Rollup merge of rust-lang#35392 - malbarbo:cell-from, r=brson

Implement From for Cell, RefCell and UnsafeCell

Considering that `From` is implemented for `Box`, `Rc` and `Arc`, it seems [reasonable](https://internals.rust-lang.org/t/implementing-from-t-for-other-std-types/3744) to implement it for `Cell`, `RefCell` and `UnsafeCell`.

Author: Manishearth

src/libcore/cell.rs

@@ -146,6 +146,7 @@
 
 use clone::Clone;
 use cmp::{PartialEq, Eq, PartialOrd, Ord, Ordering};
+use convert::From;
 use default::Default;
 use fmt::{self, Debug, Display};
 use marker::{Copy, PhantomData, Send, Sync, Sized, Unsize};
@@ -329,6 +330,13 @@ impl<T:Ord + Copy> Ord for Cell<T> {
     }
 }
 
+#[stable(feature = "cell_from", since = "1.12.0")]
+impl<T: Copy> From<T> for Cell<T> {
+    fn from(t: T) -> Cell<T> {
+        Cell::new(t)
+    }
+}
+
 /// A mutable memory location with dynamically checked borrow rules
 ///
 /// See the [module-level documentation](index.html) for more.
@@ -742,6 +750,13 @@ impl<T: ?Sized + Ord> Ord for RefCell<T> {
     }
 }
 
+#[stable(feature = "cell_from", since = "1.12.0")]
+impl<T> From<T> for RefCell<T> {
+    fn from(t: T) -> RefCell<T> {
+        RefCell::new(t)
+    }
+}
+
 struct BorrowRef<'b> {
     borrow: &'b Cell<BorrowFlag>,
 }
@@ -1064,3 +1079,10 @@ impl<T: Default> Default for UnsafeCell<T> {
         UnsafeCell::new(Default::default())
     }
 }
+
+#[stable(feature = "cell_from", since = "1.12.0")]
+impl<T> From<T> for UnsafeCell<T> {
+    fn from(t: T) -> UnsafeCell<T> {
+        UnsafeCell::new(t)
+    }
+}