Molt

Molt is a syntax-aware search and replace tool for Rust. Molt is experimental software.

Overview

Molt provides a simple pattern-matching language for the Rust programming language. It's particularly useful for mechanical refactorings, API migrations and pattern finding across large code bases.

Example

Suppose you have a main.rs containing:

fn main() {
    let x = foo.get_map().get("key").insert("value");
    let y = bar.get_map().get(CustomEnum::Key).insert(10);
    let z = baz.get(3).insert("my_string");
}

Also suppose that we want to use a different API that simplifies having to do .get(key).insert(val) with a single method call insert_at_key(key, val), but we only want to do so for those maps that were obtained via get_map(). To do so, we write a molt file:

merge_fns.molt:

let key, val: Expr;
let ident: Ident;

let expr: Expr = $ident.get_map();

let old: Expr = $expr.get($key).insert($val);
let new: Expr = $expr.insert_at_key($key, $val);

mod old -> new;

Running molt merge_fns.molt in the rust project containing main.rs will result in:

main.rs, after:

fn main() {
    let x = foo.get_map().insert_at_key("key", "value");
    let y = bar.get_map().insert_at_key(CustomEnum::Key, 10);
    let z = baz.get(3).insert("my_string"); // baz does not match `$ident.get_map()`, so it did not get rewritten
}

Attribution

The Rust parser for this project is adapted from syn.