Implement flatten for Option<&Option<T>> and Option<&mut Option<T>>

library/core/src/option.rs

@@ -2606,3 +2606,94 @@ impl<T> Option<Option<T>> {
         }
     }
 }
+
+impl<'a, T> Option<&'a Option<T>> {
+    /// Converts from `Option<&Option<T>>` to `Option<&T>`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(option_reference_flattening)]
+    ///
+    /// let x: Option<&Option<u32>> = Some(&Some(6));
+    /// assert_eq!(&Some(6), x.flatten_ref());
+    ///
+    /// let x: Option<&Option<u32>> = Some(&None);
+    /// assert_eq!(&None, x.flatten_ref());
+    ///
+    /// let x: Option<&Option<u32>> = None;
+    /// assert_eq!(&None, x.flatten_ref());
+    /// ```
+    #[inline]
+    #[unstable(feature = "option_reference_flattening", issue = "none")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn flatten_ref(self) -> &'a Option<T> {
+        match self {
+            Some(inner) => inner,
+            None => const { &None },
+        }
+    }
+}
+
+impl<'a, T> Option<&'a mut Option<T>> {
+    /// Converts from `Option<&mut Option<T>>` to `&Option<T>`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(option_reference_flattening)]
+    ///
+    /// let y = &mut Some(6);
+    /// let x: Option<&mut Option<u32>> = Some(y);
+    /// assert_eq!(&Some(6), x.flatten_ref());
+    ///
+    /// let y: &mut Option<u32> = &mut None;
+    /// let x: Option<&mut Option<u32>> = Some(y);
+    /// assert_eq!(&None, x.flatten_ref());
+    ///
+    /// let x: Option<&mut Option<u32>> = None;
+    /// assert_eq!(&None, x.flatten_ref());
+    /// ```
+    #[unstable(feature = "option_reference_flattening", issue = "none")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn flatten_ref(self) -> &'a Option<T> {
+        match self {
+            Some(inner) => inner,
+            None => const { &None },
+        }
+    }
+
+    /// Converts from `Option<&mut Option<T>>` to `Option<&mut T>`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(option_reference_flattening)]
+    ///
+    /// let y: &mut Option<u32> = &mut Some(6);
+    /// let x: Option<&mut Option<u32>> = Some(y);
+    /// assert_eq!(Some(&mut 6), x.flatten_mut());
+    ///
+    /// let y: &mut Option<u32> = &mut None;
+    /// let x: Option<&mut Option<u32>> = Some(y);
+    /// assert_eq!(None, x.flatten_mut());
+    ///
+    /// let x: Option<&mut Option<u32>> = None;
+    /// assert_eq!(None, x.flatten_mut());
+    /// ```
+    #[inline]
+    #[unstable(feature = "option_reference_flattening", issue = "none")]
+    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    pub const fn flatten_mut(self) -> Option<&'a mut T> {
+        match self {
+            Some(inner) => inner.as_mut(),
+            None => None,
+        }
+    }
+}