Skip lib definitions altogether

spec/ameba/rule/lint/useless_assign_spec.cr

@@ -998,6 +998,16 @@ module Ameba::Rule::Lint
           CRYSTAL
       end
 
+      it "doesn't report if it's referenced in a lib" do
+        expect_no_issues subject, <<-CRYSTAL
+          lib LibFoo
+            struct Foo
+              a : Int32
+            end
+          end
+          CRYSTAL
+      end
+
       it "doesn't report if it's referenced" do
         expect_no_issues subject, <<-CRYSTAL
           def foo

src/ameba/rule/lint/useless_assign.cr

@@ -39,6 +39,8 @@ module Ameba::Rule::Lint
     end
 
     def test(source, node, scope : AST::Scope)
+      return if scope.lib_def?(check_outer_scopes: true)
+
       scope.variables.each do |var|
         next if var.ignored? || var.used_in_macro? || var.captured_by_block?
         next if exclude_type_declarations? && scope.assigns_type_dec?(var.name)