Sourced from rust-i18n's releases.

v4.0.0

What's Changed

Breaking Changes

• Backend trait signature changed to use Cow<'_, str> — available_locales() now returns Vec<Cow<'_, str>> instead of Vec<&str>, and translate() now returns Option<Cow<'_, str>> instead of Option<&str>. Custom Backend implementations must be updated accordingly. (#114)
• SimpleBackend::add_translations() signature changed — Now takes Cow<'static, str> for locale and HashMap<Cow<'static, str>, Cow<'static, str>> for data, instead of &str and &HashMap<&str, &str>. (#133)
• New required method messages_for_locale on Backend trait — Returns all translation messages for a given locale as Option<Vec<(Cow<'_, str>, Cow<'_, str>)>>.
• Removed once_cell dependency — Replaced with std::sync::LazyLock from the standard library. The re-export rust_i18n::once_cell is no longer available. This raises the minimum supported Rust version (MSRV) to 1.80.0. (#131)

Performance Improvements

• Dramatically reduced allocations in the i18n! macro expansion — Previously, the macro generated one HashMap per translation pair; now it generates one HashMap per locale. For large projects, this reduces HashMap creation from thousands to just a handful (e.g., 14,000 → 32). This also fixes stack overflow crashes on Windows for projects with many translations. (#133)
• Eliminated unnecessary String allocations — By using Cow<'static, str> throughout, static translation strings are no longer cloned into heap-allocated Strings. (#114, #133)

New Features

• messages_for_locale() method on Backend — Retrieve all translation key-value pairs for a specific locale, useful for inspection and debugging.
• try_load_locales() function — A fallible alternative to load_locales() in rust-i18n-support that returns Result instead of panicking, with detailed error messages for file lookup, parsing, and I/O failures. (#127)
• Improved error messages — Parsing failures now include the specific error reason instead of a generic panic message. (#116)

Internal / Maintenance

• Replaced once_cell::sync::Lazy with std::sync::LazyLock (#131)
• Added Cargo.lock to version control (#120)
• Improved CI workflow with concurrency groups and branch filtering
• Added release workflow
• Bumped slab from 0.4.10 to 0.4.11 (#122)

Migration Guide

If you have a custom Backend implementation, update it to:

use std::borrow::Cow;
impl Backend for MyBackend {
fn available_locales(&self) -> Vec<Cow<', str>> { /* ... */ }
fn translate(&self, locale: &str, key: &str) -> Option<Cow<', str>> { /* ... / }
fn messages_for_locale(&self, locale: &str) -> Option<Vec<(Cow<', str>, Cow<', str>)>> { / ... */ }
}

If you were using rust_i18n::once_cell, switch to std::sync::LazyLock (requires Rust 1.80+).

New Contributors

• @​niclas-AH made their first contribution in #114
• @​LennDG made their first contribution in #116
• @​jayson-lennon made their first contribution in #127, #131

... (truncated)

• a931914 Version 4.0.0
• 410adde Reduce allocations create unnecessary HashMaps and Strings (#133)
• 94a91a7 docs: Update user cases. (#134)
• 90b8e37 Bump slab from 0.4.10 to 0.4.11 (#122)
• c58ecb6 Bump v4.0.0-preview1
• 2d09179 Add messages_for_locale method to get all message for locale to Backend. (#...
• 1cc7772 Add try_load_locales method to rust-i18n-support. (#127)
• e6d6170 chore: Remove once_cell, use std::sync::LazyLock. (#131)
• 8e7fb82 Bump v4.0.0-preview0 (#130)
• 0ce74a8 Convert Backend's &str to Cow instead (#114)
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