Add TrustedRandomAccessNoCoerce supertrait without requirements or guarantees about subtype coercions

Update all the TrustedRandomAccess impls to also implement the new supertrait

Author: steffahn

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

@@ -1,5 +1,5 @@
 use core::fmt;
-use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess};
+use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
 use core::ops::Try;
 
 use super::{count, wrap_index, RingSlices};
@@ -177,6 +177,10 @@ unsafe impl<T> TrustedLen for Iter<'_, T> {}
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<T> TrustedRandomAccess for Iter<'_, T> {
+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;
 }

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

@@ -1,5 +1,5 @@
 use core::fmt;
-use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess};
+use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
 use core::marker::PhantomData;
 
 use super::{count, wrap_index, RingSlices};
@@ -146,6 +146,10 @@ unsafe impl<T> TrustedLen for IterMut<'_, T> {}
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<T> TrustedRandomAccess for IterMut<'_, T> {
+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;
 }

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

@@ -1,4 +1,6 @@
-use crate::iter::adapters::{zip::try_get_unchecked, TrustedRandomAccess};
+use crate::iter::adapters::{
+    zip::try_get_unchecked, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
+};
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::ops::Try;
 
@@ -121,9 +123,13 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccess for Cloned<I>
+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>
 where
-    I: TrustedRandomAccess,
+    I: TrustedRandomAccessNoCoerce,
 {
     const MAY_HAVE_SIDE_EFFECT: bool = true;
 }

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

@@ -1,4 +1,6 @@
-use crate::iter::adapters::{zip::try_get_unchecked, TrustedRandomAccess};
+use crate::iter::adapters::{
+    zip::try_get_unchecked, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
+};
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::ops::Try;
 
@@ -137,9 +139,13 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccess for Copied<I>
+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>
 where
-    I: TrustedRandomAccess,
+    I: TrustedRandomAccessNoCoerce,
 {
     const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
 }

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

@@ -1,4 +1,6 @@
-use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
+use crate::iter::adapters::{
+    zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
+};
 use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
 use crate::ops::Try;
 
@@ -207,9 +209,13 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I> TrustedRandomAccess for Enumerate<I>
+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>
 where
-    I: TrustedRandomAccess,
+    I: TrustedRandomAccessNoCoerce,
 {
     const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
 }

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

@@ -2,6 +2,7 @@ use crate::intrinsics;
 use crate::iter::adapters::{zip::try_get_unchecked, InPlaceIterable, SourceIter};
 use crate::iter::{
     DoubleEndedIterator, ExactSizeIterator, FusedIterator, TrustedLen, TrustedRandomAccess,
+    TrustedRandomAccessNoCoerce,
 };
 use crate::ops::Try;
 
@@ -198,9 +199,13 @@ 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>
+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>
 where
-    I: TrustedRandomAccess,
+    I: TrustedRandomAccessNoCoerce,
 {
     const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
 }

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

@@ -1,5 +1,7 @@
 use crate::fmt;
-use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
+use crate::iter::adapters::{
+    zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
+};
 use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
 use crate::ops::Try;
 
@@ -187,9 +189,13 @@ where
 
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
-unsafe impl<I, F> TrustedRandomAccess for Map<I, F>
+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>
 where
-    I: TrustedRandomAccess,
+    I: TrustedRandomAccessNoCoerce,
 {
     const MAY_HAVE_SIDE_EFFECT: bool = true;
 }

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

@@ -51,6 +51,9 @@ 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;
+
 #[unstable(feature = "iter_zip", issue = "83574")]
 pub use self::zip::zip;
 

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

@@ -354,6 +354,15 @@ 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 MAY_HAVE_SIDE_EFFECT: bool = A::MAY_HAVE_SIDE_EFFECT || B::MAY_HAVE_SIDE_EFFECT;
 }
@@ -431,7 +440,7 @@ impl<A: Debug + TrustedRandomAccess, B: Debug + TrustedRandomAccess> ZipFmt<A, B
 ///
 /// The iterator's `size_hint` must be exact and cheap to call.
 ///
-/// `size` may not be overridden.
+/// `TrustedRandomAccessNoCoerce::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
@@ -455,7 +464,7 @@ impl<A: Debug + TrustedRandomAccess, B: Debug + TrustedRandomAccess> ZipFmt<A, B
 ///     * `std::iter::Iterator::size_hint`
 ///     * `std::iter::DoubleEndedIterator::next_back`
 ///     * `std::iter::Iterator::__iterator_get_unchecked`
-///     * `std::iter::TrustedRandomAccess::size`
+///     * `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
@@ -474,7 +483,15 @@ impl<A: Debug + TrustedRandomAccess, B: Debug + TrustedRandomAccess> ZipFmt<A, B
 #[doc(hidden)]
 #[unstable(feature = "trusted_random_access", issue = "none")]
 #[rustc_specialization_trait]
-pub unsafe trait TrustedRandomAccess: Sized {
+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`].
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+#[rustc_specialization_trait]
+pub unsafe trait TrustedRandomAccessNoCoerce: Sized {
     // Convenience method.
     fn size(&self) -> usize
     where

library/core/src/iter/mod.rs

@@ -407,6 +407,8 @@ pub use self::adapters::SourceIter;
 pub use self::adapters::StepBy;
 #[unstable(feature = "trusted_random_access", issue = "none")]
 pub use self::adapters::TrustedRandomAccess;
+#[unstable(feature = "trusted_random_access", issue = "none")]
+pub use self::adapters::TrustedRandomAccessNoCoerce;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::adapters::{
     Chain, Cycle, Enumerate, Filter, FilterMap, FlatMap, Fuse, Inspect, Map, Peekable, Rev, Scan,