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Skillsrust-syntax-edition-2024

rust-syntax-edition-2024

Coding & development

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Updated 10-15 22:18

About this skill

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)

  1. 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.
  2. 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.
  3. 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 = "...")].
  4. 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.
  5. 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>).
  6. 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.
  7. 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>, OnceLock<T>, LazyLock<T>, or &raw const/&raw mut.
  8. 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.
  9. 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.
  10. 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)

  1. git status must be clean. cargo build must be green on edition 2021 with rustc 1.85+.
  2. cargo fix --edition (NOT cargo fix --edition --allow-dirty unless you committed your cargo fix output deliberately).
  3. Review diff. The driver runs every lint in the rust_2024_compatibility lint group.
  4. Edit Cargo.toml: edition = "2024". Optionally set resolver = "3" if you have a workspace root.
  5. cargo build; cargo test; cargo clippy --all-targets.
  6. Manually inspect any tail_expr_drop_order warnings (no autofix); migrate or accept.
  7. Manually inspect any static_mut_refs errors (no autofix); refactor away from static mut.
  8. 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 2021
fn f<'a>(x: &'a [u8]) -> impl Sized { 42_u32 }
// `'a` NOT captured. `impl Sized` is `'static`-compatible.

// edition 2024 (same source)
fn f<'a>(x: &'a [u8]) -> impl Sized { 42_u32 }
// `'a` IS captured. The returned opaque type is bounded by `'a`.

// edition 2024, restoring 2021 semantics:
fn f<'a>(x: &'a [u8]) -> impl Sized + use<> { 42_u32 }
//                                  ^^^^^^^ explicit "capture nothing"

// edition 2024, capturing a subset:
fn g<'a, 'b, T>(x: &'a [u8], y: &'b T) -> impl Sized + 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.
fn outer<T>(x: T) -> Result<T, ()> {
    fn f<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.
fn outer<T>(x: T) -> Result<T, ()> {
    fn f<T: Default>() -> Result<T, ()> { Ok(T::default()) }
    f::<()>()?;    // explicit
    Ok(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.

Pattern: unsafe extern (item 3)

// edition 2021
extern "C" {
    pub fn sqrt(x: f64) -> f64;                        // implicitly unsafe to call
    pub unsafe fn strlen(p: *const std::ffi::c_char) -> usize;
}

// edition 2024
unsafe extern "C" {
    pub safe fn sqrt(x: f64) -> f64;                   // callable WITHOUT unsafe{}
    pub unsafe fn strlen(p: *const std::ffi::c_char) -> usize;
    pub fn free(p: *mut core::ffi::c_void);            // defaults to unsafe
}

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]
pub extern "C" fn example() {}

#[link_section = ".init_array"]
static INIT: extern "C" fn() = example;

// edition 2024
// SAFETY: no other global function exports this symbol name.
#[unsafe(no_mangle)]
pub extern "C" fn example() {}

#[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)
unsafe fn read_unchecked<T>(slice: &[T], i: usize) -> &T {
    slice.get_unchecked(i)         // unsafe op, no inner block needed
}

// edition 2024 (warns by default)
unsafe fn read_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.
fn f_2021(value: &RwLock<Option<bool>>) {
    if let Some(x) = *value.read().unwrap() {
        println!("{x}");
    } else {
        let mut w = value.write().unwrap();   // DEADLOCK in 2021
        if 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".
fn len() -> usize {
    let c = RefCell::new("..");
    c.borrow().len()
}

// edition 2024 fix:
fn len_2024() -> usize {
    let c = RefCell::new("..");
    let borrowed = 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)
static mut 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:
static mut COUNTER: i32 = 0;
let p = &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#:
fn r#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.
  • [[rust-syntax-unsafe]] : unsafe_op_in_unsafe_fn rationale, 8 unsafe superpowers, strict-provenance APIs replacing static mut refs.
  • [[rust-syntax-pattern-matching]] : match-ergonomics changes for & patterns (item 12).
  • [[rust-core-type-system]] : never-type ! and its fallback rules.
  • [[rust-impl-ffi-bindgen]] : unsafe extern blocks and the safe per-item modifier.
  • [[rust-core-language-versions]] : Rust 1.80 through 1.87 release-by-release feature index; edition 2024 is the 1.85 entry.

Reference Links

Official Sources