Correctly unpack &mut [T] into *mut T and usize

wasi-common-cbindgen/tests/array_args.rs

@@ -0,0 +1,20 @@
+extern crate wasi_common_cbindgen;
+
+pub use wasi_common_cbindgen::wasi_common_cbindgen;
+
+#[wasi_common_cbindgen]
+fn array_args(a: &mut [u8]) {
+    a[0] = 1;
+}
+
+fn main() {
+    let mut expected: &mut [u8] = &mut [0, 0];
+    array_args(&mut expected);
+
+    let given: &mut [u8] = &mut [0, 0];
+    unsafe {
+        __wasi_array_args(given.as_mut_ptr(), given.len());
+    }
+
+    assert_eq!(given, expected);
+}