#mbt_rs

Bidirectional transpiler between MoonBit and Rust.

Focuses on syntax-level conversion with standard library API mapping, ownership/lifetime stripping, Drop→defer conversion, and async syntax support. Does not aim for perfect conversion — converts as much as possible and leaves the rest as WARNING comments for manual fixing.

#Install

# rs2mbt (Rust → MoonBit) CLI
cargo install --git https://github.com/mizchi/mbt_rs rs2mbt

# mbt2rs (MoonBit → Rust) library
moon add mizchi/mbt_rs

#Usage

#Rust → MoonBit (CLI)

# Single file
rs2mbt input.rs > output.mbt

# With macro expansion (recommended for real projects)
rs2mbt --expand src/lib.rs > output.mbt

# Quality report
rs2mbt --report input.rs

# Convert + auto format
just convert input.rs

#MoonBit → Rust (Library)

// moon.pkg
import {
  "mizchi/mbt_rs/mbt2rs",
  "moonbitlang/parser",
}

let source = "fn add(x : Int, y : Int) -> Int { x + y }"
let (impls, _) = @moonbitlang/parser.parse_string(source)
let rust_code = @mbt2rs.to_rust(impls)
// → "fn add(x: i32, y: i32) -> i32 {\n    x + y\n}\n"

#Quality Report

rs2mbt --report input.rs
rs2mbt --expand --report src/lib.rs  # with macro expansion

Results on real projects:

File	Lines	Conversion	Notes
calculator.rs	211	100%	Expression tree + eval + simplify
stack.rs	114	100%	Generic stack
quality_test.rs	181	97%	Mixed patterns
linear_map.rs	606	96%	Vec-based map
hex crate	525	96%	Hex encode/decode
After cargo expand	—	99%	Macro expansion eliminates most WARNINGs

#Conversion Reference

#Types

Rust	MoonBit
i32	Int
u32	UInt
i64	Int64
u64	UInt64
f32	Float
f64	Double
bool	Bool
char	Char
u8	Byte
String / &str	String
Vec<T>	Array[T]
HashMap<K,V>	Map[K,V]
Option<T>	Option[T]
Result<T,E>	Result[T,E]
&[u8]	Bytes
Box<T> / Rc<T> / Arc<T>	T (GC'd, wrapper stripped)

#Syntax

Rust	MoonBit
fn foo<T>(x: T) -> T	fn[T] foo(x: T) -> T
impl<T> S<T> { fn m(&self) }	fn S::m(self: S[T])
trait Foo { fn bar(&self) -> T; }	trait Foo { bar(Self) -> T }
impl Foo for Bar { fn bar(&self) { } }	impl Foo for Bar with bar(self: Bar) { }
fn f(x: &dyn Trait)	fn[X: Trait] f(x: X)
#[derive(Debug, Eq)]	} derive(Show, Eq)
if let Some(x) = opt	if opt is Some(x)
expr.await	expr (no await needed)
vec![1,2,3]	[1,2,3]
format!("{} {}", a, b)	"\{a} \{b}"
assert_eq!(a, b)	assert_eq(a, b)
println!("hi")	println("hi")
matches!(x, Pat)	x is Pat
String::from("s")	"s"
Vec::new()	[]
Box::new(x)	x
x.clone()	x (GC'd)
x.len()	x.length()
x.iter().map(f).collect()	x.map(f)
x.and_then(f)	x.bind(f)
x as f64	x.to_double()
!expr	not(expr)
#[test] fn test_foo()	test "foo" { }
struct Unit;	struct Unit {}
let e = Unit;	let e = Unit::{ }
loop { }	while true { }
while let Some(x) = v.next()	for x in v { }
for (a,b) in v	for _item in v { let (a,b) = _item }

#Ownership / Lifetimes

All stripped. MoonBit is garbage-collected.

Rust	MoonBit
&T / &mut T	T
const T / mut T	T
Box<T>	T
Rc<T> / Arc<T>	T (with NOTE comment)
Cell<T> / RefCell<T>	T
'a lifetime params	removed
*expr (deref)	expr
impl Drop for X { fn drop() { body } }	defer { body } at creation site

#Iterator / Collection Trait Impls

Rust	MoonBit
impl Iterator for X	skipped (use .iter() / for-in)
impl IntoIterator	skipped
impl Clone / Copy	removed
impl Index	skipped (op_get / op_set)
impl Extend	skipped

#Option / Result

Rust	MoonBit
.map(f)	.map(f)
.and_then(f)	.bind(f)
.unwrap_or(v)	.unwrap_or(v)
.unwrap_or_else(f)	.or_else(f)
.flatten()	.flatten()
.is_some() / .is_none()	same
.is_ok() / .is_err()	same
.map_err(f)	.map_err(f)

#Unsupported (emits WARNING comments)

macro_rules! definitions — use cargo expand or rs2mbt --expand
unsafe blocks — stripped with comment
Blanket trait impls (impl<T> Foo for T)
Complex trait objects / dynamic dispatch

#Testing

# All tests
just test-all

# Individual suites
just test            # mbt2rs MoonBit tests (100)
just cargo-test      # rs2mbt Rust tests (139)
just behavioral-test # Behavioral equivalence (Rust 159 + MoonBit 161)

The behavioral test runs the same assertions in both Rust (cargo test) and MoonBit (moon test) to verify that converted code produces identical results.

#CI

GitHub Actions with 3 jobs:

MoonBit: lint + type check + mbt2rs tests
Rust: rs2mbt unit tests + roundtrip tests
Behavioral equivalence: Rust cargo test + converted MoonBit moon test

#License

MIT

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