Make SpecInPlaceCollect use TrustedRandomAccessNoCoerce

Author: steffahn

library/alloc/src/vec/source_iter_marker.rs

@@ -1,4 +1,4 @@
-use core::iter::{InPlaceIterable, SourceIter, TrustedRandomAccess};
+use core::iter::{InPlaceIterable, SourceIter, TrustedRandomAccessNoCoerce};
 use core::mem::{self, ManuallyDrop};
 use core::ptr::{self};
 
@@ -101,6 +101,8 @@ fn write_in_place_with_drop<T>(
 trait SpecInPlaceCollect<T, I>: Iterator<Item = T> {
     /// Collects an iterator (`self`) into the destination buffer (`dst`) and returns the number of items
     /// collected. `end` is the last writable element of the allocation and used for bounds checks.
+    // FIXME: Clarify safety conditions. Iterator must not be coerced to a subtype
+    // after this call due to potential use of [`TrustedRandomAccessNoCoerce`].
     fn collect_in_place(&mut self, dst: *mut T, end: *const T) -> usize;
 }
 
@@ -124,7 +126,7 @@ where
 
 impl<T, I> SpecInPlaceCollect<T, I> for I
 where
-    I: Iterator<Item = T> + TrustedRandomAccess,
+    I: Iterator<Item = T> + TrustedRandomAccessNoCoerce,
 {
     #[inline]
     fn collect_in_place(&mut self, dst_buf: *mut T, end: *const T) -> usize {

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

@@ -514,6 +514,9 @@ impl<A: Debug + TrustedRandomAccessNoCoerce, B: Debug + TrustedRandomAccessNoCoe
 ///   `self.__iterator_get_unchecked(idx)`, assuming that the required traits are implemented.
 /// * It must also be safe to drop `self` after calling `self.__iterator_get_unchecked(idx)`.
 /// * If `T` is a subtype of `Self`, then it must be safe to coerce `self` to `T`.
+//
+// FIXME: Clarify interaction with SourceIter/InPlaceIterable. Calling `SouceIter::as_inner`
+// after `__iterator_get_unchecked` is supposed to be allowed.
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 #[rustc_specialization_trait]