Add E0395 error explanation

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -1538,8 +1538,60 @@ For more information see the [opt-in builtin traits RFC](https://github.com/rust
 -lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
 "##
 
+E0395: r##"
+The value assigned to a constant expression must be known at compile time,
+which is not the case when comparing raw pointers. Erroneous code example:
+
+```
+static foo: i32 = 42;
+static bar: i32 = 43;
+
+static baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
+// error: raw pointers cannot be compared in statics!
+```
+
+To fix this error, please not assign this value to a constant expression.
+Example:
+
+```
+static foo: i32 = 42;
+static bar: i32 = 43;
+
+let baz: bool = { (&foo as *const i32) == (&bar as *const i32) };
+// baz isn't a constant expression so it's ok
+```
+"##,
+
+E0396: r##"
+The value assigned to a constant expression must be known at compile time,
+which is not the case when dereferencing raw pointers. Erroneous code
+example:
+
+```
+const foo: i32 = 42;
+const baz: *const i32 = (&foo as *const i32);
+
+const deref: i32 = *baz;
+// error: raw pointers cannot be dereferenced in constants!
+```
+
+To fix this error, please not assign this value to a constant expression.
+Example:
+
+```
+const foo: i32 = 42;
+const baz: *const i32 = (&foo as *const i32);
+
+unsafe { let deref: i32 = *baz; }
+// baz isn't a constant expression so it's ok
+```
+
+You'll also note that this assignation must be done in an unsafe block!
+"##
+
 }
 
+
 register_diagnostics! {
     E0068,
     E0074,