ICU MessageFormat Parser (Rust)

A Rust implementation of the ICU MessageFormat parser, optimized for performance and WebAssembly compilation.

Features

• Full ICU MessageFormat syntax support
• High-performance parsing - 2.6-3.7x faster than JavaScript parser
• WebAssembly-ready with wasm-bindgen
• Zero-copy parsing where possible
• Comprehensive error handling

Performance

The Rust parser (optimized build) significantly outperforms both the JavaScript parser and other Rust implementations:

$ bazel run -c opt //crates/icu_messageformat_parser:comparison_bench

Note: Always use -c opt for benchmarking to enable release optimizations.

Project Structure

• lib.rs - Main library and WASM bindings
• parser.rs - Core parser implementation
• types.rs - AST types
• error.rs - Error types
• date_time_pattern_generator.rs - Date/time pattern support
• manipulator.rs - AST manipulation utilities
• printer.rs - AST printing utilities

Building

Native Rust Library

# Run tests
bazel test //crates/icu_messageformat_parser:icu_messageformat_parser_test

# Build library
bazel build //crates/icu_messageformat_parser:icu_messageformat_parser

# Run benchmarks
bazel run //crates/icu_messageformat_parser:parser_bench

WebAssembly

The parser can be compiled to WebAssembly using Bazel's platform transition approach.

Build with Bazel

bazel build //crates/icu_messageformat_parser:formatjs_icu_messageformat_parser_wasm

This uses rust_shared_library with platform = "@rules_rust//rust/platform:wasm" to cross-compile to wasm32.

What Gets Built

The WASM build includes:

• formatjs_icu_messageformat_parser_bg.wasm - The WASM binary (~1.2MB)
• formatjs_icu_messageformat_parser.js - JavaScript glue code generated by wasm-bindgen
• formatjs_icu_messageformat_parser.d.ts - TypeScript type definitions
• formatjs_icu_messageformat_parser_bg.wasm.d.ts - WASM module types

WASM Configuration

The WASM build uses:

• crate-type: cdylib for dynamic library output
• features: wasm feature flag enables wasm-bindgen dependencies
• platform: @rules_rust//rust/platform:wasm for wasm32 target
• dependencies: wasm-bindgen and serde-wasm-bindgen for JS interop

See BUILD.bazel for the full configuration.

WASM API

When compiled to WASM, the parser exports two functions:

parse(input: string): MessageFormatElement[]

Parse ICU MessageFormat with default options.

import init, {parse} from './formatjs_icu_messageformat_parser.js'

await init()
const ast = parse('Hello {name}!')
console.log(ast)

parse_ignore_tag(input: string): MessageFormatElement[]

Parse with ignore_tag option enabled (treats HTML-like tags as literals).

import init, {parse_ignore_tag} from './formatjs_icu_messageformat_parser.js'

await init()
const ast = parse_ignore_tag('<b>Bold {name}</b>')
console.log(ast)

Both functions return the parsed AST as a JavaScript object or throw an error on parse failure.

Usage in Packages

The WASM binary is used by the @formatjs/icu-messageformat-parser-wasm npm package, which provides a convenient JavaScript wrapper:

import {parse, parseIgnoreTag} from '@formatjs/icu-messageformat-parser-wasm'

// Automatically initializes WASM on first call
const ast = await parse('Hello {name}!')

Implementation Notes

Platform Transition

The build uses Bazel's platform transition feature to cross-compile from the host platform to wasm32:

rust_shared_library(
    name = "formatjs_icu_messageformat_parser_wasm",
    platform = "@rules_rust//rust/platform:wasm",
    crate_features = ["wasm"],
    # ...
)

This approach:

• ✅ Works entirely within Bazel's hermetic build system
• ✅ No external tools (like wasm-pack) required at build time
• ✅ Leverages rules_rust's native WASM support
• ✅ Automatically uses the wasm32 dummy CC toolchain

WASM Bindgen Integration

The wasm feature flag in Cargo.toml enables:

• wasm-bindgen for JS interop
• serde-wasm-bindgen for serializing complex types to JS
• Exported parse and parse_ignore_tag functions

The Rust code uses #[cfg(feature = "wasm")] to conditionally compile WASM-specific code.

Dependencies

• icu - Unicode/ICU functionality
• regex - Pattern matching
• serde - Serialization framework
• once_cell - Lazy static initialization

WASM-only dependencies (behind wasm feature):

• wasm-bindgen - JS interop
• serde-wasm-bindgen - Serialize to JS values

Development

Regenerate Generated Files

# Regenerate time data
bazel run //crates/icu_messageformat_parser:time-data

# Regenerate regex patterns
bazel run //crates/icu_messageformat_parser:regex

Testing

# Run Rust tests
bazel test //crates/icu_messageformat_parser:icu_messageformat_parser_test

# Run benchmarks
bazel run //crates/icu_messageformat_parser:parser_bench

References

• ICU MessageFormat Syntax
• wasm-bindgen Guide
• rules_rust WASM Documentation
• Bazel Platform Transitions