Add Cow::is_borrowed and Cow::is_owned

src/liballoc/borrow.rs

@@ -207,6 +207,47 @@ fn clone_from(&mut self, source: &Self) {
 }
 
 impl<B: ?Sized + ToOwned> Cow<'_, B> {
+    /// Returns true if the data is borrowed, i.e. if `to_mut` would require additional work.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(cow_is_borrowed)]
+    /// use std::borrow::Cow;
+    ///
+    /// let cow = Cow::Borrowed("moo");
+    /// assert!(cow.is_borrowed());
+    ///
+    /// let bull: Cow<'_, str> = Cow::Owned("...moo?".to_string());
+    /// assert!(!bull.is_borrowed());
+    /// ```
+    #[unstable(feature = "cow_is_borrowed", issue = "65143")]
+    pub fn is_borrowed(&self) -> bool {
+        match *self {
+            Borrowed(_) => true,
+            Owned(_) => false,
+        }
+    }
+
+    /// Returns true if the data is owned, i.e. if `to_mut` would be a no-op.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(cow_is_borrowed)]
+    /// use std::borrow::Cow;
+    ///
+    /// let cow: Cow<'_, str> = Cow::Owned("moo".to_string());
+    /// assert!(cow.is_owned());
+    ///
+    /// let bull = Cow::Borrowed("...moo?");
+    /// assert!(!bull.is_owned());
+    /// ```
+    #[unstable(feature = "cow_is_borrowed", issue = "65143")]
+    pub fn is_owned(&self) -> bool {
+        !self.is_borrowed()
+    }
+
     /// Acquires a mutable reference to the owned form of the data.
     ///
     /// Clones the data if it is not already owned.

src/liballoc/lib.rs

@@ -85,6 +85,7 @@
 #![feature(const_generic_impls_guard)]
 #![feature(const_generics)]
 #![feature(const_in_array_repeat_expressions)]
+#![feature(cow_is_borrowed)]
 #![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
 #![feature(container_error_extra)]