Add array lang item and [T; N]::map(f: FnMut(T) -> S)

library/core/src/array/mod.rs

@@ -364,3 +364,66 @@ 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 function `f` applied to each element
+ /// in order.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(array_map)]
+ /// let x = [1, 2, 3];
+ /// let y = x.map(|v| v + 1);
+ /// assert_eq!(y, [2, 3, 4]);
+ ///
+ /// let x = [1, 2, 3];
+ /// let mut temp = 0;
+ /// let y = x.map(|v| { temp += 1; v * temp });
+ /// assert_eq!(y, [1, 4, 9]);
+ ///
+ /// let x = ["Ferris", "Bueller's", "Day", "Off"];
+ /// let y = x.map(|v| v.len());
+ /// assert_eq!(y, [6, 9, 3, 3]);
+ /// ```
+ #[unstable(feature = "array_map", issue = "75243")]
+ pub fn map<F, U>(self, mut f: F) -> [U; N]
+ where
+ F: FnMut(T) -> U,
+ {
+ use crate::mem::MaybeUninit;
+ struct Guard<T, const N: usize> {
+ dst: *mut T,
+ initialized: usize,
+ }
+
+ impl<T, const N: usize> Drop for Guard<T, N> {
+ fn drop(&mut self) {
+ debug_assert!(self.initialized <= N);
+
+ let initialized_part =
+ crate::ptr::slice_from_raw_parts_mut(self.dst, self.initialized);
+ // 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 mut dst = MaybeUninit::uninit_array::<N>();
+ let mut guard: Guard<U, N> =
+ Guard { dst: MaybeUninit::first_ptr_mut(&mut dst), initialized: 0 };
+ for (src, dst) in IntoIter::new(self).zip(&mut dst) {
+ dst.write(f(src));
+ guard.initialized += 1;
+ }
+ // FIXME: Convert to crate::mem::transmute once it works with generics.
+ // unsafe { crate::mem::transmute::<[MaybeUninit<U>; N], [U; 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 { crate::mem::transmute_copy::<_, [U; N]>(&dst) }
+ }
+}

library/core/src/lib.rs

@@ -145,6 +145,7 @@
 #![feature(abi_unadjusted)]
 #![feature(adx_target_feature)]
 #![feature(maybe_uninit_slice)]
+#![feature(maybe_uninit_extra)]
 #![feature(external_doc)]
 #![feature(associated_type_bounds)]
 #![feature(const_caller_location)]

library/core/tests/array.rs

@@ -290,3 +290,43 @@ 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]);
+
+ let a = [1u8, 2, 3];
+ let b = a.map(|v| v as u64);
+ assert_eq!(b, [1, 2, 3]);
+}
+
+// See note on above test for why `should_panic` is used.
+#[test]
+#[should_panic(expected = "test succeeded")]
+fn array_map_drop_safety() {
+ use core::sync::atomic::AtomicUsize;
+ use core::sync::atomic::Ordering;
+ static DROPPED: AtomicUsize = AtomicUsize::new(0);
+ struct DropCounter;
+ impl Drop for DropCounter {
+ fn drop(&mut self) {
+ DROPPED.fetch_add(1, Ordering::SeqCst);
+ }
+ }
+
+ let num_to_create = 5;
+ let success = std::panic::catch_unwind(|| {
+ let items = [0; 10];
+ let mut nth = 0;
+ items.map(|_| {
+ assert!(nth < num_to_create);
+ nth += 1;
+ DropCounter
+ });
+ });
+ assert!(success.is_err());
+ assert_eq!(DROPPED.load(Ordering::SeqCst), num_to_create);
+ panic!("test succeeded")
+}

library/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)]

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;

src/librustc_typeck/check/method/probe.rs

@@ -649,6 +649,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
  self.assemble_inherent_impl_for_primitive(lang_def_id);
  }
  }
+ ty::Array(_, _) => {
+ let lang_def_id = lang_items.array_impl();
+ self.assemble_inherent_impl_for_primitive(lang_def_id);
+ }
  ty::RawPtr(ty::TypeAndMut { ty: _, mutbl }) => {
  let (lang_def_id1, lang_def_id2) = match mutbl {
  hir::Mutability::Not => {

src/librustc_typeck/coherence/inherent_impls.rs

@@ -112,6 +112,16 @@ impl ItemLikeVisitor<'v> for InherentCollect<'tcx> {
  item.span,
  );
  }
+ ty::Array(_, _) => {
+ self.check_primitive_impl(
+ def_id,
+ lang_items.array_impl(),
+ None,
+ "array",
+ "[T; N]",
+ item.span,
+ );
+ }
  ty::RawPtr(ty::TypeAndMut { ty: inner, mutbl: hir::Mutability::Not })
  if matches!(inner.kind, ty::Slice(_)) =>
  {

src/librustdoc/clean/utils.rs

@@ -388,7 +388,7 @@ pub fn build_deref_target_impls(cx: &DocContext<'_>, items: &[Item], ret: &mut V
  Bool => tcx.lang_items().bool_impl(),
  Str => tcx.lang_items().str_impl(),
  Slice => tcx.lang_items().slice_impl(),
- Array => tcx.lang_items().slice_impl(),
+ Array => tcx.lang_items().array_impl(),
  Tuple => None,
  Unit => None,
  RawPointer => tcx.lang_items().const_ptr_impl(),

src/librustdoc/passes/collect_trait_impls.rs

@@ -55,6 +55,7 @@ pub fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
  lang_items.bool_impl(),
  lang_items.char_impl(),
  lang_items.str_impl(),
+ lang_items.array_impl(),
  lang_items.slice_impl(),
  lang_items.slice_u8_impl(),
  lang_items.str_alloc_impl(),