# fluent-zero
[](https://crates.io/crates/fluent-zero)
[](https://docs.rs/fluent-zero)
[](https://spdx.org/licenses/MIT)
**Zero-allocation, high-performance [Fluent](https://projectfluent.org/) localization for Rust.**
`fluent-zero` is a specialized localization loader designed for high-performance applications, such as **GUI clients** ([egui](https://egui.rs/), [iced](https://iced.rs/), [winit](https://github.com/rust-windowing/winit)) and **Game Development** ([Bevy](https://bevy.org/), [Fyrox](https://fyrox.rs/)).
Unlike other loaders that prioritize template engine integration or hot-reloading, `fluent-zero` prioritizes **runtime speed and memory efficiency**. It generates static code at build time to allow for `O(1)` lookups that return `&'static str` whenever possible, eliminating the heap allocation overhead typical of localization libraries.
## ⚡ Why fluent-zero?
Most Fluent implementations (like `fluent-templates`) wrap the standard `fluent-bundle`. When you request a translation, they look it up in a `HashMap`, parse the pattern, and allocate a new `String` on the heap to return the result—even if the text is static.
In an immediate-mode GUI (like `egui`) running at 60 FPS, looking up 50 strings per frame results in **3,000 allocations per second**. This causes allocator contention and Garbage Collection-like micro-stutters.
**`fluent-zero` solves this by pre-computing the cache at compile time.**
| **Static Text Lookup** | Heap Allocation (`String`) | **Zero Allocation** (`&'static str`) |
| **Lookup Speed** | HashMap + AST traversal | **Perfect Hash Function (PHF)** |
| **Memory Usage** | Full AST loaded on start | **Lazy / Zero-Cost Abstraction** |
| **Best For** | Web Servers (Tera/Askama) | **Desktop GUIs & Games** |
## 🚀 Usage
### 1. Installation
You need both the runtime library and the build-time code generator.
```toml
[dependencies]
fluent-zero = "0.1"
unic-langid = "0.9"
[build-dependencies]
fluent-zero-build = "0.1"
```
### 2. File Structure
Organize your Fluent files using standard locale directories:
```text
assets/
└── locales/
├── en-US/
│ └── main.ftl
├── fr-FR/
│ └── main.ftl
└── de/
└── main.ftl
```
### 3. Build Script (`build.rs`)
Configure the code generator to read your locales directory. This will generate the static PHF maps and Rust code required for the zero-allocation cache inside your `OUT_DIR`.
```rust
// build.rs
fn main() {
// Generates static_cache.rs in your OUT_DIR
fluent_zero_build::generate_static_cache("assets/locales");
}
```
### 4. Application Code
In your `lib.rs` (or `main.rs`), you must include the generated file. This brings the `CACHE` and `LOCALES` statics into scope, which the `t!` macro relies on.
```rust
use fluent_zero::{t, set_lang};
// 1. Include the generated code from build.rs
include!(concat!(env!("OUT_DIR"), "/static_cache.rs"));
fn main() {
// 2. (Optional) Set the runtime language. Defaults to en-US.
// The parse() method comes from unic_langid::LanguageIdentifier
set_lang("fr-FR".parse().expect("Invalid lang ID"));
// 3. Use the t! macro for lookups.
// CASE A: Static String
// Returns &'static str. ZERO ALLOCATION.
let title = t!("app-title");
// CASE B: Dynamic String (with variables)
// Returns Cow<'static, str>. Allocates only if variables are resolved.
let welcome = t!("welcome-user", {
"name" => "Alice",
"unread_count" => 5
});
println!("{}", title);
println!("{}", welcome);
}
```
## 📦 Library Support & Nested Translations
`fluent-zero` supports a modular architecture where libraries and dependencies manage their own translations independently, but share their end results with the caller.
If you are writing a library (e.g., a widget crate or a game engine plugin), you can include `fluent-zero-build` in your library's `build.rs`. Your library will generate its own private `CACHE` and `LOCALES` maps based on its own `.ftl` files, and your callers will automatically receive those translations via your normal APIs based on the set global language.
### Global Synchronization
While the **translation data** is isolated per crate (compile-time), the **language selection** is global (runtime).
* **`fluent_zero::set_lang(...)`**: Updates the language for the **entire application stack**.
* **`t!(...)`**: Uses the library-specific translation files but respects the globally set language.
This allows for a cohesive ecosystem where the end-user application sets the language once, and all dependencies (UI widgets, logging, error messages) switch context instantly without manual propagation.
```text
my-app/
├── Cargo.toml
├── build.rs // Generates app-specific cache
├── assets/locales/ // Contains "welcome-screen.ftl"
└── src/main.rs // Calls set_lang("fr-FR")
my-ui-library/ (Dependency)
├── Cargo.toml
├── build.rs // Generates library-specific cache
├── assets/locales/ // Contains "ok-button.ftl", "cancel.ftl"
└── src/lib.rs // Calls t!("ok-button")
```
In this example, when `my-app` calls `set_lang("fr-FR")`, the `my-ui-library` automatically begins serving French strings for its internal components.
## 🧠 How it Works
1. **Build Time**: `fluent-zero-build` scans your `.ftl` files. It identifies which messages are purely static (no variables) and which are dynamic.
2. **Code Gen**: It generates a Rust module containing **Perfect Hash Maps** (via `phf`) for every locale.
* Static messages are compiled directly into the binary's read-only data section (`.rodata`).
* Dynamic messages are stored as raw FTL strings, wrapped in `LazyLock`.
3. **Run Time**:
* When you call `t!("hello")`, `fluent-zero` checks the PHF map.
* If it finds a static entry, it returns a reference to the binary data instantly. **No parsing. No allocation.**
* If it finds a dynamic entry, it initializes the heavy `FluentBundle` (only once) and performs the variable substitution.
## 🛠️ Example: using with `egui`
This crate shines in immediate mode GUIs. Because `t!` returns `Cow<'static, str>`, you can pass the result directly to widgets without `.to_string()` clones.
```rust
impl eframe::App for MyApp {
fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
egui::CentralPanel::default().show(ctx, |ui| {
// These calls are effectively free (nanoseconds).
// They do not allocate memory.
ui.heading(t!("menu_title"));
if ui.button(t!("btn_submit")).clicked() {
// ...
}
// Only this allocates, and only when 'count' changes if the UI is smart
ui.label(t!("items_remaining", {
"count" => self.items.len()
}));
});
}
}
```
## ⚠️ Trade-offs
While `fluent-zero` is faster at runtime, it comes with trade-offs compared to `fluent-templates`:
1. **Compile Times**: Because it generates Rust code for every string in your FTL files, heavily localized applications may see increased compile times.
2. **Binary Size**: Static strings are embedded into the binary executable code.
3. **Flexibility**: You cannot easily load new FTL files from the filesystem at runtime without restarting the application (the cache is baked in).
## License
This project is licensed under the [MIT license](https://spdx.org/licenses/MIT).
## Notice
This crate is not related to [Mozilla Project Fluent](https://projectfluent.org/) in any official capacity. All usage is at your own risk.