Add long explanation for E0018

Author: GuillaumeGomez

src/librustc/diagnostics.rs

@@ -168,6 +168,11 @@ variant constructors or struct constructors (for unit or tuple structs). This
 is because Rust currently does not support compile-time function execution.
 "##,
 
+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.
+"##,
+
 E0020: r##"
 This error indicates that an attempt was made to divide by zero (or take the
 remainder of a zero divisor) in a static or constant expression.
@@ -398,7 +403,6 @@ register_diagnostics! {
     E0014,
     E0016,
     E0017,
-    E0018,
     E0019,
     E0022,
     E0079, // enum variant: expected signed integer constant