rust: Prefer primary spans

This would be great improvement to the current situation, where the
first span is used, which is quite often not the right one.

See rust-lang/rust.vim#174

Related to: neomake#2075

autoload/neomake/makers/ft/rust.vim

@@ -64,6 +64,12 @@ function! neomake#makers#ft#rust#CargoProcessOutput(context) abort
         endif
 
         let span = data.spans[0]
+        for candidate_span in data.spans
+            if candidate_span.is_primary
+                let span = candidate_span
+            end
+        endfor
+
         let expanded = 0
         let has_expansion = type(span.expansion) == type({})
                     \ && type(span.expansion.span) == type({})