diff --git a/library/alloc/src/borrow.rs b/library/alloc/src/borrow.rs
index dbfd2e74abee6..ee810339df24a 100644
--- a/library/alloc/src/borrow.rs
+++ b/library/alloc/src/borrow.rs
@@ -341,7 +341,7 @@ where
 }
 
 #[unstable(feature = "deref_pure_trait", issue = "87121")]
-unsafe impl<B: ?Sized + ToOwned> DerefPure for Cow<'_, B> where B::Owned: Borrow<B> {}
+unsafe impl<B: ?Sized + ToOwned> DerefPure for Cow<'_, B> where B::Owned: Borrow<B> + BorrowPure<B> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<B: ?Sized> Eq for Cow<'_, B> where B: Eq + ToOwned {}
diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 1ea7b731461e7..3dfe263981db2 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -183,7 +183,7 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use core::borrow::{Borrow, BorrowMut};
+use core::borrow::{Borrow, BorrowMut, BorrowPure};
 #[cfg(not(no_global_oom_handling))]
 use core::clone::CloneToUninit;
 use core::cmp::Ordering;
@@ -2046,6 +2046,9 @@ impl<T: ?Sized, A: Allocator> BorrowMut<T> for Box<T, A> {
     }
 }
 
+#[unstable(feature = "borrow_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized, A: Allocator> BorrowPure<T> for Box<T, A> {}
+
 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
 impl<T: ?Sized, A: Allocator> AsRef<T> for Box<T, A> {
     fn as_ref(&self) -> &T {
diff --git a/library/alloc/src/str.rs b/library/alloc/src/str.rs
index 6fee8d3fe3346..1ffe43f8a00b3 100644
--- a/library/alloc/src/str.rs
+++ b/library/alloc/src/str.rs
@@ -7,7 +7,7 @@
 // It's cleaner to just turn off the unused_imports warning than to fix them.
 #![allow(unused_imports)]
 
-use core::borrow::{Borrow, BorrowMut};
+use core::borrow::{Borrow, BorrowMut, BorrowPure};
 use core::iter::FusedIterator;
 use core::mem::MaybeUninit;
 #[stable(feature = "encode_utf16", since = "1.8.0")]
@@ -201,6 +201,9 @@ impl BorrowMut<str> for String {
     }
 }
 
+#[unstable(feature = "borrow_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> BorrowPure<str> for String {}
+
 #[cfg(not(no_global_oom_handling))]
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ToOwned for str {
diff --git a/library/core/src/borrow.rs b/library/core/src/borrow.rs
index ccb1cc4e974d6..2d8baa3434f27 100644
--- a/library/core/src/borrow.rs
+++ b/library/core/src/borrow.rs
@@ -240,3 +240,21 @@ impl<T: ?Sized> BorrowMut<T> for &mut T {
         &mut **self
     }
 }
+
+/// Perma-unstable marker trait. Indicates that the type has a well-behaved [`Borrow<T>`]
+/// (and, if applicable, [`BorrowMut<T>`]) implementation. This is relied on for soundness
+/// of deref patterns of [`Cow`](alloc::borrow::Cow).
+///
+/// FIXME(deref_patterns): The precise semantics are undecided; the rough idea is that
+/// successive calls to `borrow`/`borrow_mut` without intermediate mutation should be
+/// idempotent, in the sense that they return the same value as far as pattern-matching
+/// is concerned. Calls to `borrow`/`borrow_mut` must leave the pointer itself likewise
+/// unchanged.
+#[unstable(feature = "borrow_pure_trait", issue = "87121")]
+pub unsafe trait BorrowPure<T: ?Sized> {}
+
+#[unstable(feature = "borrow_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> BorrowPure<T> for &T {}
+
+#[unstable(feature = "borrow_pure_trait", issue = "87121")]
+unsafe impl<T: ?Sized> BorrowPure<T> for &mut T {}