impl From<Cow> for Rc and Arc

These forward `Borrowed`/`Owned` values to existing `From` impls.

    impl<'a, B> From<Cow<'a, B>> for Rc<B>
    where
        B: ToOwned + ?Sized,
        Rc<B>: From<&'a B> + From<B::Owned>,

    impl<'a, B> From<Cow<'a, B>> for Arc<B>
    where
        B: ToOwned + ?Sized,
        Arc<B>: From<&'a B> + From<B::Owned>,

src/liballoc/rc.rs

@@ -252,6 +252,7 @@ use core::ptr::{self, NonNull};
 use core::slice::{self, from_raw_parts_mut};
 
 use crate::alloc::{box_free, handle_alloc_error, AllocInit, AllocRef, Global, Layout};
+use crate::borrow::{Cow, ToOwned};
 use crate::string::String;
 use crate::vec::Vec;
 
@@ -1492,6 +1493,21 @@ impl<T> From<Vec<T>> for Rc<[T]> {
     }
 }
 
+#[stable(feature = "shared_from_cow", since = "1.45.0")]
+impl<'a, B> From<Cow<'a, B>> for Rc<B>
+where
+    B: ToOwned + ?Sized,
+    Rc<B>: From<&'a B> + From<B::Owned>,
+{
+    #[inline]
+    fn from(cow: Cow<'a, B>) -> Rc<B> {
+        match cow {
+            Cow::Borrowed(s) => Rc::from(s),
+            Cow::Owned(s) => Rc::from(s),
+        }
+    }
+}
+
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
 impl<T, const N: usize> TryFrom<Rc<[T]>> for Rc<[T; N]>
 where

src/liballoc/sync.rs

@@ -25,6 +25,7 @@ use core::sync::atomic;
 use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
 
 use crate::alloc::{box_free, handle_alloc_error, AllocInit, AllocRef, Global, Layout};
+use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
 use crate::rc::is_dangling;
 use crate::string::String;
@@ -2047,6 +2048,21 @@ impl<T> From<Vec<T>> for Arc<[T]> {
     }
 }
 
+#[stable(feature = "shared_from_cow", since = "1.45.0")]
+impl<'a, B> From<Cow<'a, B>> for Arc<B>
+where
+    B: ToOwned + ?Sized,
+    Arc<B>: From<&'a B> + From<B::Owned>,
+{
+    #[inline]
+    fn from(cow: Cow<'a, B>) -> Arc<B> {
+        match cow {
+            Cow::Borrowed(s) => Arc::from(s),
+            Cow::Owned(s) => Arc::from(s),
+        }
+    }
+}
+
 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
 impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]>
 where