# Vol.69 MoonBit Update

**URL:** <https://discuss.moonbitlang.com/t/vol-69-moonbit-update/355>\
**Category:** Announcement\
**Tags:** weekly-update\
**Created:** [April 7, 2025, 9:45am UTC](https://discuss.moonbitlang.com/t/vol-69-moonbit-update/355 "2025-04-07T09:45:16Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Kuki](https://discuss.moonbitlang.com/user_avatar/discuss.moonbitlang.com/kuki/32/160_2.png) [@Kuki](https://discuss.moonbitlang.com/u/Kuki)\
**Post date:** [April 7, 2025, 9:45am UTC](https://discuss.moonbitlang.com/t/vol-69-moonbit-update/355/1 "2025-04-07T09:45:16Z")

</div>

# 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.

```rust

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

```

```c

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.

```rust

#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:

```rust

/// 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:

```rust

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:

 ![md](https://discuss.moonbitlang.com/uploads/default/original/1X/ad38ecde76df84f2530494c7f210ef22edc23bd7.jpeg)

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

**Settings \> Debug: Allow Breakpoint Everywhere**

 ![setting](https://discuss.moonbitlang.com/uploads/default/original/1X/6dd93692379a31895b425586b8b2f7cf9a4d95f1.png)

1. `moon info --package` now supports fuzzy matching for package names.