Rollup merge of #51730 - MajorBreakfast:pin-get-mut-unchecked, r=withoutboats

New safe associated functions for PinMut

- Add safe `get_mut` and `map`
- Rename unsafe equivalents to `get_mut_unchecked` and `map_unchecked`

The discussion about this starts [in this comment](https://github.com/rust-lang/rust/issues/49150#issuecomment-399604573) on the tracking issue.

src/libcore/mem.rs

@@ -1119,6 +1119,12 @@ impl<'a, T: ?Sized + Unpin> PinMut<'a, T> {
     pub fn new(reference: &'a mut T) -> PinMut<'a, T> {
         PinMut { inner: reference }
     }
+
+    /// Get a mutable reference to the data inside of this `PinMut`.
+    #[unstable(feature = "pin", issue = "49150")]
+    pub fn get_mut(this: PinMut<'a, T>) -> &'a mut T {
+        this.inner
+    }
 }
 
 
@@ -1150,21 +1156,21 @@ impl<'a, T: ?Sized> PinMut<'a, T> {
     /// the data out of the mutable reference you receive when you call this
     /// function.
     #[unstable(feature = "pin", issue = "49150")]
-    pub unsafe fn get_mut(this: PinMut<'a, T>) -> &'a mut T {
+    pub unsafe fn get_mut_unchecked(this: PinMut<'a, T>) -> &'a mut T {
         this.inner
     }
 
     /// Construct a new pin by mapping the interior value.
     ///
-    /// For example, if you  wanted to get a `PinMut` of a field of something, you
-    /// could use this to get access to that field in one line of code.
+    /// For example, if you  wanted to get a `PinMut` of a field of something,
+    /// you could use this to get access to that field in one line of code.
     ///
     /// This function is unsafe. You must guarantee that the data you return
     /// will not move so long as the argument value does not move (for example,
     /// because it is one of the fields of that value), and also that you do
     /// not move out of the argument you receive to the interior function.
     #[unstable(feature = "pin", issue = "49150")]
-    pub unsafe fn map<U, F>(this: PinMut<'a, T>, f: F) -> PinMut<'a, U> where
+    pub unsafe fn map_unchecked<U, F>(this: PinMut<'a, T>, f: F) -> PinMut<'a, U> where
         F: FnOnce(&mut T) -> &mut U
     {
         PinMut { inner: f(this.inner) }

src/libcore/option.rs

@@ -275,7 +275,7 @@ impl<T> Option<T> {
     #[unstable(feature = "pin", issue = "49150")]
     pub fn as_pin_mut<'a>(self: PinMut<'a, Self>) -> Option<PinMut<'a, T>> {
         unsafe {
-            PinMut::get_mut(self).as_mut().map(|x| PinMut::new_unchecked(x))
+            PinMut::get_mut_unchecked(self).as_mut().map(|x| PinMut::new_unchecked(x))
         }
     }
 

src/libstd/future.rs

@@ -43,7 +43,7 @@ impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {}
 impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
     type Output = T::Return;
     fn poll(self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
-        set_task_cx(cx, || match unsafe { PinMut::get_mut(self).0.resume() } {
+        set_task_cx(cx, || match unsafe { PinMut::get_mut_unchecked(self).0.resume() } {
             GeneratorState::Yielded(()) => Poll::Pending,
             GeneratorState::Complete(x) => Poll::Ready(x),
         })

src/libstd/panic.rs

@@ -327,14 +327,9 @@ impl<T: fmt::Debug> fmt::Debug for AssertUnwindSafe<T> {
 impl<'a, F: Future> Future for AssertUnwindSafe<F> {
     type Output = F::Output;
 
-    fn poll(mut self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
-        unsafe {
-            let pinned_field = PinMut::new_unchecked(
-                &mut PinMut::get_mut(self.reborrow()).0
-            );
-
-            pinned_field.poll(cx)
-        }
+    fn poll(self: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
+        let pinned_field = unsafe { PinMut::map_unchecked(self, |x| &mut x.0) };
+        pinned_field.poll(cx)
     }
 }