Create lang item array and add map fn

This creates the language item for arrays, and adds the map fn which is like map in options or
iterators. It currently allocates an extra array, unfortunately.

Added fixme for transmuting

Fix typo

Add drop guard

Author: JulianKnodt

library/core/src/array/mod.rs

@@ -364,3 +364,56 @@ macro_rules! array_impl_default {
 }
 
 array_impl_default! {32, T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T}
+
+#[cfg(not(bootstrap))]
+#[lang = "array"]
+impl<T, const N: usize> [T; N] {
+    /// Returns an array of the same size as self, with `f` applied to each element.
+    ///
+    /// # Examples
+    /// ```
+    /// let x = [1,2,3];
+    /// let y = x.map(|v| v + 1);
+    /// assert_eq!(y, [2,3,4]);
+    /// ```
+    #[unstable(feature = "array_map", issue = "77777")]
+    fn map<F, S>(self, f: F) -> [S; N]
+    where
+        F: FnMut(T) -> S,
+    {
+        use crate::mem::MaybeUninit;
+        struct Guard<T, const N: usize> {
+            dst: *mut T,
+            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);
+
+                let initialized_part =
+                    crate::ptr::slice_from_raw_parts_mut(self.dst, self.curr_init);
+                // SAFETY: this raw slice will contain only initialized objects
+                // that's why, it is allowed to drop it.
+                unsafe {
+                    crate::ptr::drop_in_place(initialized_part);
+                }
+            }
+        }
+        let dst = MaybeUninit::uninit_array::<N>();
+        let mut guard = Guard::new(&mut dst);
+        for (i, e) in self.into_iter().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) }
+        unsafe { (&mut dst as *mut _ as *mut [S; N]).read() }
+    }
+}

src/librustc_hir/lang_items.rs

@@ -165,6 +165,7 @@ language_item_table! {
     BoolImplItem,                  sym::bool,               bool_impl,               Target::Impl;
     CharImplItem,                  sym::char,               char_impl,               Target::Impl;
     StrImplItem,                   sym::str,                str_impl,                Target::Impl;
+    ArrayImplItem,                 sym::array,              array_impl,              Target::Impl;
     SliceImplItem,                 sym::slice,              slice_impl,              Target::Impl;
     SliceU8ImplItem,               sym::slice_u8,           slice_u8_impl,           Target::Impl;
     StrAllocImplItem,              sym::str_alloc,          str_alloc_impl,          Target::Impl;