Make SkylarkClassObject "Bazel-specific".

This in preparation to DeclaredProviders implementation.

--
MOS_MIGRATED_REVID=129420617

20 files changed, 268 insertions, 210 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryContext.java b/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryContext.java
index ad2290adb6..f8d10cfab5 100644
--- a/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryContext.java
+++ b/src/main/java/com/google/devtools/build/lib/bazel/repository/skylark/SkylarkRepositoryContext.java
@@ -25,6 +25,7 @@ import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
 import com.google.devtools.build.lib.events.Location;
 import com.google.devtools.build.lib.packages.AggregatingAttributeMapper;
 import com.google.devtools.build.lib.packages.Rule;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.rules.repository.RepositoryFunction.RepositoryFunctionException;
 import com.google.devtools.build.lib.skyframe.FileSymlinkException;
 import com.google.devtools.build.lib.skyframe.FileValue;
@@ -33,7 +34,6 @@ import com.google.devtools.build.lib.skyframe.PackageLookupValue;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkCallable;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.Runtime;
 import com.google.devtools.build.lib.syntax.SkylarkType;
@@ -46,7 +46,6 @@ import com.google.devtools.build.lib.vfs.RootedPath;
 import com.google.devtools.build.skyframe.SkyFunction.Environment;
 import com.google.devtools.build.skyframe.SkyFunctionException.Transience;
 import com.google.devtools.build.skyframe.SkyKey;
-
 import java.io.File;
 import java.io.IOException;
 import java.io.OutputStream;
diff --git a/src/main/java/com/google/devtools/build/lib/packages/Attribute.java b/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
index c53b92af54..9088840bd5 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/Attribute.java
@@ -29,7 +29,6 @@ import com.google.devtools.build.lib.events.Location;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
 import com.google.devtools.build.lib.syntax.ClassObject;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.SkylarkCallbackFunction;
 import com.google.devtools.build.lib.syntax.Type;
diff --git a/src/main/java/com/google/devtools/build/lib/packages/ImplicitOutputsFunction.java b/src/main/java/com/google/devtools/build/lib/packages/ImplicitOutputsFunction.java
index c20cf3bcca..3789091663 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/ImplicitOutputsFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/ImplicitOutputsFunction.java
@@ -27,7 +27,6 @@ import com.google.common.escape.Escapers;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.events.Location;
 import com.google.devtools.build.lib.syntax.ClassObject;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.Runtime;
 import com.google.devtools.build.lib.syntax.SkylarkCallbackFunction;
@@ -35,7 +34,6 @@ import com.google.devtools.build.lib.syntax.Type;
 import com.google.devtools.build.lib.util.StringUtil;
 import com.google.devtools.build.lib.vfs.FileSystemUtils;
 import com.google.devtools.build.lib.vfs.PathFragment;
-
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -44,7 +42,6 @@ import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-
 import javax.annotation.Nullable;
 
 /**
diff --git a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
index 4c3138a2c6..7b8e57f500 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
@@ -1526,7 +1526,7 @@ public final class PackageFactory {
         builder.put(function.getName(), function);
       }
     }
-    return new ClassObject.SkylarkClassObject(builder.build(), "no native function or rule '%s'");
+    return new SkylarkClassObject(builder.build(), "no native function or rule '%s'");
   }
 
   private void buildPkgEnv(Environment pkgEnv, PackageContext context, RuleFactory ruleFactory) {
diff --git a/src/main/java/com/google/devtools/build/lib/packages/SkylarkClassObject.java b/src/main/java/com/google/devtools/build/lib/packages/SkylarkClassObject.java
new file mode 100644
index 0000000000..d28146a526
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/packages/SkylarkClassObject.java
@@ -0,0 +1,184 @@
+// Copyright 2016 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.packages;
+
+import com.google.common.base.Joiner;
+import com.google.common.collect.ImmutableCollection;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Ordering;
+import com.google.common.collect.Sets;
+import com.google.common.collect.Sets.SetView;
+import com.google.devtools.build.lib.events.Location;
+import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
+import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
+import com.google.devtools.build.lib.skylarkinterface.SkylarkValue;
+import com.google.devtools.build.lib.syntax.ClassObject;
+import com.google.devtools.build.lib.syntax.Concatable;
+import com.google.devtools.build.lib.syntax.EvalException;
+import com.google.devtools.build.lib.syntax.EvalUtils;
+import com.google.devtools.build.lib.syntax.Printer;
+import com.google.devtools.build.lib.syntax.SkylarkType;
+import com.google.devtools.build.lib.util.Preconditions;
+import java.io.Serializable;
+import java.util.Map;
+
+/** An implementation class of ClassObject for structs created in Skylark code. */
+@SkylarkModule(
+  name = "struct",
+  category = SkylarkModuleCategory.BUILTIN,
+  doc =
+      "A special language element to support structs (i.e. simple value objects). "
+          + "See the global <a href=\"globals.html#struct\">struct</a> function "
+          + "for more details."
+)
+public class SkylarkClassObject implements ClassObject, SkylarkValue, Concatable, Serializable {
+  /** Error message to use when errorMessage argument is null. */
+  private static final String DEFAULT_ERROR_MESSAGE = "'struct' object has no attribute '%s'";
+
+  private final ImmutableMap<String, Object> values;
+  private final Location creationLoc;
+  private final String errorMessage;
+
+  /**
+   * Primarily for testing purposes where no location is available and the default
+   * errorMessage suffices.
+   */
+  public SkylarkClassObject(Map<String, Object> values) {
+    this.values = copyValues(values);
+    this.creationLoc = null;
+    this.errorMessage = DEFAULT_ERROR_MESSAGE;
+  }
+
+  /**
+   * Creates a built-in struct (i.e. without creation loc). The errorMessage has to have
+   * exactly one '%s' parameter to substitute the struct field name.
+   */
+  public SkylarkClassObject(Map<String, Object> values, String errorMessage) {
+    this.values = copyValues(values);
+    this.creationLoc = null;
+    this.errorMessage = Preconditions.checkNotNull(errorMessage);
+  }
+
+  public SkylarkClassObject(Map<String, Object> values, Location creationLoc) {
+    this.values = copyValues(values);
+    this.creationLoc = Preconditions.checkNotNull(creationLoc);
+    this.errorMessage = DEFAULT_ERROR_MESSAGE;
+  }
+
+  // Ensure that values are all acceptable to Skylark before to stuff them in a ClassObject
+  private ImmutableMap<String, Object> copyValues(Map<String, Object> values) {
+    ImmutableMap.Builder<String, Object> builder = ImmutableMap.builder();
+    for (Map.Entry<String, Object> e : values.entrySet()) {
+      builder.put(e.getKey(), SkylarkType.convertToSkylark(e.getValue(), null));
+    }
+    return builder.build();
+  }
+
+  @Override
+  public Object getValue(String name) {
+    return values.get(name);
+  }
+
+  /**
+   *  Returns a value and try to cast it into specified type
+   */
+  public <TYPE> TYPE getValue(String key, Class<TYPE> type) throws EvalException {
+    Object obj = values.get(key);
+    if (obj == null) {
+      return null;
+    }
+    SkylarkType.checkType(obj, type, key);
+    return type.cast(obj);
+  }
+
+  @Override
+  public ImmutableCollection<String> getKeys() {
+    return values.keySet();
+  }
+
+  public Location getCreationLoc() {
+    return Preconditions.checkNotNull(creationLoc,
+        "This struct was not created in a Skylark code");
+  }
+
+  @Override
+  public Concatter getConcatter() {
+    return StructConcatter.INSTANCE;
+  }
+
+  private static class StructConcatter implements Concatter {
+    private static final StructConcatter INSTANCE = new StructConcatter();
+
+    private StructConcatter() {}
+
+    @Override
+    public SkylarkClassObject concat(
+        Concatable left, Concatable right, Location loc) throws EvalException {
+      SkylarkClassObject lval = (SkylarkClassObject) left;
+      SkylarkClassObject rval = (SkylarkClassObject) right;
+      SetView<String> commonFields = Sets
+          .intersection(lval.values.keySet(), rval.values.keySet());
+      if (!commonFields.isEmpty()) {
+        throw new EvalException(loc, "Cannot concat structs with common field(s): "
+            + Joiner.on(",").join(commonFields));
+      }
+      return new SkylarkClassObject(ImmutableMap.<String, Object>builder()
+          .putAll(lval.values).putAll(rval.values).build(), loc);
+    }
+  }
+
+  @Override
+  public String errorMessage(String name) {
+    String suffix =
+        "Available attributes: "
+            + Joiner.on(", ").join(Ordering.natural().sortedCopy(values.keySet()));
+    return String.format(errorMessage, name) + "\n" + suffix;
+  }
+
+  @Override
+  public boolean isImmutable() {
+    for (Object item : values.values()) {
+      if (!EvalUtils.isImmutable(item)) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  /**
+   * Convert the object to string using Skylark syntax. The output tries to be
+   * reversible (but there is no guarantee, it depends on the actual values).
+   */
+  @Override
+  public void write(Appendable buffer, char quotationMark) {
+    boolean first = true;
+    Printer.append(buffer, "struct(");
+    // Sort by key to ensure deterministic output.
+    for (String key : Ordering.natural().sortedCopy(values.keySet())) {
+      if (!first) {
+        Printer.append(buffer, ", ");
+      }
+      first = false;
+      Printer.append(buffer, key);
+      Printer.append(buffer, " = ");
+      Printer.write(buffer, values.get(key), quotationMark);
+    }
+    Printer.append(buffer, ")");
+  }
+
+  @Override
+  public String toString() {
+    return Printer.repr(this);
+  }
+}
diff --git a/src/main/java/com/google/devtools/build/lib/packages/WorkspaceFactory.java b/src/main/java/com/google/devtools/build/lib/packages/WorkspaceFactory.java
index 2a9892904a..54b393a7dc 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/WorkspaceFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/WorkspaceFactory.java
@@ -454,7 +454,7 @@ public class WorkspaceFactory {
     }
 
     builder.put("bazel_version", version);
-    return new ClassObject.SkylarkClassObject(builder.build(), "no native function or rule '%s'");
+    return new SkylarkClassObject(builder.build(), "no native function or rule '%s'");
   }
 
   public static ClassObject newNativeModule(RuleClassProvider ruleClassProvider, String version) {
diff --git a/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleClassFunctions.java b/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleClassFunctions.java
index 2e6dbb880b..cb22b50ba3 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleClassFunctions.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleClassFunctions.java
@@ -62,6 +62,7 @@ import com.google.devtools.build.lib.packages.RuleFactory;
 import com.google.devtools.build.lib.packages.RuleFactory.BuildLangTypedAttributeValuesMap;
 import com.google.devtools.build.lib.packages.RuleFactory.InvalidRuleException;
 import com.google.devtools.build.lib.packages.SkylarkAspect;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.packages.TargetUtils;
 import com.google.devtools.build.lib.packages.TestSize;
 import com.google.devtools.build.lib.rules.SkylarkAttr.Descriptor;
@@ -70,7 +71,6 @@ import com.google.devtools.build.lib.skylarkinterface.SkylarkSignature;
 import com.google.devtools.build.lib.syntax.BaseFunction;
 import com.google.devtools.build.lib.syntax.BuiltinFunction;
 import com.google.devtools.build.lib.syntax.ClassObject;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.Environment;
 import com.google.devtools.build.lib.syntax.Environment.NoSuchVariableException;
 import com.google.devtools.build.lib.syntax.EvalException;
@@ -88,7 +88,6 @@ import com.google.devtools.build.lib.syntax.Type.ConversionException;
 import com.google.devtools.build.lib.util.Pair;
 import com.google.devtools.build.lib.util.Preconditions;
 import com.google.protobuf.TextFormat;
-
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -193,6 +192,22 @@ public class SkylarkRuleClassFunctions {
         .build();
   }
 
+  @SkylarkSignature(name = "struct", returnType = SkylarkClassObject.class, doc =
+      "Creates an immutable struct using the keyword arguments as attributes. It is used to group "
+          + "multiple values together.Example:<br>"
+          + "<pre class=\"language-python\">s = struct(x = 2, y = 3)\n"
+          + "return s.x + getattr(s, \"y\")  # returns 5</pre>",
+      extraKeywords = @Param(name = "kwargs", doc = "the struct attributes"),
+      useLocation = true)
+  private static final BuiltinFunction struct = new BuiltinFunction("struct") {
+    @SuppressWarnings("unchecked")
+    public SkylarkClassObject invoke(SkylarkDict<String, Object> kwargs, Location loc)
+        throws EvalException {
+      return new SkylarkClassObject(kwargs, loc);
+    }
+  };
+
+
   // TODO(bazel-team): implement attribute copy and other rule properties
   @SkylarkSignature(name = "rule", doc =
       "Creates a new rule. Store it in a global value, so that it can be loaded and called "
diff --git a/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleConfiguredTargetBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleConfiguredTargetBuilder.java
index 0cc409fd74..52e4750c7c 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleConfiguredTargetBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleConfiguredTargetBuilder.java
@@ -28,6 +28,7 @@ import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.events.Location;
 import com.google.devtools.build.lib.packages.Rule;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.packages.TargetUtils;
 import com.google.devtools.build.lib.rules.SkylarkRuleContext.Kind;
 import com.google.devtools.build.lib.rules.test.InstrumentedFilesCollector;
@@ -36,7 +37,6 @@ import com.google.devtools.build.lib.rules.test.InstrumentedFilesProvider;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkValue;
 import com.google.devtools.build.lib.syntax.BaseFunction;
 import com.google.devtools.build.lib.syntax.ClassObject;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.Environment;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.EvalExceptionWithStackTrace;
@@ -49,7 +49,6 @@ import com.google.devtools.build.lib.syntax.SkylarkType;
 import com.google.devtools.build.lib.syntax.Type;
 import com.google.devtools.build.lib.util.FileType;
 import com.google.devtools.build.lib.util.FileTypeSet;
-
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
diff --git a/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleContext.java b/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleContext.java
index 8112e02119..248bbfd33b 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleContext.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleContext.java
@@ -42,12 +42,12 @@ import com.google.devtools.build.lib.packages.ImplicitOutputsFunction.SkylarkImp
 import com.google.devtools.build.lib.packages.OutputFile;
 import com.google.devtools.build.lib.packages.Package;
 import com.google.devtools.build.lib.packages.RawAttributeMapper;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.shell.ShellUtils;
 import com.google.devtools.build.lib.shell.ShellUtils.TokenizationException;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkCallable;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.FuncallExpression.FuncallException;
 import com.google.devtools.build.lib.syntax.Runtime;
@@ -58,7 +58,6 @@ import com.google.devtools.build.lib.syntax.SkylarkType;
 import com.google.devtools.build.lib.syntax.Type;
 import com.google.devtools.build.lib.util.Preconditions;
 import com.google.devtools.build.lib.vfs.PathFragment;
-
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
@@ -67,7 +66,6 @@ import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-
 import javax.annotation.Nullable;
 
 /** A Skylark API for the ruleContext. */
diff --git a/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProvider.java b/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProvider.java
index 63ad021223..8249fd121a 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProvider.java
@@ -26,11 +26,12 @@ import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.collect.nestedset.Order;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.rules.cpp.CppModuleMap;
 import com.google.devtools.build.lib.rules.cpp.LinkerInputs;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
+import com.google.devtools.build.lib.syntax.Concatable.Concatter;
 import com.google.devtools.build.lib.syntax.EvalUtils;
 import com.google.devtools.build.lib.util.Preconditions;
 import com.google.devtools.build.lib.vfs.PathFragment;
@@ -426,6 +427,11 @@ public final class ObjcProvider extends SkylarkClassObject implements Transitive
     this.strictDependencyItems = Preconditions.checkNotNull(strictDependencyItems);
   }
 
+  @Override
+  public Concatter getConcatter() {
+    return null;
+  }
+
   /**
    * All artifacts, bundleable files, etc. of the type specified by {@code key}.
    */
diff --git a/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProviderSkylarkConverters.java b/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProviderSkylarkConverters.java
index 8d379e7f1c..727891e820 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProviderSkylarkConverters.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/objc/ObjcProviderSkylarkConverters.java
@@ -24,8 +24,8 @@ import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.rules.objc.ObjcProvider.Key;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.EvalUtils;
 import com.google.devtools.build.lib.syntax.SkylarkNestedSet;
diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java b/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java
index 74abb33451..842e9b7d7b 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java
@@ -43,11 +43,11 @@ import com.google.devtools.build.lib.collect.nestedset.Order;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
 import com.google.devtools.build.lib.packages.BuildType;
 import com.google.devtools.build.lib.packages.Rule;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.rules.cpp.CppFileTypes;
 import com.google.devtools.build.lib.rules.test.InstrumentedFilesCollector;
 import com.google.devtools.build.lib.rules.test.InstrumentedFilesCollector.LocalMetadataCollector;
 import com.google.devtools.build.lib.rules.test.InstrumentedFilesProvider;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.EvalUtils;
 import com.google.devtools.build.lib.syntax.SkylarkNestedSet;
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/SkylarkAspectFactory.java b/src/main/java/com/google/devtools/build/lib/skyframe/SkylarkAspectFactory.java
index 33b544bb34..450015804b 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/SkylarkAspectFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/SkylarkAspectFactory.java
@@ -24,16 +24,15 @@ import com.google.devtools.build.lib.analysis.SkylarkProviderValidationUtil;
 import com.google.devtools.build.lib.events.Location;
 import com.google.devtools.build.lib.packages.AspectParameters;
 import com.google.devtools.build.lib.packages.SkylarkAspect;
+import com.google.devtools.build.lib.packages.SkylarkClassObject;
 import com.google.devtools.build.lib.rules.SkylarkRuleContext;
 import com.google.devtools.build.lib.rules.SkylarkRuleContext.Kind;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.Environment;
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.EvalExceptionWithStackTrace;
 import com.google.devtools.build.lib.syntax.Mutability;
 import com.google.devtools.build.lib.syntax.SkylarkNestedSet;
 import com.google.devtools.build.lib.syntax.SkylarkType;
-
 import java.util.Map;
 
 /**
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java b/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
index a31b037601..131ee54772 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/BinaryOperatorExpression.java
@@ -18,7 +18,7 @@ import static com.google.devtools.build.lib.syntax.compiler.ByteCodeUtils.append
 import com.google.common.base.Strings;
 import com.google.common.collect.Iterables;
 import com.google.devtools.build.lib.events.Location;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
+import com.google.devtools.build.lib.syntax.Concatable.Concatter;
 import com.google.devtools.build.lib.syntax.SkylarkList.MutableList;
 import com.google.devtools.build.lib.syntax.SkylarkList.Tuple;
 import com.google.devtools.build.lib.syntax.compiler.ByteCodeMethodCalls;
@@ -29,17 +29,15 @@ import com.google.devtools.build.lib.syntax.compiler.Jump;
 import com.google.devtools.build.lib.syntax.compiler.Jump.PrimitiveComparison;
 import com.google.devtools.build.lib.syntax.compiler.LabelAdder;
 import com.google.devtools.build.lib.syntax.compiler.VariableScope;
-
-import net.bytebuddy.implementation.bytecode.ByteCodeAppender;
-import net.bytebuddy.implementation.bytecode.Duplication;
-import net.bytebuddy.implementation.bytecode.Removal;
-import net.bytebuddy.implementation.bytecode.StackManipulation;
-
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.IllegalFormatException;
 import java.util.List;
+import net.bytebuddy.implementation.bytecode.ByteCodeAppender;
+import net.bytebuddy.implementation.bytecode.Duplication;
+import net.bytebuddy.implementation.bytecode.Removal;
+import net.bytebuddy.implementation.bytecode.StackManipulation;
 
 /**
  * Syntax node for a binary operator expression.
@@ -338,9 +336,15 @@ public final class BinaryOperatorExpression extends Expression {
       return SkylarkDict.plus((SkylarkDict<?, ?>) lval, (SkylarkDict<?, ?>) rval, env);
     }
 
-    if (lval instanceof SkylarkClassObject && rval instanceof SkylarkClassObject) {
-      return SkylarkClassObject.concat(
-          (SkylarkClassObject) lval, (SkylarkClassObject) rval, location);
+    if (lval instanceof Concatable && rval instanceof Concatable) {
+      Concatable lobj = (Concatable) lval;
+      Concatable robj = (Concatable) rval;
+      Concatter concatter = lobj.getConcatter();
+      if (concatter != null && concatter.equals(robj.getConcatter())) {
+        return concatter.concat(lobj, robj, location);
+      } else {
+        throw typeException(lval, rval, Operator.PLUS, location);
+      }
     }
 
     // TODO(bazel-team): Remove this case. Union of sets should use '|' instead of '+'.
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java b/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java
index 79c19610e0..8ea9aa1372 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java
@@ -13,21 +13,7 @@
 // limitations under the License.
 package com.google.devtools.build.lib.syntax;
 
-import com.google.common.base.Joiner;
 import com.google.common.collect.ImmutableCollection;
-import com.google.common.collect.ImmutableMap;
-import com.google.common.collect.Ordering;
-import com.google.common.collect.Sets;
-import com.google.common.collect.Sets.SetView;
-import com.google.devtools.build.lib.events.Location;
-import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
-import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
-import com.google.devtools.build.lib.skylarkinterface.SkylarkValue;
-import com.google.devtools.build.lib.util.Preconditions;
-
-import java.io.Serializable;
-import java.util.Map;
-
 import javax.annotation.Nullable;
 
 /**
@@ -53,140 +39,4 @@ public interface ClassObject {
    * of this struct, or returns null to use the default error message.
    */
   @Nullable String errorMessage(String name);
-
-  /** An implementation class of ClassObject for structs created in Skylark code. */
-  // TODO(bazel-team): maybe move the SkylarkModule annotation to the ClassObject interface?
-  @SkylarkModule(
-    name = "struct",
-    category = SkylarkModuleCategory.BUILTIN,
-    doc =
-        "A special language element to support structs (i.e. simple value objects). "
-            + "See the global <a href=\"globals.html#struct\">struct</a> function "
-            + "for more details."
-  )
-  public class SkylarkClassObject implements ClassObject, SkylarkValue, Serializable {
-    /** Error message to use when errorMessage argument is null. */
-    private static final String DEFAULT_ERROR_MESSAGE = "'struct' object has no attribute '%s'";
-
-    private final ImmutableMap<String, Object> values;
-    private final Location creationLoc;
-    private final String errorMessage;
-
-    /**
-     * Primarily for testing purposes where no location is available and the default
-     * errorMessage suffices.
-     */
-    public SkylarkClassObject(Map<String, Object> values) {
-      this.values = copyValues(values);
-      this.creationLoc = null;
-      this.errorMessage = DEFAULT_ERROR_MESSAGE;
-    }
-
-    /**
-     * Creates a built-in struct (i.e. without creation loc). The errorMessage has to have
-     * exactly one '%s' parameter to substitute the struct field name.
-     */
-    public SkylarkClassObject(Map<String, Object> values, String errorMessage) {
-      this.values = copyValues(values);
-      this.creationLoc = null;
-      this.errorMessage = Preconditions.checkNotNull(errorMessage);
-    }
-
-    public SkylarkClassObject(Map<String, Object> values, Location creationLoc) {
-      this.values = copyValues(values);
-      this.creationLoc = Preconditions.checkNotNull(creationLoc);
-      this.errorMessage = DEFAULT_ERROR_MESSAGE;
-    }
-
-    // Ensure that values are all acceptable to Skylark before to stuff them in a ClassObject
-    private ImmutableMap<String, Object> copyValues(Map<String, Object> values) {
-      ImmutableMap.Builder<String, Object> builder = ImmutableMap.builder();
-      for (Map.Entry<String, Object> e : values.entrySet()) {
-        builder.put(e.getKey(), SkylarkType.convertToSkylark(e.getValue(), null));
-      }
-      return builder.build();
-    }
-
-    @Override
-    public Object getValue(String name) {
-      return values.get(name);
-    }
-
-    /**
-     *  Returns a value and try to cast it into specified type
-     */
-    public <TYPE> TYPE getValue(String key, Class<TYPE> type) throws EvalException {
-      Object obj = values.get(key);
-      if (obj == null) {
-        return null;
-      }
-      SkylarkType.checkType(obj, type, key);
-      return type.cast(obj);
-    }
-
-    @Override
-    public ImmutableCollection<String> getKeys() {
-      return values.keySet();
-    }
-
-    public Location getCreationLoc() {
-      return Preconditions.checkNotNull(creationLoc,
-          "This struct was not created in a Skylark code");
-    }
-
-    static SkylarkClassObject concat(
-        SkylarkClassObject lval, SkylarkClassObject rval, Location loc) throws EvalException {
-      SetView<String> commonFields = Sets.intersection(lval.values.keySet(), rval.values.keySet());
-      if (!commonFields.isEmpty()) {
-        throw new EvalException(loc, "Cannot concat structs with common field(s): "
-            + Joiner.on(",").join(commonFields));
-      }
-      return new SkylarkClassObject(ImmutableMap.<String, Object>builder()
-          .putAll(lval.values).putAll(rval.values).build(), loc);
-    }
-
-    @Override
-    public String errorMessage(String name) {
-      String suffix =
-          "Available attributes: "
-              + Joiner.on(", ").join(Ordering.natural().sortedCopy(values.keySet()));
-      return String.format(errorMessage, name) + "\n" + suffix;
-    }
-
-    @Override
-    public boolean isImmutable() {
-      for (Object item : values.values()) {
-        if (!EvalUtils.isImmutable(item)) {
-          return false;
-        }
-      }
-      return true;
-    }
-
-    /**
-     * Convert the object to string using Skylark syntax. The output tries to be
-     * reversible (but there is no guarantee, it depends on the actual values).
-     */
-    @Override
-    public void write(Appendable buffer, char quotationMark) {
-      boolean first = true;
-      Printer.append(buffer, "struct(");
-      // Sort by key to ensure deterministic output.
-      for (String key : Ordering.natural().sortedCopy(values.keySet())) {
-        if (!first) {
-          Printer.append(buffer, ", ");
-        }
-        first = false;
-        Printer.append(buffer, key);
-        Printer.append(buffer, " = ");
-        Printer.write(buffer, values.get(key), quotationMark);
-      }
-      Printer.append(buffer, ")");
-    }
-
-    @Override
-    public String toString() {
-      return Printer.repr(this);
-    }
-  }
 }
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Concatable.java b/src/main/java/com/google/devtools/build/lib/syntax/Concatable.java
new file mode 100644
index 0000000000..81f489f7bf
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Concatable.java
@@ -0,0 +1,36 @@
+// Copyright 2016 The Bazel Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.syntax;
+
+import com.google.devtools.build.lib.events.Location;
+import javax.annotation.Nullable;
+
+/**
+ * Skylark values that support '+' operator should implement this interface.
+ */
+public interface Concatable {
+
+  /**
+   * Implements 'plus' operator on ClassObjects.
+   */
+  interface Concatter {
+    Concatable concat(Concatable lval, Concatable rval, Location loc) throws EvalException;
+  }
+
+  /* Returns a concatter for this {@link Concatable}.
+   * Two {@link Concatable}s can be added together if their concatters are equal.
+   */
+  @Nullable
+  Concatter getConcatter();
+}
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
index 4a661896b6..386a9aefeb 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/EvalUtils.java
@@ -26,12 +26,10 @@ import com.google.devtools.build.lib.skylarkinterface.SkylarkValue;
 import com.google.devtools.build.lib.syntax.compiler.ByteCodeUtils;
 import com.google.devtools.build.lib.util.Preconditions;
 import com.google.devtools.build.lib.vfs.PathFragment;
-
-import net.bytebuddy.implementation.bytecode.StackManipulation;
-
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import net.bytebuddy.implementation.bytecode.StackManipulation;
 
 /**
  * Utilities used by the evaluator.
@@ -280,8 +278,6 @@ public final class EvalUtils {
     } else if (NestedSet.class.isAssignableFrom(c) || SkylarkNestedSet.class.isAssignableFrom(c)) {
       // TODO(bazel-team): no one should be seeing naked NestedSet at all.
       return "set";
-    } else if (ClassObject.SkylarkClassObject.class.isAssignableFrom(c)) {
-      return "struct";
     } else {
       if (c.getSimpleName().isEmpty()) {
         return c.getName();
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
index f103211379..4b39a7d1c6 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/MethodLibrary.java
@@ -29,11 +29,9 @@ import com.google.devtools.build.lib.skylarkinterface.Param;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkSignature;
-import com.google.devtools.build.lib.syntax.ClassObject.SkylarkClassObject;
 import com.google.devtools.build.lib.syntax.SkylarkList.MutableList;
 import com.google.devtools.build.lib.syntax.SkylarkList.Tuple;
 import com.google.devtools.build.lib.syntax.Type.ConversionException;
-
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.Iterator;
@@ -1850,21 +1848,6 @@ public class MethodLibrary {
         }
       };
 
-  @SkylarkSignature(name = "struct", returnType = SkylarkClassObject.class, doc =
-      "Creates an immutable struct using the keyword arguments as attributes. It is used to group "
-      + "multiple values together.Example:<br>"
-      + "<pre class=\"language-python\">s = struct(x = 2, y = 3)\n"
-      + "return s.x + getattr(s, \"y\")  # returns 5</pre>",
-      extraKeywords = @Param(name = "kwargs", doc = "the struct attributes"),
-      useLocation = true)
-  private static final BuiltinFunction struct = new BuiltinFunction("struct") {
-    @SuppressWarnings("unchecked")
-    public SkylarkClassObject invoke(SkylarkDict<String, Object> kwargs, Location loc)
-        throws EvalException {
-      return new SkylarkClassObject(kwargs, loc);
-    }
-  };
-
   @SkylarkSignature(
       name = "set",
       returnType = SkylarkNestedSet.class,
@@ -2345,7 +2328,7 @@ public class MethodLibrary {
   static final List<BaseFunction> skylarkGlobalFunctions =
       ImmutableList.<BaseFunction>builder()
           .addAll(buildGlobalFunctions)
-          .add(dir, fail, getattr, hasattr, hash, print, struct, type)
+          .add(dir, fail, getattr, hasattr, hash, print, type)
           .build();
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java
index 0f8a3d31bc..2a64d7931d 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java
@@ -24,13 +24,11 @@ import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkValue;
 import com.google.devtools.build.lib.util.Preconditions;
-
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Set;
-
 import javax.annotation.Nullable;
 
 /** A generic type safe NestedSet wrapper for Skylark. */
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
index f662ea13b4..e73672a93b 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkType.java
@@ -27,7 +27,6 @@ import com.google.devtools.build.lib.skylarkinterface.SkylarkValue;
 import com.google.devtools.build.lib.syntax.SkylarkList.MutableList;
 import com.google.devtools.build.lib.syntax.SkylarkList.Tuple;
 import com.google.devtools.build.lib.util.Preconditions;
-
 import java.io.Serializable;
 import java.lang.reflect.Method;
 import java.lang.reflect.ParameterizedType;
@@ -37,7 +36,6 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
-
 import javax.annotation.Nullable;
 
 /**
@@ -168,9 +166,6 @@ public abstract class SkylarkType implements Serializable {
   /** The BOOLEAN type, that contains TRUE and FALSE */
   public static final Simple BOOL = Simple.of(Boolean.class);
 
-  /** The STRUCT type, for all Struct's */
-  public static final Simple STRUCT = Simple.of(ClassObject.SkylarkClassObject.class);
-
   /** The FUNCTION type, that contains all functions, otherwise dynamically typed at call-time */
   public static final SkylarkFunctionType FUNCTION = new SkylarkFunctionType("unknown", TOP);