ANTLR4 Runtime for Rust

antlr-rust-runtime is a pure Rust runtime and metadata generator for ANTLR v4 lexers and parsers. It is a clean-room implementation written from scratch from the public ANTLR runtime contract; it does not vendor or fork an older Rust ANTLR runtime.

First Steps

1. Install ANTLR4

Follow the ANTLR getting-started guide and install the ANTLR tool jar. The runtime tests currently validate against ANTLR 4.13.2.

2. Install the Rust ANTLR runtime tools

Each ANTLR target language needs a runtime package used by generated parsers. For Rust projects, add the runtime crate:

[dependencies]
antlr-rust-runtime = "0.3"

The library crate is imported as antlr4_runtime:

use antlr4_runtime::{CommonTokenStream, InputStream};

Install the companion generator binary:

cargo install antlr-rust-runtime

This installs antlr4-rust-gen, which turns ANTLR .interp metadata into Rust lexer and parser modules.

3. Generate your parser

The current release uses a metadata-first generation path:

1. run the official ANTLR tool to produce .interp files,
2. run antlr4-rust-gen to emit Rust modules,
3. compile those modules against antlr4_runtime.

For a split lexer/parser grammar:

antlr4 MyGrammarLexer.g4 MyGrammarParser.g4

antlr4-rust-gen \
  --lexer MyGrammarLexer.interp \
  --parser MyGrammarParser.interp \
  --out-dir src/generated

The checked-in ANTLR RustTarget/StringTemplate shell is kept in tool/ and will be expanded around the same runtime contracts.

Alternative: Generate metadata with antlr-ng

antlr-ng is a TypeScript/npm parser generator based on ANTLR 4.13.2. It does not currently ship a Rust target, but it can produce the same .interp metadata that antlr4-rust-gen uses.

Install it with npm or run it through npx:

npx -y antlr-ng -Dlanguage=Java -o build/antlr --exact-output-dir true JSON.g4

The -Dlanguage=Java option selects one of antlr-ng's bundled code-generation targets only so the tool emits grammar artifacts, including JSONLexer.interp and JSON.interp. The Java files can be ignored; Rust code still comes from antlr4-rust-gen:

antlr4-rust-gen \
  --lexer build/antlr/JSONLexer.interp \
  --parser build/antlr/JSON.interp \
  --out-dir src/generated

For local tooling, antlr-ng requires Node.js 20 or newer. See the antlr-ng getting-started guide for CLI installation and option details.

Complete Example

Suppose you are using the JSON grammar from antlr/grammars-v4/json.

Fetch or copy JSON.g4, then generate ANTLR metadata:

antlr4 JSON.g4

Generate Rust modules:

antlr4-rust-gen \
  --lexer JSONLexer.interp \
  --parser JSON.interp \
  --out-dir src/generated

Declare the generated modules in your crate:

mod generated {
    #![allow(dead_code)]

    pub mod json;
    pub mod json_lexer;
}

Call the generated lexer and parser:

use antlr4_runtime::{CommonTokenStream, InputStream};
use generated::json::Json;
use generated::json_lexer::JsonLexer;

fn main() -> Result<(), antlr4_runtime::AntlrError> {
    let lexer = JsonLexer::new(InputStream::new(r#"{"a":1}"#));
    let tokens = CommonTokenStream::new(lexer);
    let mut parser = Json::new(tokens);
    let tree = parser.json()?;

    println!("{}", tree.text());
    Ok(())
}

Technical Notes

• Pure Rust runtime implementation.
• Written from scratch as a clean-room implementation.
• Supports ANTLR serialized ATN deserialization.
• Supports lexer and parser execution through generated Rust wrappers.
• Supports real split lexer/parser grammars, including Kotlin smoke builds.
• Passes every upstream ANTLR runtime-testsuite descriptor discovered by the harness: 357 passed, 0 failed, 0 skipped, 357 run.
• Licensed under BSD-3-Clause for compatibility with ANTLR's runtime licensing pattern and downstream open-source applications.

The runtime contains:

• IntStream and CharStream
• UTF-8 input as Unicode scalar values
• Token, CommonToken, token factories, and TokenSource
• buffered, channel-aware CommonTokenStream
• Vocabulary
• recognizer metadata and error listener plumbing
• parse tree node types, rule contexts, terminal nodes, error nodes, and walkers
• ANTLR v4 serialized ATN deserialization
• lexer ATN recognition with longest-match/rule-priority behavior and lexer actions
• parser ATN rule recognition with backtracking over token stream indices
• antlr4-rust-gen, a Rust generator that consumes ANTLR .interp metadata and emits Rust modules
• antlr4-runtime-testsuite, a harness for running upstream ANTLR runtime-test descriptors through the Rust metadata path

See docs/kotlin-build.md for the Kotlin smoke workflow. See docs/runtime-testsuite.md for the upstream runtime-testsuite harness.

Runtime Testsuite

On the maintainer checkout, where the ANTLR jar and upstream runtime-testsuite live under /tmp/antlr-cleanroom, run the full sweep with:

cargo run --quiet --bin antlr4-runtime-testsuite

Run a specific descriptor:

cargo run --bin antlr4-runtime-testsuite -- \
  --antlr-jar path/to/antlr-4.13.2-complete.jar \
  --descriptors path/to/antlr4/runtime-testsuite \
  --case LexerExec/KeywordID

Performance

tools/parse-bench/ benchmarks parse throughput of the generated Rust parsers against the upstream Go runtime (github.com/antlr4-go/antlr/v4) — and optionally the reference Python runtime and tree-sitter — on real-world Kotlin, C#, Java, and Trino SQL fixtures. See tools/parse-bench/README.md for setup (the ANTLR jar, the grammars-v4 sparse checkout, and the Python dependencies).

Run the Rust-vs-Go comparison across all fixture languages:

python3 tools/parse-bench/run.py \
  --languages kotlin,csharp,java,trino \
  --runtimes rust-antlr,go-antlr \
  --quick \
  --json target/parse-bench/results.json \
  --markdown target/parse-bench/results.md

The report prints min/avg parse time and a ratio against rust-antlr for every fixture. Drop --quick (or add --iters/--warmups) for longer, lower variance runs; add --runtimes rust-antlr,go-antlr,python-antlr,tree-sitter to include the other runtimes.

Current results

Relative parse speed of this runtime versus the Go runtime, summarized as the geometric mean of the per-fixture go ÷ rust parse-time ratios in each language group (> 1.0 means Rust is faster than Go; < 1.0 means slower):

Language	Fixtures	Rust vs Go (parse time)
Kotlin	4	~10× faster
Java	4	~0.9× (roughly on par)
C#	4	~0.45× (Go ~2.2× faster)
Trino SQL	5	~0.4× (Go ~2.6× faster)

Rust is dramatically faster on Kotlin (expression-ladder memoization in the generated walker) and near parity on Java; C# and Trino remain ahead for Go and are the focus of ongoing prediction/closure optimization. Numbers are quick-mode (--quick, best-of-min) on an Apple M3 Pro and are indicative — re-run the benchmark on your own hardware for authoritative figures.

Useful Information

• ANTLR: https://www.antlr.org/
• ANTLR documentation: https://github.com/antlr/antlr4/blob/dev/doc/index.md
• Grammars v4: https://github.com/antlr/grammars-v4