Work around stack overflow in array_from_fn (#876)

It should fix a crash in JikesRVM caused by overflown stack overwriting
objects in the VM space.

Fixes: #874

---------

Co-authored-by: Yi Lin <qinsoon@gmail.com>

src/util/rust_util/mod.rs

@@ -127,14 +127,24 @@ mod initialize_once_tests {
 }
 
 /// This implements `std::array::from_fn` introduced in Rust 1.63.
-/// We should replace this with the standard counterpart after bumping MSRV.
-pub(crate) fn array_from_fn<T, const N: usize, F>(cb: F) -> [T; N]
+/// We should replace this with the standard counterpart after bumping MSRV,
+/// but we also need to evaluate whether it would use too much stack space (see code comments).
+pub(crate) fn array_from_fn<T, const N: usize, F>(mut cb: F) -> [T; N]
 where
     F: FnMut(usize) -> T,
 {
-    let mut index_array = [0; N];
-    for (index, item) in index_array.iter_mut().enumerate() {
-        *item = index;
+    // Note on unsafety: An alternative to the unsafe implementation below is creating a fixed
+    // array of `[0, 1, 2, ..., N-1]` and using the `.map(cb)` method to create the result.
+    // However, the `array::map` method implemented in the standard library consumes a surprisingly
+    // large amount of stack space.  For VMs that run on confined stacks, such as JikesRVM, that
+    // would cause stack overflow.  Therefore we implement it manually using unsafe primitives.
+    let mut result_array: MaybeUninit<[T; N]> = MaybeUninit::zeroed();
+    let array_ptr = result_array.as_mut_ptr();
+    for index in 0..N {
+        let item = cb(index);
+        unsafe {
+            std::ptr::addr_of_mut!((*array_ptr)[index]).write(item);
+        }
     }
-    index_array.map(cb)
+    unsafe { result_array.assume_init() }
 }