Add copy_to_uninit() to all TypedArrays

CHANGELOG.md

@@ -3,6 +3,11 @@
 
 ## Unreleased
 
+### Added
+
+* Add a `copy_to_uninit()` method to all `TypedArray`s. It takes `&mut [MaybeUninit<T>]` and returns `&mut [T]`.
+  [#4340](https://github.com/rustwasm/wasm-bindgen/pull/4340)
+
 ### Changed
 
 * Optional parameters are now typed as `T | undefined | null` to reflect the actual JS behavior.

crates/js-sys/src/lib.rs

@@ -32,7 +32,7 @@ use core::convert::{self, Infallible, TryFrom};
 use core::f64;
 use core::fmt;
 use core::iter::{self, Product, Sum};
-use core::mem;
+use core::mem::{self, MaybeUninit};
 use core::ops::{Add, BitAnd, BitOr, BitXor, Div, Mul, Neg, Not, Rem, Shl, Shr, Sub};
 use core::str;
 use core::str::FromStr;
@@ -6336,6 +6336,23 @@ macro_rules! arrays {
                 unsafe { self.raw_copy_to_ptr(dst.as_mut_ptr()); }
             }
 
+            /// Copy the contents of this JS typed array into the destination
+            /// Rust slice.
+            ///
+            /// This function will efficiently copy the memory from a typed
+            /// array into this Wasm module's own linear memory, initializing
+            /// the memory destination provided.
+            ///
+            /// # Panics
+            ///
+            /// This function will panic if this typed array's length is
+            /// different than the length of the provided `dst` array.
+            pub fn copy_to_uninit<'dst>(&self, dst: &'dst mut [MaybeUninit<$ty>]) -> &'dst mut [$ty] {
+                core::assert_eq!(self.length() as usize, dst.len());
+                unsafe { self.raw_copy_to_ptr(dst.as_mut_ptr().cast()); }
+                unsafe { &mut *(dst as *mut [MaybeUninit<$ty>] as *mut [$ty]) }
+            }
+
             /// Copy the contents of the source Rust slice into this
             /// JS typed array.
             ///

crates/js-sys/tests/wasm/TypedArray.rs

@@ -1,3 +1,5 @@
+use std::mem::MaybeUninit;
+
 use js_sys::*;
 use wasm_bindgen::prelude::*;
 use wasm_bindgen_test::*;
@@ -174,6 +176,17 @@ fn copy_to() {
     }
 }
 
+#[wasm_bindgen_test]
+fn copy_to_uninit() {
+    let mut x = [MaybeUninit::uninit(); 10];
+    let array = Int32Array::new(&10.into());
+    array.fill(5, 0, 10);
+    let x = array.copy_to_uninit(&mut x);
+    for i in x.iter() {
+        assert_eq!(*i, 5);
+    }
+}
+
 #[wasm_bindgen_test]
 fn copy_from() {
     let x = [1, 2, 3];