simplify iter::Zip

This removes all TrustedRandomAccess specialization from Zip, which means Zip::next no
longer gets optimized as well. But since for loop desugaring uses TRA we retain most of the benefits.

We're still losing optimizations on fold, try_fold and reverse iteration. But those can
be re-specialized more easily.

Author: the8472

library/alloc/src/collections/vec_deque/iter.rs

@@ -210,7 +210,6 @@ unsafe impl<T> TrustedRandomAccess for Iter<'_, T> {}
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<T> TrustedRandomAccessNoCoerce for Iter<'_, T> {
-    const MAY_HAVE_SIDE_EFFECT: bool = false;
     const NEEDS_CLEANUP: bool = false;
 
     fn cleanup(&mut self, num: usize, forward: bool) {

library/alloc/src/collections/vec_deque/iter_mut.rs

@@ -159,7 +159,6 @@ unsafe impl<T> TrustedRandomAccess for IterMut<'_, T> {}
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 unsafe impl<T> TrustedRandomAccessNoCoerce for IterMut<'_, T> {
-    const MAY_HAVE_SIDE_EFFECT: bool = false;
     const NEEDS_CLEANUP: bool = false;
 
     fn cleanup(&mut self, num: usize, forward: bool) {

library/alloc/src/vec/into_iter.rs

@@ -290,7 +290,6 @@ unsafe impl<T, A: Allocator> TrustedRandomAccessNoCoerce for IntoIter<T, A>
 where
     T: NonDrop,
 {
-    const MAY_HAVE_SIDE_EFFECT: bool = false;
     const NEEDS_CLEANUP: bool = true;
 
     fn cleanup(&mut self, num: usize, forward: bool) {

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

@@ -131,8 +131,6 @@ unsafe impl<I> TrustedRandomAccessNoCoerce for Cloned<I>
 where
     I: TrustedRandomAccessNoCoerce,
 {
-    const MAY_HAVE_SIDE_EFFECT: bool = true;
-
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 
     #[inline]

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

@@ -157,8 +157,6 @@ unsafe impl<I> TrustedRandomAccessNoCoerce for Copied<I>
 where
     I: TrustedRandomAccessNoCoerce,
 {
-    const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
-
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 
     #[inline]

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

@@ -240,8 +240,6 @@ unsafe impl<I> TrustedRandomAccessNoCoerce for Enumerate<I>
 where
     I: TrustedRandomAccessNoCoerce,
 {
-    const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
-
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 
     #[inline]

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

@@ -230,8 +230,6 @@ unsafe impl<I> TrustedRandomAccessNoCoerce for Fuse<I>
 where
     I: TrustedRandomAccessNoCoerce,
 {
-    const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
-
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 
     #[inline]

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

@@ -198,8 +198,6 @@ unsafe impl<I, F> TrustedRandomAccessNoCoerce for Map<I, F>
 where
     I: TrustedRandomAccessNoCoerce,
 {
-    const MAY_HAVE_SIDE_EFFECT: bool = true;
-
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 
     #[inline]