remove TrustedRandomAccess vs. TrustedRandomAccessNoCoerce distinction

TRA is no longer used for single-step (`next()`) specializations, only loops and after each
loop we bring the iterator back into a safe state, meaning coercions are now safe because
the user code will not have access to an iterator instance that would be unsafe to coerce.

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

@@ -1,5 +1,5 @@
 use core::fmt;
-use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
+use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess};
 use core::mem::MaybeUninit;
 use core::ops::Try;
 
@@ -205,11 +205,7 @@ unsafe impl<T> TrustedLen for Iter<'_, T> {}
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<T> TrustedRandomAccess for Iter<'_, T> {}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<T> TrustedRandomAccessNoCoerce for Iter<'_, T> {
+unsafe impl<T> TrustedRandomAccess for Iter<'_, T> {
     const NEEDS_CLEANUP: bool = false;
 
     fn cleanup(&mut self, num: usize, forward: bool) {

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

@@ -1,5 +1,5 @@
 use core::fmt;
-use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
+use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess};
 use core::marker::PhantomData;
 
 use super::{count, wrap_index, RingSlices};
@@ -154,11 +154,7 @@ unsafe impl<T> TrustedLen for IterMut<'_, T> {}
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<T> TrustedRandomAccess for IterMut<'_, T> {}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<T> TrustedRandomAccessNoCoerce for IterMut<'_, T> {
+unsafe impl<T> TrustedRandomAccess for IterMut<'_, T> {
     const NEEDS_CLEANUP: bool = false;
 
     fn cleanup(&mut self, num: usize, forward: bool) {

library/alloc/src/vec/in_place_collect.rs

@@ -60,7 +60,7 @@
 //! # O(1) collect
 //!
 //! The main iteration itself is further specialized when the iterator implements
-//! [`TrustedRandomAccessNoCoerce`] to let the optimizer see that it is a counted loop with a single
+//! [`TrustedRandomAccess`] to let the optimizer see that it is a counted loop with a single
 //! [induction variable]. This can turn some iterators into a noop, i.e. it reduces them from O(n) to
 //! O(1). This particular optimization is quite fickle and doesn't always work, see [#79308]
 //!
@@ -70,7 +70,7 @@
 //! Since unchecked accesses through that trait do not advance the read pointer of `IntoIter`
 //! this would interact unsoundly with the requirements about dropping the tail described above.
 //! But since the normal `Drop` implementation of `IntoIter` would suffer from the same problem it
-//! is only correct for `TrustedRandomAccessNoCoerce` to be implemented when the items don't
+//! is only correct for `TrustedRandomAccess` to be implemented when the items don't
 //! have a destructor. Thus that implicit requirement also makes the specialization safe to use for
 //! in-place collection.
 //! Note that this safety concern is about the correctness of `impl Drop for IntoIter`,
@@ -134,7 +134,7 @@
 //! }
 //! vec.truncate(write_idx);
 //! ```
-use core::iter::{InPlaceIterable, SourceIter, TrustedRandomAccessNoCoerce};
+use core::iter::{InPlaceIterable, SourceIter, TrustedRandomAccess};
 use core::mem::{self, ManuallyDrop};
 use core::ptr::{self};
 
@@ -195,7 +195,7 @@ where
         // itself once IntoIter goes out of scope.
         // If the drop panics then we also leak any elements collected into dst_buf.
         //
-        // Note: This access to the source wouldn't be allowed by the TrustedRandomIteratorNoCoerce
+        // Note: This access to the source wouldn't be allowed by the TrustedRandomIterator
         // contract (used by SpecInPlaceCollect below). But see the "O(1) collect" section in the
         // module documenttation why this is ok anyway.
         src.forget_allocation_drop_remaining();
@@ -230,7 +230,7 @@ trait SpecInPlaceCollect<T, I>: Iterator<Item = T> {
     /// collected. `end` is the last writable element of the allocation and used for bounds checks.
     ///
     /// This method is specialized and one of its implementations makes use of
-    /// `Iterator::__iterator_get_unchecked` calls with a `TrustedRandomAccessNoCoerce` bound
+    /// `Iterator::__iterator_get_unchecked` calls with a `TrustedRandomAccess` bound
     /// on `I` which means the caller of this method must take the safety conditions
     /// of that trait into consideration.
     fn collect_in_place(&mut self, dst: *mut T, end: *const T) -> usize;
@@ -256,7 +256,7 @@ where
 
 impl<T, I> SpecInPlaceCollect<T, I> for I
 where
-    I: Iterator<Item = T> + TrustedRandomAccessNoCoerce,
+    I: Iterator<Item = T> + TrustedRandomAccess,
 {
     #[inline]
     fn collect_in_place(&mut self, dst_buf: *mut T, end: *const T) -> usize {

library/alloc/src/vec/into_iter.rs

@@ -4,9 +4,7 @@ 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, TrustedRandomAccessNoCoerce,
-};
+use core::iter::{FusedIterator, InPlaceIterable, SourceIter, TrustedLen, TrustedRandomAccess};
 use core::marker::PhantomData;
 use core::mem::{self, ManuallyDrop};
 use core::ops::Deref;
@@ -200,7 +198,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, i: usize) -> Self::Item
     where
-        Self: TrustedRandomAccessNoCoerce,
+        Self: TrustedRandomAccess,
     {
         // 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)`
@@ -284,9 +282,7 @@ impl<T: Copy> NonDrop for T {}
 
 #[doc(hidden)]
 #[unstable(issue = "none", feature = "std_internals")]
-// TrustedRandomAccess (without NoCoerce) must not be implemented because
-// subtypes/supertypes of `T` might not be `NonDrop`
-unsafe impl<T, A: Allocator> TrustedRandomAccessNoCoerce for IntoIter<T, A>
+unsafe impl<T, A: Allocator> TrustedRandomAccess for IntoIter<T, A>
 where
     T: NonDrop,
 {

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

@@ -1,6 +1,4 @@
-use crate::iter::adapters::{
-    zip::try_get_unchecked, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
-};
+use crate::iter::adapters::{zip::try_get_unchecked, TrustedRandomAccess};
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::ops::Try;
 
@@ -63,7 +61,7 @@ where
     #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> T
     where
-        Self: TrustedRandomAccessNoCoerce,
+        Self: TrustedRandomAccess,
     {
         // SAFETY: the caller must uphold the contract for
         // `Iterator::__iterator_get_unchecked`.
@@ -123,13 +121,9 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccess for Cloned<I> where I: TrustedRandomAccess {}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccessNoCoerce for Cloned<I>
+unsafe impl<I> TrustedRandomAccess for Cloned<I>
 where
-    I: TrustedRandomAccessNoCoerce,
+    I: TrustedRandomAccess,
 {
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 

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

@@ -1,6 +1,4 @@
-use crate::iter::adapters::{
-    zip::try_get_unchecked, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
-};
+use crate::iter::adapters::{zip::try_get_unchecked, TrustedRandomAccess};
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::ops::Try;
 
@@ -84,7 +82,7 @@ where
     #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> T
     where
-        Self: TrustedRandomAccessNoCoerce,
+        Self: TrustedRandomAccess,
     {
         // SAFETY: the caller must uphold the contract for
         // `Iterator::__iterator_get_unchecked`.
@@ -149,13 +147,9 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccess for Copied<I> where I: TrustedRandomAccess {}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccessNoCoerce for Copied<I>
+unsafe impl<I> TrustedRandomAccess for Copied<I>
 where
-    I: TrustedRandomAccessNoCoerce,
+    I: TrustedRandomAccess,
 {
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 

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

@@ -1,6 +1,4 @@
-use crate::iter::adapters::{
-    zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
-};
+use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
 use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
 use crate::ops::Try;
 
@@ -132,7 +130,7 @@ where
     #[inline]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
     where
-        Self: TrustedRandomAccessNoCoerce,
+        Self: TrustedRandomAccess,
     {
         // SAFETY: the caller must uphold the contract for
         // `Iterator::__iterator_get_unchecked`.
@@ -232,13 +230,9 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccess for Enumerate<I> where I: TrustedRandomAccess {}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccessNoCoerce for Enumerate<I>
+unsafe impl<I> TrustedRandomAccess for Enumerate<I>
 where
-    I: TrustedRandomAccessNoCoerce,
+    I: TrustedRandomAccess,
 {
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 

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

@@ -2,7 +2,6 @@ use crate::intrinsics;
 use crate::iter::adapters::zip::try_get_unchecked;
 use crate::iter::{
     DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedLen, TrustedRandomAccess,
-    TrustedRandomAccessNoCoerce,
 };
 use crate::ops::Try;
 
@@ -132,7 +131,7 @@ where
     #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
     where
-        Self: TrustedRandomAccessNoCoerce,
+        Self: TrustedRandomAccess,
     {
         match self.iter {
             // SAFETY: the caller must uphold the contract for
@@ -222,13 +221,9 @@ unsafe impl<I> TrustedLen for Fuse<I> where I: TrustedLen {}
 //
 // This is safe to implement as `Fuse` is just forwarding these to the wrapped iterator `I`, which
 // preserves these properties.
-unsafe impl<I> TrustedRandomAccess for Fuse<I> where I: TrustedRandomAccess {}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccessNoCoerce for Fuse<I>
+unsafe impl<I> TrustedRandomAccess for Fuse<I>
 where
-    I: TrustedRandomAccessNoCoerce,
+    I: TrustedRandomAccess,
 {
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 

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

@@ -1,7 +1,5 @@
 use crate::fmt;
-use crate::iter::adapters::{
-    zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
-};
+use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
 use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
 use crate::ops::Try;
 
@@ -128,7 +126,7 @@ where
     #[inline]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> B
     where
-        Self: TrustedRandomAccessNoCoerce,
+        Self: TrustedRandomAccess,
     {
         // SAFETY: the caller must uphold the contract for
         // `Iterator::__iterator_get_unchecked`.
@@ -190,13 +188,9 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I, F> TrustedRandomAccess for Map<I, F> where I: TrustedRandomAccess {}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I, F> TrustedRandomAccessNoCoerce for Map<I, F>
+unsafe impl<I, F> TrustedRandomAccess for Map<I, F>
 where
-    I: TrustedRandomAccessNoCoerce,
+    I: TrustedRandomAccess,
 {
     const NEEDS_CLEANUP: bool = I::NEEDS_CLEANUP;
 

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

@@ -55,9 +55,6 @@ pub use self::map_while::MapWhile;
 #[unstable(feature = "trusted_random_access", issue = "none")]
 pub use self::zip::TrustedRandomAccess;
 
-#[unstable(feature = "trusted_random_access", issue = "none")]
-pub use self::zip::TrustedRandomAccessNoCoerce;
-
 #[stable(feature = "iter_zip", since = "1.59.0")]
 pub use self::zip::zip;
 

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

@@ -92,7 +92,7 @@ where
     #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
     where
-        Self: TrustedRandomAccessNoCoerce,
+        Self: TrustedRandomAccess,
     {
         // SAFETY: the caller must uphold the contract for
         // `Iterator::__iterator_get_unchecked`.
@@ -257,15 +257,6 @@ unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>
 where
     A: TrustedRandomAccess,
     B: TrustedRandomAccess,
-{
-}
-
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<A, B> TrustedRandomAccessNoCoerce for Zip<A, B>
-where
-    A: TrustedRandomAccessNoCoerce,
-    B: TrustedRandomAccessNoCoerce,
 {
     const NEEDS_CLEANUP: bool = A::NEEDS_CLEANUP || B::NEEDS_CLEANUP;
 
@@ -328,9 +319,7 @@ impl<A: Debug, B: Debug> ZipFmt<A, B> for Zip<A, B> {
     }
 }
 
-impl<A: Debug + TrustedRandomAccessNoCoerce, B: Debug + TrustedRandomAccessNoCoerce> ZipFmt<A, B>
-    for Zip<A, B>
-{
+impl<A: Debug + TrustedRandomAccess, B: Debug + TrustedRandomAccess> ZipFmt<A, B> for Zip<A, B> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // It's *not safe* to call fmt on the contained iterators, since once
         // we start iterating they're in strange, potentially unsafe, states.
@@ -344,7 +333,7 @@ impl<A: Debug + TrustedRandomAccessNoCoerce, B: Debug + TrustedRandomAccessNoCoe
 ///
 /// The iterator's `size_hint` must be exact and cheap to call.
 ///
-/// `TrustedRandomAccessNoCoerce::size` may not be overridden.
+/// `TrustedRandomAccess::size` may not be overridden.
 ///
 /// All subtypes and all supertypes of `Self` must also implement `TrustedRandomAccess`.
 /// In particular, this means that types with non-invariant parameters usually can not have
@@ -372,42 +361,21 @@ impl<A: Debug + TrustedRandomAccessNoCoerce, B: Debug + TrustedRandomAccessNoCoe
 ///     * `std::iter::DoubleEndedIterator::next_back`
 ///     * `std::iter::ExactSizeIterator::len`
 ///     * `std::iter::Iterator::__iterator_get_unchecked`
-///     * `std::iter::TrustedRandomAccessNoCoerce::size`
-/// 5. If `T` is a subtype of `Self`, then `self` is allowed to be coerced
-///    to `T`. If `self` is coerced to `T` after `self.__iterator_get_unchecked(idx)` has already
-///    been called, then no methods except for the ones listed under 4. are allowed to be called
-///    on the resulting value of type `T`, either. Multiple such coercion steps are allowed.
-///    Regarding 2. and 3., the number of times `__iterator_get_unchecked(idx)` or `next_back()` is
-///    called on `self` and the resulting value of type `T` (and on further coercion results with
-///    sub-subtypes) are added together and their sums must not exceed the specified bounds.
+///     * `std::iter::TrustedRandomAccess::size`
 ///
 /// Further, given that these conditions are met, it must guarantee that:
 ///
 /// * It does not change the value returned from `size_hint`
 /// * It must be safe to call the methods listed above on `self` after calling
 ///   `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 `SourceIter::as_inner`
 // after `__iterator_get_unchecked` is supposed to be allowed.
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 #[rustc_specialization_trait]
-pub unsafe trait TrustedRandomAccess: TrustedRandomAccessNoCoerce {}
-
-/// Like [`TrustedRandomAccess`] but without any of the requirements / guarantees around
-/// coercions to subtypes after `__iterator_get_unchecked` (they aren’t allowed here!), and
-/// without the requirement that subtypes / supertypes implement `TrustedRandomAccessNoCoerce`.
-///
-/// This trait was created in PR #85874 to fix soundness issue #85873 without performance regressions.
-/// It is subject to change as we might want to build a more generally useful (for performance
-/// optimizations) and more sophisticated trait or trait hierarchy that replaces or extends
-/// [`TrustedRandomAccess`] and `TrustedRandomAccessNoCoerce`.
-#[doc(hidden)]
-#[unstable(feature = "trusted_random_access", issue = "none")]
-#[rustc_specialization_trait]
-pub unsafe trait TrustedRandomAccessNoCoerce: Sized {
+pub unsafe trait TrustedRandomAccess: Sized {
     // Convenience method.
     fn size(&self) -> usize
     where
@@ -450,7 +418,7 @@ unsafe impl<I: Iterator> SpecTrustedRandomAccess for I {
     }
 }
 
-unsafe impl<I: Iterator + TrustedRandomAccessNoCoerce> SpecTrustedRandomAccess for I {
+unsafe impl<I: Iterator + TrustedRandomAccess> SpecTrustedRandomAccess for I {
     #[inline]
     unsafe fn try_get_unchecked(&mut self, index: usize) -> Self::Item {
         // SAFETY: the caller must uphold the contract for