Tweak multiple allocators error

Author: estebank

src/librustc_metadata/creader.rs

@@ -698,7 +698,9 @@ impl<'a> CrateLoader<'a> {
         let has_global_allocator = match &*global_allocator_spans(krate) {
             [span1, span2, ..] => {
                 self.sess.struct_span_err(*span2, "cannot define multiple global allocators")
-                         .span_note(*span1, "the previous global allocator is defined here").emit();
+                    .span_label(*span2, "cannot define a new global allocator")
+                    .span_label(*span1, "previous global allocator is defined here")
+                    .emit();
                 true
             }
             spans => !spans.is_empty()

src/test/ui/allocator/two-allocators.stderr

@@ -1,14 +1,11 @@
 error: cannot define multiple global allocators
   --> $DIR/two-allocators.rs:6:1
    |
-LL | static B: System = System;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-note: the previous global allocator is defined here
-  --> $DIR/two-allocators.rs:4:1
-   |
 LL | static A: System = System;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | -------------------------- previous global allocator is defined here
+LL | #[global_allocator]
+LL | static B: System = System;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot define a new global allocator
 
 error: aborting due to previous error