Override Iterator::advance(_back)_by for array::IntoIter

Because I happened to notice that `nth` is currently getting codegen'd as a loop even for `Copy` types: <https://rust.godbolt.org/z/fPqv7Gvs7>

Author: scottmcm

library/core/src/array/iter.rs

@@ -1,7 +1,7 @@
 //! Defines the `IntoIter` owned iterator for arrays.
 
 use crate::{
-    fmt,
+    cmp, fmt,
     iter::{self, ExactSizeIterator, FusedIterator, TrustedLen},
     mem::{self, MaybeUninit},
     ops::Range,
@@ -150,6 +150,27 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
     fn last(mut self) -> Option<Self::Item> {
         self.next_back()
     }
+
+    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+        let len = self.len();
+
+        // The number of elements to drop.  Always in-bounds by construction.
+        let delta = cmp::min(n, len);
+
+        let range_to_drop = self.alive.start..(self.alive.start + delta);
+
+        // Moving the start marks them as conceptually "dropped", so if anything
+        // goes bad then our drop impl won't double-free them.
+        self.alive.start += delta;
+
+        // SAFETY: These elements are currently initialized, so it's fine to drop them.
+        unsafe {
+            let slice = self.data.get_unchecked_mut(range_to_drop);
+            ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
+        }
+
+        if n > len { Err(len) } else { Ok(()) }
+    }
 }
 
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
@@ -170,6 +191,27 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
             unsafe { self.data.get_unchecked(idx).assume_init_read() }
         })
     }
+
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        let len = self.len();
+
+        // The number of elements to drop.  Always in-bounds by construction.
+        let delta = cmp::min(n, len);
+
+        let range_to_drop = (self.alive.end - delta)..self.alive.end;
+
+        // Moving the end marks them as conceptually "dropped", so if anything
+        // goes bad then our drop impl won't double-free them.
+        self.alive.end -= delta;
+
+        // SAFETY: These elements are currently initialized, so it's fine to drop them.
+        unsafe {
+            let slice = self.data.get_unchecked_mut(range_to_drop);
+            ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
+        }
+
+        if n > len { Err(len) } else { Ok(()) }
+    }
 }
 
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]

library/core/tests/array.rs

@@ -474,3 +474,109 @@ fn array_split_array_mut_out_of_bounds() {
 
     v.split_array_mut::<7>();
 }
+
+#[test]
+fn array_intoiter_advance_by() {
+    use std::cell::Cell;
+    struct DropCounter<'a>(usize, &'a Cell<usize>);
+    impl Drop for DropCounter<'_> {
+        fn drop(&mut self) {
+            let x = self.1.get();
+            self.1.set(x + 1);
+        }
+    }
+
+    let counter = Cell::new(0);
+    let a: [_; 100] = std::array::from_fn(|i| DropCounter(i, &counter));
+    let mut it = IntoIterator::into_iter(a);
+
+    let r = it.advance_by(1);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 99);
+    assert_eq!(counter.get(), 1);
+
+    let r = it.advance_by(0);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 99);
+    assert_eq!(counter.get(), 1);
+
+    let r = it.advance_by(11);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 88);
+    assert_eq!(counter.get(), 12);
+
+    let x = it.next();
+    assert_eq!(x.as_ref().map(|x| x.0), Some(12));
+    assert_eq!(it.len(), 87);
+    assert_eq!(counter.get(), 12);
+    drop(x);
+    assert_eq!(counter.get(), 13);
+
+    let r = it.advance_by(123456);
+    assert_eq!(r, Err(87));
+    assert_eq!(it.len(), 0);
+    assert_eq!(counter.get(), 100);
+
+    let r = it.advance_by(0);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 0);
+    assert_eq!(counter.get(), 100);
+
+    let r = it.advance_by(10);
+    assert_eq!(r, Err(0));
+    assert_eq!(it.len(), 0);
+    assert_eq!(counter.get(), 100);
+}
+
+#[test]
+fn array_intoiter_advance_back_by() {
+    use std::cell::Cell;
+    struct DropCounter<'a>(usize, &'a Cell<usize>);
+    impl Drop for DropCounter<'_> {
+        fn drop(&mut self) {
+            let x = self.1.get();
+            self.1.set(x + 1);
+        }
+    }
+
+    let counter = Cell::new(0);
+    let a: [_; 100] = std::array::from_fn(|i| DropCounter(i, &counter));
+    let mut it = IntoIterator::into_iter(a);
+
+    let r = it.advance_back_by(1);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 99);
+    assert_eq!(counter.get(), 1);
+
+    let r = it.advance_back_by(0);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 99);
+    assert_eq!(counter.get(), 1);
+
+    let r = it.advance_back_by(11);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 88);
+    assert_eq!(counter.get(), 12);
+
+    let x = it.next_back();
+    assert_eq!(x.as_ref().map(|x| x.0), Some(87));
+    assert_eq!(it.len(), 87);
+    assert_eq!(counter.get(), 12);
+    drop(x);
+    assert_eq!(counter.get(), 13);
+
+    let r = it.advance_back_by(123456);
+    assert_eq!(r, Err(87));
+    assert_eq!(it.len(), 0);
+    assert_eq!(counter.get(), 100);
+
+    let r = it.advance_back_by(0);
+    assert_eq!(r, Ok(()));
+    assert_eq!(it.len(), 0);
+    assert_eq!(counter.get(), 100);
+
+    let r = it.advance_back_by(10);
+    assert_eq!(r, Err(0));
+    assert_eq!(it.len(), 0);
+    assert_eq!(counter.get(), 100);
+}