Add workaround for hidden outer attribute

If the snippet is empty, it's an attribute that was inserted during macro
expansion and we want to ignore those, because they could come from external
sources that the user has no control over.
For some reason these attributes don't have any expansion info on them, so
we have to check it this way until there is a better way.

clippy_lints/src/attrs.rs

@@ -263,6 +263,10 @@ fn check_attrs(cx: &LateContext, span: Span, name: &Name, attrs: &[Attribute]) {
 
     for attr in attrs {
         if attr.style == AttrStyle::Outer {
+            if !is_present_in_source(cx, attr.span) {
+                return;
+            }
+
             let attr_to_item_span = Span::new(attr.span.lo(), span.lo(), span.ctxt());
 
             if let Some(snippet) = snippet_opt(cx, attr_to_item_span) {
@@ -319,3 +323,17 @@ fn is_word(nmi: &NestedMetaItem, expected: &str) -> bool {
         false
     }
 }
+
+// If the snippet is empty, it's an attribute that was inserted during macro
+// expansion and we want to ignore those, because they could come from external
+// sources that the user has no control over.
+// For some reason these attributes don't have any expansion info on them, so
+// we have to check it this way until there is a better way.
+fn is_present_in_source(cx: &LateContext, span: Span) -> bool {
+    if let Some(snippet) = snippet_opt(cx, span) {
+        if snippet.is_empty() {
+            return false;
+        }
+    }
+    true
+}