
unsafe extern { ... } mandatory
YES
missing_unsafe_on_extern
4
#[unsafe(no_mangle)] etc. required
YES
unsafe_attr_outside_unsafe
5
unsafe_op_in_unsafe_fn warn-by-default
YES
unsafe_op_in_unsafe_fn
6
if let scrutinee dropped before else
YES (rewrite to match)
if_let_rescope
7
Tail-expression temporaries drop before locals
NO
tail_expr_drop_order
8
static mut refs deny-by-default
NO
static_mut_refs
9
expr fragment matches const {} and _
YES (rewrite to expr_2021)
edition_2024_expr_fragment_specifier
10
gen reserved keyword
YES (r#gen)
keyword_idents_2024
11
Reserved guarded string literal syntax
YES
rust_2024_guarded_string_incompatible_syntax
12
Match-ergonomics restrictions for & patterns
YES
mismatched_lifetime_syntaxes (closely related)
13
Prelude adds Future, IntoFuture
YES (FQN rewrite)
rust_2024_prelude_collisions
ALL of the above except items 2 (partial), 7, and 8 are fully covered by cargo fix --edition. Items 7 and 8 ALWAYS require human review.
Patterns
Pattern: RPIT lifetime capture (item 1)
In edition 2024, every in-scope lifetime and type parameter is implicitly captured by -> impl Trait. Pre-2024 only types/lifetimes that syntactically appeared in trait bounds were captured.
Side-by-side:
// edition 2021fnf<'a>(x: &'a [u8]) ->implSized { 42_u32 }
// `'a` NOT captured. `impl Sized` is `'static`-compatible.// edition 2024 (same source)fnf<'a>(x: &'a [u8]) ->implSized { 42_u32 }
// `'a` IS captured. The returned opaque type is bounded by `'a`.// edition 2024, restoring 2021 semantics:fnf<'a>(x: &'a [u8]) ->implSized + use<> { 42_u32 }
// ^^^^^^^ explicit "capture nothing"// edition 2024, capturing a subset:fng<'a, 'b, T>(x: &'a [u8], y: &'b T) ->implSized + use<'a, T> { 42_u32 }
// ^^^^^^^^^^ explicit list
When the caller requires 'static from the returned opaque type, 2024 overcapture is a compile error; add + use<> to opt out. The impl_trait_overcaptures lint flags every such case; cargo fix --edition inserts + use<> automatically. See [[rust-syntax-lifetimes]] for the broader use<> story.
Pattern: never-type fallback (item 2)
// edition 2021: `T` falls back to `()` when otherwise unconstrained.fnouter<T>(x: T) ->Result<T, ()> {
fnf<T: Default>() ->Result<T, ()> { Ok(T::default()) }
f()?; // T = ()Ok(x)
}
// edition 2024: `T` falls back to `!`. `!: Default` does NOT hold, so error.// Fix: annotate the turbofish.fnouter<T>(x: T) ->Result<T, ()> {
fnf<T: Default>() ->Result<T, ()> { Ok(T::default()) }
f::<()>()?; // explicitOk(x)
}
The never_type_fallback_flowing_into_unsafe lint is deny-by-default in 2024 because the change interacts with unsafe code paths in dangerous ways. See references/methods.md for the safety rationale.
The safe modifier per item is a 2024 feature: it lets you declare bindings whose ABI you have audited and whose preconditions hold unconditionally. See [[rust-impl-ffi-bindgen]].
Pattern: unsafe attributes (item 4)
// edition 2021#[no_mangle]pubextern"C"fnexample() {}
#[link_section = ".init_array"]static INIT: extern"C"fn() = example;
// edition 2024// SAFETY: no other global function exports this symbol name.#[unsafe(no_mangle)]pubextern"C"fnexample() {}
#[unsafe(link_section = ".init_array")]static INIT: extern"C"fn() = example;
ALWAYS write a // SAFETY: comment above each #[unsafe(...)] attribute. The compiler does not enforce it, but reviewers will reject changes without it.
Pattern: unsafe_op_in_unsafe_fn (item 5)
// edition 2021 (warns only if you opt-in to the lint)unsafefnread_unchecked<T>(slice: &[T], i: usize) -> &T {
slice.get_unchecked(i) // unsafe op, no inner block needed
}
// edition 2024 (warns by default)unsafefnread_unchecked<T>(slice: &[T], i: usize) -> &T {
// SAFETY: caller guarantees `i < slice.len()`.unsafe { slice.get_unchecked(i) }
}
See [[rust-syntax-unsafe]] for the broader two-purposes-of-unsafe-fn discussion.
Pattern: if let rescope (item 6) and tail expr drop order (item 7)
use std::sync::RwLock;
// edition 2021: read lock held through the `else` block -> deadlock when// `value.write()` is called.fnf_2021(value: &RwLock<Option<bool>>) {
ifletSome(x) = *value.read().unwrap() {
println!("{x}");
} else {
letmut w = value.write().unwrap(); // DEADLOCK in 2021if w.is_none() { *w = Some(true); }
}
}
// edition 2024: read lock dropped before entering `else`. No deadlock.// Same source compiles; behaviour changes.
Tail-expression drop order pairs with this. A RefCell::borrow() chained off a local goes wrong in 2024 because the borrow temporary now drops BEFORE the local:
use std::cell::RefCell;
// edition 2021: compiles. Temporary outlives `c`.// edition 2024: error E0597 "`c` does not live long enough".fnlen() ->usize {
letc = RefCell::new("..");
c.borrow().len()
}
// edition 2024 fix:fnlen_2024() ->usize {
letc = RefCell::new("..");
letborrowed = c.borrow();
borrowed.len()
}
Items 6 and 7 are the two most common 2024-migration footguns. The if_let_rescope lint suggests rewriting as match (preserves 2021 semantics); the tail_expr_drop_order lint flags potential issues but provides NO autofix.
Pattern: static mut references (item 8)
// edition 2021 (warns)staticmut COUNTER: i32 = 0;
unsafe { let_r = &COUNTER; } // shared ref to static mut// edition 2024 (`deny`-by-default; hard error in practice)// Migrate to one of:use std::sync::atomic::{AtomicI32, Ordering};
static COUNTER: AtomicI32 = AtomicI32::new(0);
COUNTER.fetch_add(1, Ordering::Relaxed);
// Or, when raw pointer access is genuinely required:staticmut COUNTER: i32 = 0;
letp = &raw const COUNTER; // 1.82+ syntax, NO reference taken
There is NO automatic migration for static_mut_refs. ALWAYS refactor toward atomics, Mutex, OnceLock, or &raw const/&raw mut. See [[rust-syntax-unsafe]] for strict-provenance APIs.
Pattern: expr fragment widening (item 9), gen keyword (item 10), prelude additions (item 13)
// item 9: `expr` now matches `const { ... }` and `_`. Use `expr_2021` for the// strict pre-2024 grammar inside macros that already match `const $e:expr`.macro_rules! takes_either {
($e:expr) => { /* matches now also const blocks and _ */ };
(const $e:expr_2021) => { /* preserves pre-2024 grammar */ };
}
// item 10: `gen` is reserved. Rename identifiers OR escape with r#:fnr#gen() ->i32 { 0 }
// item 13: Future / IntoFuture are now in the prelude.// If you defined a trait with a `poll` method previously, you may now see// ambiguity errors. `cargo fix --edition` rewrites the call site to FQN:// <_ as MyTrait>::poll(&pinned)
See [[rust-syntax-pattern-matching]] for the related match-ergonomics tightening (item 12).
Edge Cases
The unsafe(...) attribute syntax (item 4) is independently stable since Rust 1.82, so you can adopt it BEFORE migrating the edition. Doing so reduces churn in the migration commit.
The precise-capturing + use<...> syntax (item 1) is stable since Rust 1.82 as well; the EDITION change is the implicit-capture default, not the syntax availability.
cargo new defaults to edition = "2024" AND resolver = "3" since 1.85. If you scaffold subprojects inside a workspace whose root uses resolver = "2", Cargo warns and uses the workspace setting; declare resolver explicitly at the workspace root to avoid surprises. See [[rust-impl-cargo-project]].
The rust_2024_compatibility lint group is a UMBRELLA group: enabling it during 2021 development surfaces every 2024-incompatibility before you migrate. ALWAYS add #![warn(rust_2024_compatibility)] to crate roots that target a future migration.
Edition is a per-CRATE property, NOT a per-workspace property. A workspace MAY mix 2021 and 2024 crates. Public APIs across the boundary are stable; only the source-level idioms differ.
Cross-References
[[rust-syntax-lifetimes]] : RPIT lifetime capture deep dive, use<> semantics, variance.
