Add better explanations

Author: GuillaumeGomez

src/librustc/diagnostics.rs

@@ -169,8 +169,26 @@ 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.
+The value of static and const variables must be known at compile time. You
+can't cast an integer as a pointer because we can't know what value will be
+at the address.
+
+However, pointers to other constants' address are allowed in constants,
+example:
+
+```
+const X: u32 = 50;
+const Y: *const u32 = &X;
+```
+
+Therefore, casting one of these non-constant pointers to an integer results
+in a non-constant integer whichs lead to this error. Example:
+
+```
+const X: u32 = 50;
+const Y: *const u32 = &X;
+println!("{:?}", Y);
+```
 "##,
 
 E0020: r##"