updated E0396 to new error format

Author: clementmiao

src/librustc_mir/transform/qualify_consts.rs

@@ -490,9 +490,13 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                             if let ty::TyRawPtr(_) = base_ty.sty {
                                 this.add(Qualif::NOT_CONST);
                                 if this.mode != Mode::Fn {
-                                    span_err!(this.tcx.sess, this.span, E0396,
-                                              "raw pointers cannot be dereferenced in {}s",
-                                              this.mode);
+                                    struct_span_err!(this.tcx.sess,
+                                        this.span, E0396,
+                                        "raw pointers cannot be dereferenced in {}s",
+                                        this.mode)
+                                    .span_label(this.span,
+                                        &format!("dereference of raw pointer in constant"))
+                                    .emit();
                                 }
                             }
                         }

src/test/compile-fail/E0396.rs

@@ -11,6 +11,7 @@
 const REG_ADDR: *const u8 = 0x5f3759df as *const u8;
 
 const VALUE: u8 = unsafe { *REG_ADDR }; //~ ERROR E0396
+                  //~| NOTE dereference of raw pointer in constant
 
 fn main() {
 }

src/test/compile-fail/const-deref-ptr.rs

@@ -12,5 +12,6 @@
 
 fn main() {
     static C: u64 = unsafe {*(0xdeadbeef as *const u64)}; //~ ERROR E0396
+                    //~| NOTE dereference of raw pointer in constant
     println!("{}", C);
 }