Add long explanation for E0018

Author: GuillaumeGomez

src/librustc/diagnostics.rs

@@ -112,6 +112,11 @@ reference when using guards or refactor the entire expression, perhaps by
 putting the condition inside the body of the arm.
 "##,
 
+E0018: r##"
+The value of static and const variables must be known at compile time, which is
+not possible with a pointer or with a value pointed by a pointer.
+"##,
+
 E0152: r##"
 Lang items are already implemented in the standard library. Unless you are
 writing a free-standing application (e.g. a kernel), you do not need to provide