Update: 2019-08-10: Fixed rerun directive in build.rs

Tree-sitter is a parser generator tool and parsing library. It generates portable parsers that can be used in several languages including Rust. Tree-sitter grammars are available for several languages.

This is a game changer because it lowers the barrier to entry for writing language tooling. You no longer need to write your own parser. With Tree-sitter, you can now simply use an existing parser.


Tree-sitter grammars are written in Javascript. The grammars are executed using Node to generate the grammar JSON. The Tree-sitter CLI uses the grammar JSON to generate a C-based parser. The parser is compiled into a Rust binary and used via the Rust Tree-sitter bindings.

Install the Dependencies

Node is required to generate the grammar JSON. Install node:

sudo apt-get install nodejs

NOTE: This article assumes you have Rust and a C compiler installed.

Create a New Rust Project

cargo new tree-sitter-verilog-test
cd tree-sitter-verilog-test
git init
git add .
git commit -m "Initial commit"

Obtain the Grammar

A list of existing Tree-sitter grammars is available at https://tree-sitter.github.io/tree-sitter.

I’ll be using the Verilog grammar. Obtain the grammar:

git submodule add https://github.com/tree-sitter/tree-sitter-verilog.git
git commit -m "Add tree-sitter-verilog"

Generate the Parser

cd tree-sitter-verilog
npm install

This installs the Tree-sitter CLI and runs tree-sitter generate which executes the grammar Javascript to generate the grammar JSON then generates the C-based parser. The parser is written to the tree-sitter-verilog/src/ directory.

Compile the Parser

A Cargo build script is needed to compile and link the parser into the Rust binary.

We need the cc crate for compiling C code into our Rust binary. Add the cc crate to the build-dependencies section of Cargo.toml:

cc = "1.0"

Create a build.rs build script with the contents:

fn main() {
    let language = "verilog";
    let package = format!("tree-sitter-{}", language);
    let source_directory = format!("{}/src", package);
    let source_file = format!("{}/parser.c", source_directory);

    println!("cargo:rerun-if-changed={}", source_file); // <1>

        .compile(&package); // <2>
  1. Tells Cargo to only re-run the build script if the parser source has changed.
  2. Compiles the parser C code into the Rust binary.

NOTE: We could instead rerun on a change in the grammar Javascript and add a call to npm install to fully automate the building of the parser.

Use the Parser

We’ll be using the parser via the Rust Tree-sitter bindings provided by the tree-sitter crate. Add the tree-sitter crate to the dependencies section of Cargo.toml:

tree-sitter = "0.3"

Edit the contents of src/main.rs to be the following:

use tree_sitter::{Parser, Language};

extern "C" { fn tree_sitter_verilog() -> Language; }

fn main() {
    println!("Hello, world!");

fn test_parser() {
    let language = unsafe { tree_sitter_verilog() };
    let mut parser = Parser::new();

    let source_code = "module mymodule(); endmodule";
    let tree = parser.parse(source_code, None).unwrap();

    assert_eq!(tree.root_node().to_sexp(), "(source_file (module_declaration (module_header (module_keyword) (module_identifier (simple_identifier))) (module_nonansi_header (list_of_ports))))");

Running cargo test should then give the following output:

$ cargo test
   Compiling tree-sitter-verilog-test v0.1.0 (/home/rfdonnelly/repos/tree-sitter-verilog-test)
    Finished dev [unoptimized + debuginfo] target(s) in 6.27s
     Running target/debug/deps/tree_sitter_verilog_test-3a23b31b32f84a74

running 1 test
test test_parser ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Finally commit the necessary files:

git add -f Cargo.lock Cargo.toml build.rs src/main.rs
git commit -m "Add Tree-sitter parser"