Add Array Impl Lang Item in various places

Add basic test

And also run fmt which is where the other changes are from

Fix mut issues

These only appear when running tests, so resolved by adding mut

Swap order of forget

Add pub and rm guard impl

Add explicit type to guard

Add safety note

Change guard type from T to S

It should never have been T, as it guards over [MaybeUninit<S>; N]
Also add feature to test

Author: JulianKnodt

core/src/array/mod.rs

@@ -377,7 +377,7 @@ impl<T, const N: usize> [T; N] {
     /// assert_eq!(y, [2,3,4]);
     /// ```
     #[unstable(feature = "array_map", issue = "77777")]
-    fn map<F, S>(self, f: F) -> [S; N]
+    pub fn map<F, S>(self, mut f: F) -> [S; N]
     where
         F: FnMut(T) -> S,
     {
@@ -387,12 +387,6 @@ impl<T, const N: usize> [T; N] {
             curr_init: usize,
         }
 
-        impl<T, const N: usize> Guard<T, N> {
-            fn new(dst: &mut [MaybeUninit<T>; N]) -> Self {
-                Guard { dst: dst as *mut _ as *mut T, curr_init: 0 }
-            }
-        }
-
         impl<T, const N: usize> Drop for Guard<T, N> {
             fn drop(&mut self) {
                 debug_assert!(self.curr_init <= N);
@@ -406,14 +400,17 @@ impl<T, const N: usize> [T; N] {
                 }
             }
         }
-        let dst = MaybeUninit::uninit_array::<N>();
-        let mut guard = Guard::new(&mut dst);
-        for (i, e) in self.into_iter().enumerate() {
+        let mut dst = MaybeUninit::uninit_array::<N>();
+        let mut guard: Guard<S, N> = Guard { dst: &mut dst as *mut _ as *mut S, curr_init: 0 };
+        for (i, e) in IntoIter::new(self).enumerate() {
             dst[i] = MaybeUninit::new(f(e));
             guard.curr_init += 1;
         }
         // FIXME convert to crate::mem::transmute when works with generics
         // unsafe { crate::mem::transmute::<[MaybeUninit<S>; N], [S; N]>(dst) }
+        crate::mem::forget(guard);
+        // SAFETY: At this point we've properly initialized the whole array
+        // and we just need to cast it to the correct type
         unsafe { (&mut dst as *mut _ as *mut [S; N]).read() }
     }
 }

core/tests/array.rs

@@ -290,3 +290,10 @@ fn empty_array_is_always_default() {
 
     let _arr = <[DoesNotImplDefault; 0]>::default();
 }
+
+#[test]
+fn array_map() {
+    let a = [1, 2, 3];
+    let b = a.map(|v| v + 1);
+    assert_eq!(b, [2, 3, 4]);
+}

core/tests/lib.rs

@@ -1,5 +1,6 @@
 #![feature(alloc_layout_extra)]
 #![feature(array_chunks)]
+#![feature(array_map)]
 #![feature(bool_to_option)]
 #![feature(bound_cloned)]
 #![feature(box_syntax)]