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

compiler/rustc_feature/src/active.rs

@@ -478,6 +478,10 @@ declare_features! (
  (active, object_safe_for_dispatch, "1.40.0", Some(43561), None),
  /// Allows using `#[optimize(X)]`.
  (active, optimize_attribute, "1.34.0", Some(54882), None),
+ /// Allows using `Option<&Option<T>>::flatten_ref()`,
+ /// Option<&mut Option<T>>::flatten_ref()` and
+ /// Option<&mut Option<T>>::flatten_mut()`
+ (active, option_reference_flattening, "CURRENT_RUSTC_VERSION", None, None),
  /// Allows `extern "platform-intrinsic" { ... }`.
  (active, platform_intrinsics, "1.4.0", Some(27731), None),
  /// Allows using `#![plugin(myplugin)]`.

compiler/rustc_span/src/symbol.rs

@@ -1043,6 +1043,7 @@ symbols! {
  optin_builtin_traits,
  option,
  option_env,
+ option_reference_flattening,
  options,
  or,
  or_patterns,

library/core/src/option.rs

@@ -2606,3 +2606,92 @@ 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 x: Option<&mut Option<u32>> = Some(&mut Some(6));
+ /// assert_eq!(&Some(6), x.flatten_ref());
+ ///
+ /// let x: Option<&mut Option<u32>> = Some(&mut None);
+ /// 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,
+ }
+ }
+}