Rollup merge of #123480 - Nadrieril:impl-all-derefpures, r=compiler-errors

deref patterns: impl `DerefPure` for more std types

Context: [deref patterns](https://github.com/rust-lang/rust/issues/87121). The requirements of `DerefPure` aren't precise yet, but these types unambiguously satisfy them.

Interestingly, a hypothetical `impl DerefMut for Cow` that does a `Clone` would *not* be eligible for `DerefPure` if we allow mixing deref patterns with normal patterns. If the following is exhaustive then the `DerefMut` would cause UB:
```rust
match &mut Cow::Borrowed(&()) {
    Cow::Owned(_) => ..., // Doesn't match
    deref!(_x) if false => ..., // Causes the variant to switch to `Owned`
    Cow::Borrowed(_) => ..., // Doesn't match
    // We reach unreachable
}
```

4 files changed, 19 insertions, 4 deletions

diff --git a/library/alloc/src/borrow.rs b/library/alloc/src/borrow.rs
index b6b6246baa6..42f8a08a9e4 100644
--- a/library/alloc/src/borrow.rs
+++ b/library/alloc/src/borrow.rs
@@ -4,9 +4,9 @@
 
 use core::cmp::Ordering;
 use core::hash::{Hash, Hasher};
-use core::ops::Deref;
 #[cfg(not(no_global_oom_handling))]
 use core::ops::{Add, AddAssign};
+use core::ops::{Deref, DerefPure};
 
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::borrow::{Borrow, BorrowMut};
@@ -343,6 +343,9 @@ where
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<B: ?Sized + ToOwned> DerefPure for Cow<'_, B> where B::Owned: Borrow<B> {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
 
diff --git a/library/core/src/cell.rs b/library/core/src/cell.rs
index ae0436bc520..4b491ffdafa 100644
--- a/library/core/src/cell.rs
+++ b/library/core/src/cell.rs
@@ -239,7 +239,7 @@ use crate::cmp::Ordering;
 use crate::fmt::{self, Debug, Display};
 use crate::marker::{PhantomData, Unsize};
 use crate::mem::{self, size_of};
-use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn};
+use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
 use crate::ptr::{self, NonNull};
 
 mod lazy;
@@ -1452,6 +1452,9 @@ impl<T: ?Sized> Deref for Ref<'_, T> {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> DerefPure for Ref<'_, T> {}
+
 impl<'b, T: ?Sized> Ref<'b, T> {
     /// Copies a `Ref`.
     ///
@@ -1844,6 +1847,9 @@ impl<T: ?Sized> DerefMut for RefMut<'_, T> {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> DerefPure for RefMut<'_, T> {}
+
 #[unstable(feature = "coerce_unsized", issue = "18598")]
 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
 
diff --git a/library/core/src/mem/manually_drop.rs b/library/core/src/mem/manually_drop.rs
index 98cff3493a7..955efb9b0f9 100644
--- a/library/core/src/mem/manually_drop.rs
+++ b/library/core/src/mem/manually_drop.rs
@@ -1,4 +1,4 @@
-use crate::ops::{Deref, DerefMut};
+use crate::ops::{Deref, DerefMut, DerefPure};
 use crate::ptr;
 
 /// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
@@ -161,3 +161,6 @@ impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
         &mut self.value
     }
 }
+
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> DerefPure for ManuallyDrop<T> {}
diff --git a/library/core/src/pin.rs b/library/core/src/pin.rs
index f7463d170fd..d8fc3b7177f 100644
--- a/library/core/src/pin.rs
+++ b/library/core/src/pin.rs
@@ -923,7 +923,7 @@
 use crate::cmp;
 use crate::fmt;
 use crate::hash::{Hash, Hasher};
-use crate::ops::{CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Receiver};
+use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, Receiver};
 
 #[allow(unused_imports)]
 use crate::{
@@ -1684,6 +1684,9 @@ impl<Ptr: DerefMut<Target: Unpin>> DerefMut for Pin<Ptr> {
     }
 }
 
+#[unstable(feature = "deref_pure_trait", issue = "87121")]
+unsafe impl<Ptr: DerefPure> DerefPure for Pin<Ptr> {}
+
 #[unstable(feature = "receiver_trait", issue = "none")]
 impl<Ptr: Receiver> Receiver for Pin<Ptr> {}