Add back TrustedRandomAccess-specialization for Vec, but only without coercions

Author: steffahn

library/alloc/src/vec/into_iter.rs

@@ -2,7 +2,7 @@ use crate::alloc::{Allocator, Global};
 use crate::raw_vec::RawVec;
 use core::fmt;
 use core::intrinsics::arith_offset;
-use core::iter::{FusedIterator, InPlaceIterable, SourceIter, TrustedLen};
+use core::iter::{FusedIterator, InPlaceIterable, SourceIter, TrustedLen, TrustedRandomAccessNoCoerce};
 use core::marker::PhantomData;
 use core::mem::{self};
 use core::ptr::{self, NonNull};
@@ -162,6 +162,24 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     fn count(self) -> usize {
         self.len()
     }
+
+    #[doc(hidden)]
+    unsafe fn __iterator_get_unchecked(&mut self, i: usize) -> Self::Item
+    where
+        Self: TrustedRandomAccessNoCoerce,
+    {
+        // SAFETY: the caller must guarantee that `i` is in bounds of the
+        // `Vec<T>`, so `i` cannot overflow an `isize`, and the `self.ptr.add(i)`
+        // is guaranteed to pointer to an element of the `Vec<T>` and
+        // thus guaranteed to be valid to dereference.
+        //
+        // Also note the implementation of `Self: TrustedRandomAccess` requires
+        // that `T: Copy` so reading elements from the buffer doesn't invalidate
+        // them for `Drop`.
+        unsafe {
+            if mem::size_of::<T>() == 0 { mem::zeroed() } else { ptr::read(self.ptr.add(i)) }
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -197,6 +215,20 @@ impl<T, A: Allocator> FusedIterator for IntoIter<T, A> {}
 #[unstable(feature = "trusted_len", issue = "37572")]
 unsafe impl<T, A: Allocator> TrustedLen for IntoIter<T, A> {}
 
+#[doc(hidden)]
+#[unstable(issue = "none", feature = "std_internals")]
+// T: Copy as approximation for !Drop since get_unchecked does not advance self.ptr
+// and thus we can't implement drop-handling
+//
+// TrustedRandomAccess (without NoCoerce) must not be implemented because
+// subtypes/supertypes of `T` might not be `Copy`
+unsafe impl<T, A: Allocator> TrustedRandomAccessNoCoerce for IntoIter<T, A>
+where
+    T: Copy,
+{
+    const MAY_HAVE_SIDE_EFFECT: bool = false;
+}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "vec_into_iter_clone", since = "1.8.0")]
 impl<T: Clone, A: Allocator + Clone> Clone for IntoIter<T, A> {