From 9fb6730c80eb6ae0ad60708fb8a2ff9ef08d4e2b Mon Sep 17 00:00:00 2001
From: Ohad Ravid <ohad.rv@gmail.com>
Date: Thu, 21 Apr 2022 23:02:38 +0300
Subject: [PATCH] Make `Cow` covariant w.r.t `Owned`

---
 library/alloc/src/borrow.rs   |  4 ++--
 library/alloc/tests/borrow.rs | 11 +++++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/library/alloc/src/borrow.rs b/library/alloc/src/borrow.rs
index 8b13e36c4b3c7..a3e661172059a 100644
--- a/library/alloc/src/borrow.rs
+++ b/library/alloc/src/borrow.rs
@@ -178,7 +178,7 @@ where
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Cow")]
-pub enum Cow<'a, B: ?Sized + 'a>
+pub enum Cow<'a, B: ?Sized + 'a, O = <B as ToOwned>::Owned>
 where
     B: ToOwned,
 {
@@ -188,7 +188,7 @@ where
 
     /// Owned data.
     #[stable(feature = "rust1", since = "1.0.0")]
-    Owned(#[stable(feature = "rust1", since = "1.0.0")] <B as ToOwned>::Owned),
+    Owned(#[stable(feature = "rust1", since = "1.0.0")] O),
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/library/alloc/tests/borrow.rs b/library/alloc/tests/borrow.rs
index 57976aa6cdfdf..75b5867496e15 100644
--- a/library/alloc/tests/borrow.rs
+++ b/library/alloc/tests/borrow.rs
@@ -58,3 +58,14 @@ fn cow_const() {
     const IS_OWNED: bool = COW.is_owned();
     assert!(!IS_OWNED);
 }
+
+#[allow(dead_code)]
+fn assert_covariance() {
+    fn cow<'new>(c: Cow<'static, str>) -> Cow<'new, str> {
+        c
+    }
+
+    fn cow_cow<'new>(c: Cow<'static, Cow<'static, str>>) -> Cow<'new, Cow<'new, str>> {
+        c
+    }
+}