Flatten dstr nesting that results from implicit string concatenation

russell-stripe · mbj · commit 7df6c76456ae · 2015-07-29T14:42:35.000-07:00
This extra nesting happens when a dstr is generated from implicit string
concatenation and one of the strings being concatenated is itself a dstr.

Example: "foo" "bar #{baz}"

Without this preprocessor, this turns into: "foo#{"bar#{baz}"}"
With this preprocessor, this turns into: "foobar #{baz}"
diff --git a/lib/unparser/preprocessor.rb b/lib/unparser/preprocessor.rb
@@ -149,7 +149,44 @@ def chunked_children
 
     end # CollapseStrChildren
 
-    # Preprocessor eliminating unneded dstr nodes
+    # Preprocessor flattening unneeded dstr nesting. This happens when a dstr is generated from
+    # implicit string concatenation, and one of the strings being concatenated is itself a dstr.
+    #
+    # Example: "foo" "bar #{baz}"
+    #
+    # Without this preprocessor, this turns into: "foo#{"bar#{baz}"}"
+    # With this preprocessor, this turns into: "foobar #{baz}"
+    class FlattenImplicitDSTR < self
+
+      # No need to register :dsym, because Ruby doesn't do implicit concatenation of symbols
+      register :dstr
+
+      # Return preprocessor result
+      #
+      # @return [Parser::AST::Node]
+      #
+      # @api private
+      #
+      def result
+        # This should only ever be true for DSTR nodes that are a result of implicit concatenation
+        # (see the comments on this class). Any other DSTR node would have a :begin node as a child.
+        if children.any? && children.all? { |child| [:dstr, :str].include?(child.type) }
+          flat_children = children.flat_map do |child|
+            if child.type.equal?(:str)
+              child
+            else # child.type is dstr
+              child.children
+            end
+          end
+          node.updated(nil, flat_children)
+        else
+          node
+        end
+      end
+
+    end # FlattenImplicitDSTR
+
+    # Preprocessor eliminating unneeded dstr nodes
     class CompactDSTR < self
 
       register :dstr
diff --git a/spec/unit/unparser_spec.rb b/spec/unit/unparser_spec.rb
@@ -139,6 +139,7 @@ def foo(bar:, baz: "value")
       context 'string' do
         assert_generates '?c', '"c"'
         assert_generates '"foo" "bar"', '"foobar"'
+        assert_generates '"foo" "bar #{baz}"', '"foobar #{baz}"'
         assert_generates '%Q(foo"#{@bar})', '"foo\\"#{@bar}"'
         assert_terminated '"\""'
         assert_terminated '"foo#{1}bar"'
