String interpolation: combine contiguous string literals

spec/compiler/normalize/string_interpolation_spec.cr

@@ -5,6 +5,10 @@ describe "Normalize: string interpolation" do
     assert_expand %("foo\#{bar}baz"), %(::String.interpolation("foo", bar, "baz"))
   end
 
+  it "normalizes string interpolation with multiple lines" do
+    assert_expand %("foo\n\#{bar}\nbaz\nqux\nfox"), %(::String.interpolation("foo\\n", bar, "\\nbaz\\nqux\\nfox"))
+  end
+
   it "normalizes heredoc" do
     assert_normalize "<<-FOO\nhello\nFOO", %("hello")
   end

src/compiler/crystal/semantic/literal_expander.cr

@@ -293,7 +293,33 @@ module Crystal
     #
     #     String.interpolation("foo", bar, "baz", qux)
     def expand(node : StringInterpolation)
-      Call.new(Path.global("String").at(node), "interpolation", node.expressions).at(node)
+      # We could do `node.expressions.dup` for more purity,
+      # but the string interpolation isn't used later on so this is fine,
+      # and having pieces in a different representation but same end
+      # result is just fine.
+      pieces = node.expressions
+      combine_contiguous_string_literals(pieces)
+      Call.new(Path.global("String").at(node), "interpolation", pieces).at(node)
+    end
+
+    private def combine_contiguous_string_literals(pieces)
+      i = 0
+      pieces.reject! do |piece|
+        delete =
+          if i < pieces.size - 1
+            next_piece = pieces[i + 1]
+            if piece.is_a?(StringLiteral) && next_piece.is_a?(StringLiteral)
+              pieces[i + 1] = StringLiteral.new(piece.value + next_piece.value)
+              true
+            else
+              false
+            end
+          else
+            false
+          end
+        i += 1
+        delete
+      end
     end
 
     # Convert a Case into a series of if ... elseif ... end: