Vol.69 MoonBit Update

[Kuki]

2025-04-07

Language Updates

Wasm Backend: extern type T Now Storable in Arrays and Data Structures

The wasm backend now supports storing values of extern type T in arrays and other data structures. At FFI boundaries (import/export function signatures), extern type T is still compiled to WebAssembly’s externref type.

C FFI Supports borrow

C FFI now supports the borrow attribute. You can use #borrow(args, ...) above an extern "c" function to customize how MoonBit manages the lifetime of certain arguments, where args is a subset of the C FFI function’s parameter names.

By default, MoonBit assumes C FFI functions are responsible for releasing the passed arguments. This often requires writing helper functions to manually release values.

fn open(path: Bytes, flags: Int, mode: Int) -> Int = "open"

int open_wrapper(moonbit_bytes_t path, int flags, int mode) {

int rc = open(path, flags, mode);

moonbit_decref(path);

return rc;

}

With the borrow attribute, you can instruct MoonBit not to generate reference counting instructions for the specified arguments, allowing direct binding to C library functions without the need for extra helper functions.

#borrow(path)

fn open(path: Bytes, flags: Int, mode: Int) -> Int = "open"

Thanks to #borrow, MoonBit will automatically release path after calling open.

type & trait Support the #deprecated Attribute

type and trait now support the #deprecated attribute. In the next release, the old pragma-based mechanism will be removed. It’s recommended to use the attribute instead:

/// the @alert pragma is deprecated

/// @alert deprecated "message"

fn f() -> Unit { ... }

/// use the #deprecated attribute instead

#deprecated("message")

fn f() -> Unit { ... }

Backend Consistency Checks for Declarations of FFI extern Functions

Backend consistency checks have been added to declarations of FFI extern functions. For example, the following function will raise an error when building with non-Native backends:

extern "c" fn open(path: Bytes, flags: Int, mode: Int) -> Int = "open"

Toolchain Updates

1. Starting this week, toolchain releases will move from Monday to Thursday.

2. Fixed a bug in the test explorer and added support for testing and debugging .mbt.md files:

You can enable the following setting to allow setting breakpoints inside Markdown files:

Settings > Debug: Allow Breakpoint Everywhere

3. moon info --package now supports fuzzy matching for package names.