antlr-rust-runtime 0.15.0

High performance Rust runtime and target support for ANTLR v4 generated parsers
Documentation

ANTLR4 Runtime for Rust

Crates.io Version ANTLR Runtime Testsuite

antlr-rust-runtime is a pure Rust runtime and source 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. Get an ANTLR4 grammar

Use your own .g4 files or a grammar from antlr/grammars-v4. Rust generation does not require Java, Node.js, the ANTLR tool jar, or an intermediate .interp file. The repository's differential tests use ANTLR 4.13.2 only as an explicit compatibility oracle.

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 = "0.15.0"

The library crate is imported as antlr4_runtime:

use antlr4_runtime::{CommonTokenStream, InputStream};

Install the companion generator binary:

cargo install antlr-rust-runtime --features codegen --bin antlr4-rust-gen

This installs antlr4-rust-gen, which compiles ANTLR .g4 source into Rust lexer and parser modules. During generation it also compiles the lexer's DFA ahead of time and embeds the tables in the generated lexer, so tokenization runs at full speed from the first character with no per-process warmup.

3. Generate your parser

Pass one or more root grammars directly. Imports and tokenVocab dependencies are resolved from each root's directory and any additional --lib/-I directories.

For a split lexer/parser grammar:

antlr4-rust-gen \
  MyGrammarLexer.g4 \
  MyGrammarParser.g4 \
  --lib . \
  --out-dir src/generated

Use multiple roots when a build should emit several independent recognizers in one deterministic source-set compilation.

Complete Example

Suppose you are using JSON.g4 from antlr/grammars-v4/json. Generate both recognizers directly from the combined grammar:

antlr4-rust-gen \
  JSON.g4 \
  --lib . \
  --out-dir src/generated

Declare the generated modules in your crate:

mod generated {
    #![allow(dead_code)]

    pub mod json_lexer;
    pub mod json_parser;
}

Typed listeners and visitors

Parser generation emits a typed <Grammar>Listener and <Grammar>TreeWalker by default. A listener can start a grammar-typed walk directly. Listener callbacks return Result<(), E>, where E defaults to Infallible, so domain errors can stop traversal without a side channel:

listener.walk(parsed.tree())?;

Use --no-listener to omit that surface. Add --visitor to emit a typed <Grammar>Visitor; visitors choose an associated Result type, define its initial value with default_result(), and drive recursion explicitly:

type Result = Result<i64, MissingChildError>;

fn default_result(&mut self) -> Self::Result {
    Ok(0)
}

fn visit_add_label(&mut self, ctx: &AddLabelContext) -> Self::Result {
    let left = self.visit(ctx.left()?)?;
    let right = self.visit(ctx.right()?)?;
    Ok(left + right)
}

Generated child accessors follow grammar cardinality. Required children return Result<T, MissingChildError>, optional children return Option<T>, and repeated children are lazy iterators. Rule labels keep their grammar names (left()), while token accessors use snake_case names such as int_token() and comma_tokens().

--no-visitor disables visitor generation. The generator also accepts ANTLR's single-dash spellings (-listener, -no-listener, -visitor, -no-visitor).

Call the generated parser helper for the compact path:

use generated::json_lexer::JsonLexer;
use generated::json_parser::{self, JsonParser};

fn main() -> Result<(), antlr4_runtime::AntlrError> {
    let parsed =
        json_parser::parse(r#"{"a":1}"#, JsonLexer::new, JsonParser::json)?;

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

Use parse_with_parser when you want the compact setup path and also need the parser afterward for diagnostics:

use antlr4_runtime::Parser;
use generated::json_lexer::JsonLexer;
use generated::json_parser::{self, JsonParser};

fn main() -> Result<(), antlr4_runtime::AntlrError> {
    let output = json_parser::parse_with_parser(
        r#"{"a":1}"#,
        JsonLexer::new,
        JsonParser::json,
    )?;
    let syntax_errors = output.parser.number_of_syntax_errors();
    let json_parser::JsonParserParseOutput {
        result: tree,
        parser,
    } = output;
    let parsed = parser.into_parsed_file(tree);

    println!(
        "{} errors across {} tokens",
        syntax_errors,
        parsed.tokens().len()
    );
    println!("{}", parsed.tree().text());
    Ok(())
}

Or construct each layer explicitly when you need to set source names, parser options, or custom error handling before invoking the entry rule:

use antlr4_runtime::{CommonTokenStream, InputStream};
use generated::json_lexer::JsonLexer;
use generated::json_parser::JsonParser;

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

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

Generated recognizers install a ConsoleErrorListener by default. Remove it from both the lexer and parser to suppress recovery output, as above, or call add_error_listener after removal to redirect diagnostics to a replacement.

Reusing Recognizers

Generated recognizers can be re-fed without reconstructing the lexer or parser. The token stream owns its lexer, so use the mutable accessors to update that nested source and rebuild the token buffer:

let lexer = JsonLexer::new(InputStream::new(""));
let tokens = CommonTokenStream::new(lexer);
let mut parser = JsonParser::new(tokens);

for input in [r#"{"a":1}"#, r#"{"b":2}"#] {
    let tokens = parser.token_stream_mut();
    tokens
        .token_source_mut()
        .set_input_stream(InputStream::new(input));
    tokens.refill();
    parser.reset();

    let tree = parser.json()?;
    println!("{}", parser.node(tree).text());
}

CommonTokenStream::set_token_source and generated Parser::set_token_stream replace whole layers when ownership is already available. clear_dfa() on generated lexers and parsers drops learned fallback and decision DFA state for cold measurements or memory control; immutable ahead-of-time lexer DFA tables remain embedded generated data.

Choosing Parser Entry Rules

Generated parsers expose one public method per grammar rule. Call the method that matches the grammar's intended top-level rule for the input; the generator can identify rules that are not called by other rules, but it cannot infer the semantic choice between multiple top-level forms. The generated parser rustdoc lists likely entry methods first, followed by all rule methods.

For the JSON grammar above, json() is the natural entry. Larger grammars may have several top-level forms, so confirm the intended entry rule against that grammar's documentation. Calling the wrong rule can still recover and return a parse tree with error nodes, so check parser diagnostics when adding a new input form.

Technical Notes

  • Pure Rust runtime implementation.
  • Written from scratch as a clean-room implementation.
  • Compiles ANTLR grammar source into lexer ATNs and packed parser runtime tables without an external generator.
  • 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
  • compact TokenId/TokenView access, TokenSource, and one canonical TokenStore
  • 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 lexer ATN deserialization
  • lexer ATN recognition with longest-match/rule-priority behavior and lexer actions
  • ahead-of-time compiled lexer DFA tables, built by antlr4-rust-gen and embedded in generated lexers, with per-token escape to ATN interpretation for constructs a finite DFA cannot represent (semantic predicates, recursive lexer rules)
  • versioned, packed parser ATN tables embedded directly in generated parsers, with rule recognition over borrowing state/transition views
  • canonical ContextId prediction graphs pooled with learned parser DFA state
  • antlr4-rust-gen, a source-only Rust generator that compiles .g4 roots and their import graph into Rust modules
  • antlr4-runtime-testsuite, a harness for running upstream ANTLR runtime-test descriptors through the direct Rust source compiler

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

Semantic Predicates and Actions: the Compatibility Boundary

ANTLR grammars may embed target-language semantic predicates and actions ({isTypeName()}?, {this.count++;}). The direct compiler preserves their structural owner, source span, and finalized ATN coordinate, but cannot make arbitrary code written for another target language executable as Rust. The boundary is:

  • Target-agnostic grammars — no embedded code, or only built-in lexer commands (skip, channel(...), mode(...), type(...)) — are fully supported.
  • Recognized predicate/action shapes — a library of common idioms (constant predicates, lookahead text/type checks, integer member counters, column predicates, and the upstream testsuite's action templates) — are translated into SemIR by antlr4-rust-gen.
  • User pattern files — --sem-patterns file.toml can add exact predicate rewrites, helper-call rewrites, and per-coordinate hook / assume-true / assume-false / error dispositions without changing the generator.
  • Everything else is not silently guessed. Each generator run writes a semantics.json manifest next to the generated modules listing every predicate/action coordinate with its grammar source span, body, and disposition (translated, hooked, assume-true, assume-false, ignored, synthetic, or error). A synthetic action is one ANTLR inserts itself (e.g. during left-recursion elimination); it has no grammar-author source, is a runtime no-op, and is exempt from the error gate — only actions the author actually wrote in the grammar can fail it.

The same manifest inventories top-level grammar options. Options implemented by the source compiler (tokenVocab and caseInsensitive) are recorded without a warning. Target extension options such as superClass and contextSuperClass warn because the Rust backend cannot inherit their target-language implementation automatically. If caller-owned Rust hooks provide that behavior, acknowledge the exact option:

antlr4-rust-gen L.g4 \
    --option-hook superClass=MyLexerBase --out-dir src/generated

Acknowledged options have the hooked disposition. Unacknowledged target options have the unsupported disposition and make --require-full-semantics fail.

Unknown coordinates are governed by --sem-unknown:

antlr4-rust-gen L.g4 P.g4 --lib . \
    --out-dir src/generated --sem-unknown error

  • assume-true (current default, deprecated): unknown predicates pass, unknown actions are no-ops — the historical behavior. A future minor release changes the default to error.

  • hook: unknown parser predicates are routed to SemanticHooks and fail if the hook does not handle them.

  • assume-false: unknown predicates fail, removing the guarded alternatives.

  • error: generation fails, naming each coordinate:

    unsupported semantic predicate: rule=s(0) pred_index=0 at 2:4: {isTypeName()}

At runtime the same policy exists as ParserRuntimeOptions::unknown_predicate_policy (UnknownSemanticPolicy::{AssumeTrue, AssumeFalse, Error}); under Error, evaluating an unknown predicate coordinate fails the parse with AntlrError::Unsupported instead of producing a tree whose shape silently depended on a guess.

Generated parsers also expose a parser-side hook escape hatch: MyParser::with_hooks(tokens, hooks), where hooks implements SemanticHooks. Unknown parser predicates are offered to SemanticHooks::sempred before the fallback policy is applied, and unhandled parser action events are offered to SemanticHooks::action after the committed parse path is selected. Predicate hooks may run speculatively during prediction, so they must be replay-safe.

For bare helper-call predicates, generated parsers also emit a typed hook adapter (MyParserHooks plus MyParserTypedHooks<T>) that maps stable manifest coordinates to named Rust methods. Lexer callers can use LexerSemCtx with atn::lexer::next_token_with_semantic_hooks or the compiled-DFA variant to route lexer predicates/actions through the same SemanticHooks trait. On the committed action path, LexerSemCtx exposes the pending token type/channel, character lookahead and consumption, and mode mutators. Actions can also queue a prefix token and advance the current token start, allowing one lexer match to return multiple tokens while each TokenSource::next_token call still appends exactly one token.

Lexer behavior that has no ATN action/predicate coordinate uses LexerLifecycleCtx. A hook may implement lexer_before_token, lexer_after_accept, lexer_reset, and the existing lexer_token_emitted observer. The post-accept callback runs after portable and custom actions but before the token span is emitted, including for rules with no semantic transition. Generated lexers expose reset() to clear runtime-owned pending tokens and invoke extension-owned cleanup. Lexers built with new() retain the direct compiled-DFA path; with_hooks() opts into the lifecycle dispatch path.

Generated lexers also own optional hook state and emit typed lexer adapters when a semantic pattern maps lexer helper calls to hooks. The official grammars-v4 JavaScript and TypeScript grammars are complete examples, including checked-in Rust lexer/parser base modules and strict build commands; see docs/javascript-build.md and docs/typescript-build.md.

Use --require-full-semantics in CI when every coordinate and target extension option must be either translated, metadata-backed, or explicitly hooked; policy fallbacks and unsupported options fail generation.

Embedded target-language actions are not portable — including in official ANTLR

A grammar that embeds a target-language action (a { ... } block of Java/C#/etc. code, rather than a portable lexer command) is only usable with the language it was written for. This is a limitation of ANTLR itself, not of this runtime: the official ANTLR tool does not translate embedded actions between targets — it copies the source text verbatim into the generated code.

For example, the official Kotlin/kotlin-spec KotlinLexer.g4 contains a Java-only action:

RCURL: '}' { if (!_modeStack.isEmpty()) { popMode(); } };

Generating a Go parser from it with the official tool (antlr4 -Dlanguage=Go KotlinLexer.g4) emits the Java verbatim:

func (l *KotlinLexer) RCURL_Action(localctx antlr.RuleContext, actionIndex int) {
	switch actionIndex {
	case 0:
		if !_modeStack.isEmpty() { // undefined in Go — does not compile
			popMode()              // undefined in Go — does not compile
		}
	}
}

The generated Go fails to compile (undefined: _modeStack, undefined: popMode), and ANTLR offers no supported way to fix it beyond hand-editing the grammar — the grammar even carries a comment telling non-Java users to replace the snippet manually. Every non-Java ANTLR target has this gap.

This runtime does better in two ways:

  1. It recognizes a library of common embedded idioms (e.g. the guarded popMode() above) and maps them to the equivalent portable operation, so many real grammars generate as-is.
  2. For anything it does not recognize, --sem-unknown=error fails loudly at generation time, naming the coordinate, instead of silently emitting uncompilable or no-op code. The fix is to express the action as a portable lexer command (-> popMode, -> pushMode(X), -> type(X), -> channel(HIDDEN)), add a --sem-patterns rewrite, or route it through a SemanticHooks implementation.

Portable lexer commands and the recognized idioms are the target-agnostic subset; prefer them when authoring grammars intended for multiple runtimes.

Grammars whose { ... } blocks are already Rust can skip translation entirely: antlr4-rust-gen Foo.g4 --actions embedded splices the bodies verbatim (after $-attribute translation) into the generated parser, inline at their ATN action/predicate coordinates. This is the mode the conformance harness uses after rendering descriptor grammars through Rust.test.stg (see below).

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 --release --quiet --bin antlr4-runtime-testsuite

The harness runs descriptors the way every official ANTLR target does: each descriptor grammar is rendered through .conformance-review/Rust.test.stg with the real StringTemplate engine, so its actions and predicates become real Rust code. The rendered .g4 source graph is then compiled directly by antlr4-rust-gen, and the resulting code is executed inline.

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 \
  --iters 10 \
  --warmups 2 \
  --json target/parse-bench/results.json \
  --markdown target/parse-bench/results.md

Add --ast-check to require byte-identical Rust/Go parse trees (no error nodes) before timing. Prefer that gate for fair comparisons; some C# Mono fixtures still diverge today (see tools/parse-bench/README.md). For a clean smoke:

python3 tools/parse-bench/run.py \
  --languages kotlin,trino \
  --runtimes rust-antlr,go-antlr \
  --ast-check \
  --quick

The report prints min/avg parse time and a ratio against rust-antlr for every fixture. Use --quick for a 3-iteration/1-warmup smoke run, or adjust --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 30.336x
Java 4 3.166x
C# 4 2.177x
Trino SQL 5 3.311x

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