Use Result and rename to filter_map

The use of Result allows for making use of a reconstructed original value on failed
projections.

library/core/src/cell.rs

@@ -1261,34 +1261,38 @@ impl<'b, T: ?Sized> Ref<'b, T> {
         Ref { value: f(orig.value), borrow: orig.borrow }
     }
 
-    /// Makes a new `Ref` for an optional component of the borrowed data.
+    /// Makes a new `Ref` for an optional component of the borrowed data. The
+    /// original guard is returned as an `Err(..)` if the closure returns
+    /// `None`.
     ///
     /// The `RefCell` is already immutably borrowed, so this cannot fail.
     ///
     /// This is an associated function that needs to be used as
-    /// `Ref::try_map(...)`. A method would interfere with methods of the same
+    /// `Ref::filter_map(...)`. A method would interfere with methods of the same
     /// name on the contents of a `RefCell` used through `Deref`.
     ///
     /// # Examples
     ///
     /// ```
-    /// #![feature(cell_try_map)]
+    /// #![feature(cell_filter_map)]
     ///
     /// use std::cell::{RefCell, Ref};
     ///
     /// let c = RefCell::new(vec![1, 2, 3]);
     /// let b1: Ref<Vec<u32>> = c.borrow();
-    /// let b2: Option<Ref<u32>> = Ref::try_map(b1, |v| v.get(1));
-    /// assert_eq!(b2.as_deref(), Some(&2))
+    /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
+    /// assert_eq!(*b2.unwrap(), 2);
     /// ```
-    #[unstable(feature = "cell_try_map", reason = "recently added", issue = "none")]
+    #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "none")]
     #[inline]
-    pub fn try_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
+    pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
     where
         F: FnOnce(&T) -> Option<&U>,
     {
-        let value = f(orig.value)?;
-        Some(Ref { value, borrow: orig.borrow })
+        match f(orig.value) {
+            Some(value) => Ok(Ref { value, borrow: orig.borrow }),
+            None => Err(orig),
+        }
     }
 
     /// Splits a `Ref` into multiple `Ref`s for different components of the
@@ -1402,44 +1406,56 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
         RefMut { value: f(value), borrow }
     }
 
-    /// Makes a new `RefMut` for an optional component of the borrowed data.
+    /// Makes a new `RefMut` for an optional component of the borrowed data. The
+    /// original guard is returned as an `Err(..)` if the closure returns
+    /// `None`.
     ///
     /// The `RefCell` is already mutably borrowed, so this cannot fail.
     ///
     /// This is an associated function that needs to be used as
-    /// `RefMut::try_map(...)`. A method would interfere with methods of the
+    /// `RefMut::filter_map(...)`. A method would interfere with methods of the
     /// same name on the contents of a `RefCell` used through `Deref`.
     ///
     /// # Examples
     ///
     /// ```
-    /// #![feature(cell_try_map)]
+    /// #![feature(cell_filter_map)]
     ///
     /// use std::cell::{RefCell, RefMut};
     ///
     /// let c = RefCell::new(vec![1, 2, 3]);
     ///
     /// {
     ///     let b1: RefMut<Vec<u32>> = c.borrow_mut();
-    ///     let mut b2: Option<RefMut<u32>> = RefMut::try_map(b1, |v| v.get_mut(1));
+    ///     let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
     ///
-    ///     if let Some(mut b2) = b2 {
+    ///     if let Ok(mut b2) = b2 {
     ///         *b2 += 2;
     ///     }
     /// }
     ///
     /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
     /// ```
-    #[unstable(feature = "cell_try_map", reason = "recently added", issue = "none")]
+    #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "none")]
     #[inline]
-    pub fn try_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
+    pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
     where
         F: FnOnce(&mut T) -> Option<&mut U>,
     {
         // FIXME(nll-rfc#40): fix borrow-check
         let RefMut { value, borrow } = orig;
-        let value = f(value)?;
-        Some(RefMut { value, borrow })
+        let value = value as *mut T;
+        // SAFETY: function holds onto an exclusive reference for the duration
+        // of its call through `orig`, and the pointer is only de-referenced
+        // inside of the function call never allowing the exclusive reference to
+        // escape.
+        match f(unsafe { &mut *value }) {
+            Some(value) => Ok(RefMut { value, borrow }),
+            None => {
+                // SAFETY: same as above.
+                Err(RefMut { value: unsafe { &mut *value }, borrow })
+            }
+        }
     }
 
     /// Splits a `RefMut` into multiple `RefMut`s for different components of the