Add long description and test for E0311

Adds a long description and unit test for the E0311 compiler error.

compiler/rustc_error_codes/src/error_codes/E0311.md

@@ -0,0 +1,49 @@
+E0311 occurs when there is insufficient information for the rust compiler to
+prove that some time has a long enough lifetime.
+
+Erroneous code example:
+
+```compile_fail, E0311
+use std::borrow::BorrowMut;
+
+trait NestedBorrowMut<U, V> {
+    fn nested_borrow_mut(&mut self) -> &mut V;
+}
+
+impl<T, U, V> NestedBorrowMut<U, V> for T
+where
+    T: BorrowMut<U>,
+    U: BorrowMut<V>,  // missing lifetime specifier here --> compile fail
+{
+    fn nested_borrow_mut(&mut self) -> &mut V {
+        self.borrow_mut().borrow_mut()
+    }
+}
+```
+
+In this example we have a trait that borrows some inner data element of type V
+from an outer type T, through an intermediate type U. The compiler is unable to
+prove that the livetime of U is long enough to support the reference, so it
+throws E0311. To fix the issue we can explicitly add lifetime specifiers to the
+trait, which link the lifetimes of the various data types and allow the code
+to compile.
+
+Working implementation of the `NestedBorrowMut` trait:
+
+```
+use std::borrow::BorrowMut;
+
+trait NestedBorrowMut<'a, U, V> {
+    fn nested_borrow_mut(& 'a mut self) -> &'a mut V;
+}
+
+impl<'a, T, U, V> NestedBorrowMut<'a, U, V> for T
+where
+    T: BorrowMut<U>,
+    U: BorrowMut<V> + 'a,
+{
+    fn nested_borrow_mut(&'a mut self) -> &'a mut V {
+        self.borrow_mut().borrow_mut()
+    }
+}
+```