Allow canonicalizing the array::map loop in trusted cases

library/core/src/iter/adapters/array_chunks.rs

@@ -1,6 +1,5 @@
 use crate::array;
 use crate::iter::{ByRefSized, FusedIterator, Iterator, TrustedRandomAccessNoCoerce};
-use crate::mem::{self, MaybeUninit};
 use crate::ops::{ControlFlow, NeverShortCircuit, Try};
 
 /// An iterator over `N` elements of the iterator at a time.
@@ -212,19 +211,14 @@ where
         let mut i = 0;
         // Use a while loop because (0..len).step_by(N) doesn't optimize well.
         while inner_len - i >= N {
-            let mut chunk = MaybeUninit::uninit_array();
-            let mut guard = array::Guard { array_mut: &mut chunk, initialized: 0 };
-            while guard.initialized < N {
+            let chunk = crate::array::from_fn(|local| {
                 // SAFETY: The method consumes the iterator and the loop condition ensures that
                 // all accesses are in bounds and only happen once.
                 unsafe {
-                    let idx = i + guard.initialized;
-                    guard.push_unchecked(self.iter.__iterator_get_unchecked(idx));
+                    let idx = i + local;
+                    self.iter.__iterator_get_unchecked(idx)
                 }
-            }
-            mem::forget(guard);
-            // SAFETY: The loop above initialized all elements
-            let chunk = unsafe { MaybeUninit::array_assume_init(chunk) };
+            });
             accum = f(accum, chunk);
             i += N;
         }

library/core/src/iter/adapters/cloned.rs

@@ -1,7 +1,7 @@
 use crate::iter::adapters::{
     zip::try_get_unchecked, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
 };
-use crate::iter::{FusedIterator, TrustedLen};
+use crate::iter::{FusedIterator, TrustedLen, UncheckedIterator};
 use crate::ops::Try;
 
 /// An iterator that clones the elements of an underlying iterator.
@@ -140,3 +140,16 @@ where
     T: Clone,
 {
 }
+
+impl<'a, I, T: 'a> UncheckedIterator for Cloned<I>
+where
+    I: UncheckedIterator<Item = &'a T>,
+    T: Clone,
+{
+    unsafe fn next_unchecked(&mut self) -> T {
+        // SAFETY: `Cloned` is 1:1 with the inner iterator, so if the caller promised
+        // that there's an element left, the inner iterator has one too.
+        let item = unsafe { self.it.next_unchecked() };
+        item.clone()
+    }
+}

library/core/src/iter/adapters/map.rs

@@ -2,7 +2,7 @@ use crate::fmt;
 use crate::iter::adapters::{
     zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
 };
-use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
+use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen, UncheckedIterator};
 use crate::ops::Try;
 
 /// An iterator that maps the values of `iter` with `f`.
@@ -187,6 +187,19 @@ where
 {
 }
 
+impl<B, I, F> UncheckedIterator for Map<I, F>
+where
+    I: UncheckedIterator,
+    F: FnMut(I::Item) -> B,
+{
+    unsafe fn next_unchecked(&mut self) -> B {
+        // SAFETY: `Map` is 1:1 with the inner iterator, so if the caller promised
+        // that there's an element left, the inner iterator has one too.
+        let item = unsafe { self.iter.next_unchecked() };
+        (self.f)(item)
+    }
+}
+
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<I, F> TrustedRandomAccess for Map<I, F> where I: TrustedRandomAccess {}

library/core/src/iter/adapters/zip.rs

@@ -1,7 +1,7 @@
 use crate::cmp;
 use crate::fmt::{self, Debug};
 use crate::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator};
-use crate::iter::{InPlaceIterable, SourceIter, TrustedLen};
+use crate::iter::{InPlaceIterable, SourceIter, TrustedLen, UncheckedIterator};
 
 /// An iterator that iterates two other iterators simultaneously.
 ///
@@ -417,6 +417,13 @@ where
 {
 }
 
+impl<A, B> UncheckedIterator for Zip<A, B>
+where
+    A: UncheckedIterator,
+    B: UncheckedIterator,
+{
+}
+
 // Arbitrarily selects the left side of the zip iteration as extractable "source"
 // it would require negative trait bounds to be able to try both
 #[unstable(issue = "none", feature = "inplace_iteration")]

library/core/src/iter/mod.rs

@@ -450,6 +450,7 @@ pub use self::adapters::{
 pub use self::adapters::{Intersperse, IntersperseWith};
 
 pub(crate) use self::adapters::try_process;
+pub(crate) use self::traits::UncheckedIterator;
 
 mod adapters;
 mod range;

library/core/src/iter/traits/mod.rs

@@ -4,6 +4,7 @@ mod double_ended;
 mod exact_size;
 mod iterator;
 mod marker;
+mod unchecked_iterator;
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::{
@@ -19,3 +20,5 @@ pub use self::{
 pub use self::marker::InPlaceIterable;
 #[unstable(feature = "trusted_step", issue = "85731")]
 pub use self::marker::TrustedStep;
+
+pub(crate) use self::unchecked_iterator::UncheckedIterator;

library/core/src/iter/traits/unchecked_iterator.rs

@@ -0,0 +1,36 @@
+use crate::iter::TrustedLen;
+
+/// [`TrustedLen`] cannot have methods, so this allows augmenting it.
+///
+/// It currently requires `TrustedLen` because it's unclear whether it's
+/// reasonably possible to depend on the `size_hint` of anything else.
+pub(crate) trait UncheckedIterator: TrustedLen {
+    /// Gets the next item from a non-empty iterator.
+    ///
+    /// Because there's always a value to return, that means it can return
+    /// the `Item` type directly, without wrapping it in an `Option`.
+    ///
+    /// # Safety
+    ///
+    /// This can only be called if `size_hint().0 != 0`, guaranteeing that
+    /// there's at least one item available.
+    ///
+    /// Otherwise (aka when `size_hint().1 == Some(0)`), this is UB.
+    ///
+    /// # Note to Implementers
+    ///
+    /// This has a default implementation using [`Option::unwrap_unchecked`].
+    /// That's probably sufficient if your `next` *always* returns `Some`,
+    /// such as for infinite iterators.  In more complicated situations, however,
+    /// sometimes there can still be `insertvalue`/`assume`/`extractvalue`
+    /// instructions remaining in the IR from the `Option` handling, at which
+    /// point you might want to implement this manually instead.
+    #[unstable(feature = "trusted_len_next_unchecked", issue = "37572")]
+    #[inline]
+    unsafe fn next_unchecked(&mut self) -> Self::Item {
+        let opt = self.next();
+        // SAFETY: The caller promised that we're not empty, and
+        // `Self: TrustedLen` so we can actually trust the `size_hint`.
+        unsafe { opt.unwrap_unchecked() }
+    }
+}