Add error explanation for E0014

Author: GuillaumeGomez

src/librustc/diagnostics.rs

@@ -195,6 +195,23 @@ const Y: i32 = A;
 ```
 "##,
 
+E0014: r##"
+Constant variables' value can only be a constant value or a function. Bad
+example:
+
+```
+const FOO: i32 = { let x = 0; x }; // 'x' isn't a constant nor a function!
+```
+
+To avoid it, you have to replace the non-constant value:
+
+```
+const FOO: i32 = { const X : i32 = 0; X };
+// or even:
+const FOO: i32 = { 0 }; // but brackets are useless here
+```
+"##,
+
 E0015: r##"
 The only function calls allowed in static or constant expressions are enum
 variant constructors or struct constructors (for unit or tuple structs). This
@@ -801,7 +818,6 @@ struct Foo<T: 'static> {
 
 register_diagnostics! {
     E0011,
-    E0014,
     E0016,
     E0017,
     E0019,