Make UnsafeCell::into_inner safe

This fixes #35067. It will require a Crater run as discussed in that
issue.

src/libcore/cell.rs

@@ -450,7 +450,7 @@ impl<T> Cell<T> {
     /// ```
     #[stable(feature = "move_cell", since = "1.17.0")]
     pub fn into_inner(self) -> T {
-        unsafe { self.value.into_inner() }
+        self.value.into_inner()
     }
 }
 
@@ -569,7 +569,7 @@ impl<T> RefCell<T> {
         // compiler statically verifies that it is not currently borrowed.
         // Therefore the following assertion is just a `debug_assert!`.
         debug_assert!(self.borrow.get() == UNUSED);
-        unsafe { self.value.into_inner() }
+        self.value.into_inner()
     }
 
     /// Replaces the wrapped value with a new one, returning the old value,
@@ -1220,23 +1220,18 @@ impl<T> UnsafeCell<T> {
 
     /// Unwraps the value.
     ///
-    /// # Safety
-    ///
-    /// This function is unsafe because this thread or another thread may currently be
-    /// inspecting the inner value.
-    ///
     /// # Examples
     ///
     /// ```
     /// use std::cell::UnsafeCell;
     ///
     /// let uc = UnsafeCell::new(5);
     ///
-    /// let five = unsafe { uc.into_inner() };
+    /// let five = uc.into_inner();
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub unsafe fn into_inner(self) -> T {
+    pub fn into_inner(self) -> T {
         self.value
     }
 }

src/libcore/sync/atomic.rs

@@ -285,7 +285,7 @@ impl AtomicBool {
     #[inline]
     #[stable(feature = "atomic_access", since = "1.15.0")]
     pub fn into_inner(self) -> bool {
-        unsafe { self.v.into_inner() != 0 }
+        self.v.into_inner() != 0
     }
 
     /// Loads a value from the bool.
@@ -695,7 +695,7 @@ impl<T> AtomicPtr<T> {
     #[inline]
     #[stable(feature = "atomic_access", since = "1.15.0")]
     pub fn into_inner(self) -> *mut T {
-        unsafe { self.p.into_inner() }
+        self.p.into_inner()
     }
 
     /// Loads a value from the pointer.
@@ -1050,7 +1050,7 @@ macro_rules! atomic_int {
             #[inline]
             #[$stable_access]
             pub fn into_inner(self) -> $int_type {
-                unsafe { self.v.into_inner() }
+                self.v.into_inner()
             }
 
             /// Loads a value from the atomic integer.