Rust Editions

Guide to Rust editions and migration based on The Edition Guide.

When to Use This Skill

• Migrating between Rust editions
• Understanding edition differences
• Using edition-specific features
• Updating legacy code

Core References

• The Edition Guide

Edition Overview

2021 and 2024 are both stable. New Cargo projects typically use 2024. Existing crates may stay on 2021 until migrated.

Edition 2015

Original edition. Key features:

• Ownership, borrowing, lifetimes
• Traits and generics
• Default when edition is omitted from Cargo.toml

Edition 2018

Major improvements:

• ? for Option and Result
• Path/module system changes
• Raw identifiers (r#match)
• dyn Trait for trait objects
• New keywords (async, await, dyn, try reserved)

fn foo(x: Option<i32>) -> Option<i32> {
    Some(x? + 1)
}

let r#match = 42;
let x: Box<dyn Trait> = Box::new(Concrete::new());

Edition 2021

Stable edition (still widely used). Key changes:

• Disjoint closure capture
• Or-patterns in macro_rules!
• Prelude adds TryFrom, TryInto, FromIterator
• IntoIterator for arrays (edition-gated .into_iter() semantics)
• Panic macro consistency
• Cargo feature resolver v2 by default
• Reserved prefixes (ident#… forms)

match value {
    Some(1) | Some(2) | Some(3) => println!("one two or three"),
    _ => println!("other"),
}

Edition 2024

Stable as of Rust 1.85.0 (RFC 3501). Prefer for new crates.

RPIT lifetime capture

Without an explicit use<..> bound, in-scope lifetime parameters are captured in return-position impl Trait (was edition-dependent before). Precise capturing (use<'a, T>) works in all editions since 1.82.

// 2024: captures `'_` implicitly (like async fn / trait RPIT)
fn f(_: &()) -> impl Sized {}

// Opt out of capturing lifetimes when needed:
fn g<'a>(x: &'a ()) -> impl Sized + use<> { *x }

Migrate with cargo fix --edition (impl_trait_overcaptures lint). Prefer use<..> over old Captures / outlives tricks.

if let temporary scope

Temporaries from the if let scrutinee drop before the else branch (shorter than 2021). Avoids common RwLock deadlock patterns; code that relied on long-lived temps may need match instead.

Reserved gen keyword

gen is reserved for future gen blocks (RFC 3513). Identifiers named gen become r#gen via keyword_idents_2024. Gen blocks themselves are not stabilized by the edition.

Unsafe attributes and unsafe extern

These attributes require unsafe(...) in 2024:

• #[unsafe(no_mangle)]
• #[unsafe(export_name = "...")]
• #[unsafe(link_section = "...")]

extern blocks must be unsafe extern "C" { ... } (items can be marked safe / unsafe).

Prelude: Future and IntoFuture

Both are in the 2024 prelude. Method-name collisions (e.g. custom poll) need fully qualified syntax; rust_2024_prelude_collisions migrates them.

IntoIterator for Box<[T]>

IntoIterator for boxed slices exists in all editions (1.80+). Method call .into_iter() yields values in 2024; pre-2024 method resolution still yielded &T. Migration rewrites .into_iter() → .iter() when references were intended (boxed_slice_into_iter).

Other notable 2024 changes

• Tail-expression temporary scope (block tail temps drop sooner)
• let chains in if / while
• unsafe_op_in_unsafe_fn warn-by-default
• Disallow references to static mut
• Never-type fallback change
• Macro fragment specifier tightening
• Cargo: rust-version-aware resolver; table/key consistency
• Rustdoc combined doctests; rustfmt style edition

Deeper notes and chapter links: references/edition-2024.md.

Migration

Official flow (Edition Guide):

cargo update
cargo fix --edition
# then edit Cargo.toml:

[package]
edition = "2024"

cargo build   # or cargo test
cargo fmt

cargo fix --edition applies the rust-2024-compatibility lint group against the next edition while your manifest still names the old one; then bump edition = "2024".

Requires a toolchain that supports 2024 (Rust 1.85+).

Common post-migration review

• RPIT: confirm use<> bounds match intended capture
• if let → match rewrites: keep only if you need 2021 temp lifetimes
• Unsafe attrs / unsafe extern: review soundness; autofix cannot verify it
• Boxed-slice .into_iter() vs .iter()
• Prelude collisions on poll / into_future

Reference Map

• references/edition-2015.md — 2015 edition
• references/edition-2018.md — 2018 edition
• references/edition-2021.md — 2021 edition
• references/edition-2024.md — 2024 edition (deep dive + chapter links)
• references/migration.md — Migration commands and pitfalls

Key Commands

cargo fix --edition
# Cargo.toml: edition = "2024"
cargo build
cargo test
cargo fmt

Key References

• The Edition Guide
• Rust 2024
• Transitioning editions