// Copyright 2014 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.rules;

import static com.google.devtools.build.lib.analysis.BaseRuleClasses.RUN_UNDER;
import static com.google.devtools.build.lib.packages.Attribute.ConfigurationTransition.DATA;
import static com.google.devtools.build.lib.packages.Attribute.ConfigurationTransition.HOST;
import static com.google.devtools.build.lib.packages.Attribute.attr;
import static com.google.devtools.build.lib.packages.BuildType.LABEL;
import static com.google.devtools.build.lib.packages.BuildType.LABEL_LIST;
import static com.google.devtools.build.lib.packages.BuildType.LICENSE;
import static com.google.devtools.build.lib.syntax.SkylarkType.castMap;
import static com.google.devtools.build.lib.syntax.Type.BOOLEAN;
import static com.google.devtools.build.lib.syntax.Type.INTEGER;
import static com.google.devtools.build.lib.syntax.Type.STRING;
import static com.google.devtools.build.lib.syntax.Type.STRING_LIST;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.devtools.build.lib.actions.Artifact;
import com.google.devtools.build.lib.analysis.ActionsProvider;
import com.google.devtools.build.lib.analysis.BaseRuleClasses;
import com.google.devtools.build.lib.analysis.DefaultProvider;
import com.google.devtools.build.lib.analysis.OutputGroupProvider;
import com.google.devtools.build.lib.analysis.PlatformSemantics;
import com.google.devtools.build.lib.analysis.TransitiveInfoCollection;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
import com.google.devtools.build.lib.cmdline.LabelValidator;
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.events.Location;
import com.google.devtools.build.lib.packages.Attribute;
import com.google.devtools.build.lib.packages.AttributeMap;
import com.google.devtools.build.lib.packages.AttributeValueSource;
import com.google.devtools.build.lib.packages.ClassObjectConstructor;
import com.google.devtools.build.lib.packages.ImplicitOutputsFunction.SkylarkImplicitOutputsFunctionWithCallback;
import com.google.devtools.build.lib.packages.ImplicitOutputsFunction.SkylarkImplicitOutputsFunctionWithMap;
import com.google.devtools.build.lib.packages.NativeClassObjectConstructor;
import com.google.devtools.build.lib.packages.Package.NameConflictException;
import com.google.devtools.build.lib.packages.PackageFactory;
import com.google.devtools.build.lib.packages.PackageFactory.PackageContext;
import com.google.devtools.build.lib.packages.PredicateWithMessage;
import com.google.devtools.build.lib.packages.RuleClass;
import com.google.devtools.build.lib.packages.RuleClass.Builder;
import com.google.devtools.build.lib.packages.RuleClass.Builder.RuleClassType;
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.SkylarkClassObjectConstructor;
import com.google.devtools.build.lib.packages.SkylarkExportable;
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;
import com.google.devtools.build.lib.skylarkinterface.Param;
import com.google.devtools.build.lib.skylarkinterface.SkylarkPrinter;
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.Environment;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.syntax.EvalUtils;
import com.google.devtools.build.lib.syntax.FuncallExpression;
import com.google.devtools.build.lib.syntax.FunctionSignature;
import com.google.devtools.build.lib.syntax.Runtime;
import com.google.devtools.build.lib.syntax.SkylarkCallbackFunction;
import com.google.devtools.build.lib.syntax.SkylarkDict;
import com.google.devtools.build.lib.syntax.SkylarkList;
import com.google.devtools.build.lib.syntax.SkylarkNestedSet;
import com.google.devtools.build.lib.syntax.SkylarkSignatureProcessor;
import com.google.devtools.build.lib.syntax.SkylarkUtils;
import com.google.devtools.build.lib.syntax.Type;
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.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ExecutionException;

/**
 * A helper class to provide an easier API for Skylark rule definitions.
 */
public class SkylarkRuleClassFunctions {

  // TODO(bazel-team): Copied from ConfiguredRuleClassProvider for the transition from built-in
  // rules to skylark extensions. Using the same instance would require a large refactoring.
  // If we don't want to support old built-in rules and Skylark simultaneously
  // (except for transition phase) it's probably OK.
  private static final LoadingCache<String, Label> labelCache =
      CacheBuilder.newBuilder().build(new CacheLoader<String, Label>() {
        @Override
        public Label load(String from) throws Exception {
          try {
            return Label.parseAbsolute(from, false);
          } catch (LabelSyntaxException e) {
            throw new Exception(from);
          }
        }
      });

  // TODO(bazel-team): Remove the code duplication (BaseRuleClasses and this class).
  /** Parent rule class for non-executable non-test Skylark rules. */
  public static final RuleClass baseRule =
      BaseRuleClasses.commonCoreAndSkylarkAttributes(
              PlatformSemantics.platformAttributes(
                  BaseRuleClasses.nameAttribute(
                          new RuleClass.Builder("$base_rule", RuleClassType.ABSTRACT, true))
                      .add(attr("expect_failure", STRING))))
          .build();

  /** Parent rule class for executable non-test Skylark rules. */
  public static final RuleClass binaryBaseRule =
      new RuleClass.Builder("$binary_base_rule", RuleClassType.ABSTRACT, true, baseRule)
          .add(attr("args", STRING_LIST))
          .add(attr("output_licenses", LICENSE))
          .build();

  /** Parent rule class for test Skylark rules. */
  public static final RuleClass getTestBaseRule(String toolsRepository) {
    return new RuleClass.Builder("$test_base_rule", RuleClassType.ABSTRACT, true, baseRule)
        .add(attr("size", STRING).value("medium").taggable()
            .nonconfigurable("used in loading phase rule validation logic"))
        .add(attr("timeout", STRING).taggable()
            .nonconfigurable("used in loading phase rule validation logic").value(
                new Attribute.ComputedDefault() {
                  @Override
                  public Object getDefault(AttributeMap rule) {
                    TestSize size = TestSize.getTestSize(rule.get("size", Type.STRING));
                    if (size != null) {
                      String timeout = size.getDefaultTimeout().toString();
                      if (timeout != null) {
                        return timeout;
                      }
                    }
                    return "illegal";
                  }
                }))
        .add(attr("flaky", BOOLEAN).value(false).taggable()
            .nonconfigurable("taggable - called in Rule.getRuleTags"))
        .add(attr("shard_count", INTEGER).value(-1))
        .add(attr("local", BOOLEAN).value(false).taggable()
            .nonconfigurable("policy decision: this should be consistent across configurations"))
        .add(attr("args", STRING_LIST))
        // Input files for every test action
        .add(attr("$test_runtime", LABEL_LIST).cfg(HOST).value(ImmutableList.of(
            labelCache.getUnchecked(toolsRepository + "//tools/test:runtime"))))
        // Input files for test actions collecting code coverage
        .add(attr("$coverage_support", LABEL)
            .cfg(HOST)
            .value(labelCache.getUnchecked("//tools/defaults:coverage_support")))
        // Used in the one-per-build coverage report generation action.
        .add(attr("$coverage_report_generator", LABEL)
            .cfg(HOST)
            .value(labelCache.getUnchecked("//tools/defaults:coverage_report_generator"))
            .singleArtifact())
        .add(attr(":run_under", LABEL).cfg(DATA).value(RUN_UNDER))
        .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 ClassObjectConstructor struct = NativeClassObjectConstructor.STRUCT;

  @SkylarkSignature(
    name = "DefaultInfo",
    returnType = ClassObjectConstructor.class,
    doc =
        "A provider that is provided by every rule, even if it is not returned explicitly. "
            + "A <code>DefaultInfo</code> accepts the following parameters:"
            + "<ul>"
            + "<li><code>files</code></li>"
            + "<li><code>runfiles</code></li>"
            + "<li><code>data_runfiles</code></li>"
            + "<li><code>default_runfiles</code></li>"
            + "</ul>"
            + "Each instance of the default provider contains the following standard "
            + "fields: "
            + "<ul>"
            + "<li><code>files</code></li>"
            + "<li><code>files_to_run</code></li>"
            + "<li><code>data_runfiles</code></li>"
            + "<li><code>default_runfiles</code></li>"
            + "</ul>"
  )
  private static final ClassObjectConstructor defaultInfo = DefaultProvider.SKYLARK_CONSTRUCTOR;

  @SkylarkSignature(
    name = "OutputGroupInfo",
    returnType = ClassObjectConstructor.class,
    doc =
        "Provides information about output groups the rule provides.<br>"
            + "Instantiate this provider with <br>"
            + "<pre class=language-python>"
            + "OutputGroupInfo(group1 = &lt;files&gt;, group2 = &lt;files&gt;...)</pre>"
            + "See <a href=\"../rules.html#output-groups\">Output Groups</a> for more information"
  )
  private static final ClassObjectConstructor outputGroupInfo =
      OutputGroupProvider.SKYLARK_CONSTRUCTOR;

  // TODO(bazel-team): Move to a "testing" namespace module. Normally we'd pass an objectType
  // to @SkylarkSignature to do this, but that doesn't work here because we're exposing an already-
  // configured BaseFunction, rather than defining a new BuiltinFunction. This should wait for
  // better support from the Skylark/Java interface, or perhaps support for first-class modules.
  @SkylarkSignature(name = "Actions", returnType = SkylarkClassObjectConstructor.class, doc =
      "<i>(Note: This is a provider type. Don't instantiate it yourself; use it to retrieve a "
          + "provider object from a <a href=\"Target.html\">Target</a>.)</i>"
          + "<br/><br/>"
          + "Provides access to the <a href=\"Action.html\">actions</a> generated by a rule. There "
          + "is one field, <code>by_file</code>, which is a dictionary from an output of the rule "
          + "to its corresponding generating action. "
          + "<br/><br/>"
          + "This is designed for testing rules, and should not be accessed outside of test logic. "
          + "This provider is only available for targets generated by rules that have "
          + "<a href=\"globals.html#rule._skylark_testable\">_skylark_testable</a> set to "
          + "<code>True</code>."
  )
  private static final ClassObjectConstructor actions = ActionsProvider.SKYLARK_CONSTRUCTOR;

  @SkylarkSignature(
    name = "provider",
    returnType = ClassObjectConstructor.class,
    doc =
        "Creates a declared provider 'constructor'. The return value of this "
            + "function can be used to create \"struct-like\" values. Example:<br>"
            + "<pre class=\"language-python\">data = provider()\n"
            + "d = data(x = 2, y = 3)\n"
            + "print(d.x + d.y) # prints 5</pre>",
    parameters = {
      @Param(
        name = "doc",
        type = String.class,
        defaultValue = "''",
        doc =
            "A description of the provider that can be extracted by documentation generating tools."
      )
    },
    useLocation = true
  )
  private static final BuiltinFunction provider =
      new BuiltinFunction("provider") {
        public ClassObjectConstructor invoke(String doc, Location location) {
          return new SkylarkClassObjectConstructor(
              "<no name>", // name is set on export.
              location);
        }
      };

  // 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 "
            + "from BUILD files.",
    returnType = BaseFunction.class,
    parameters = {
      @Param(
        name = "implementation",
        type = BaseFunction.class,
        doc =
            "the function implementing this rule, must have exactly one parameter: "
                + "<a href=\"ctx.html\">ctx</a>. The function is called during the analysis "
                + "phase for each instance of the rule. It can access the attributes "
                + "provided by the user. It must create actions to generate all the declared "
                + "outputs."
      ),
      @Param(
        name = "test",
        type = Boolean.class,
        defaultValue = "False",
        doc =
            "Whether this rule is a test rule. "
                + "If True, the rule must end with <code>_test</code> (otherwise it must "
                + "not), and there must be an action that generates "
                + "<code>ctx.outputs.executable</code>."
      ),
      @Param(
        name = "attrs",
        type = SkylarkDict.class,
        noneable = true,
        defaultValue = "None",
        doc =
            "dictionary to declare all the attributes of the rule. It maps from an attribute "
                + "name to an attribute object (see <a href=\"attr.html\">attr</a> module). "
                + "Attributes starting with <code>_</code> are private, and can be used to "
                + "add an implicit dependency on a label. The attribute <code>name</code> is "
                + "implicitly added and must not be specified. Attributes "
                + "<code>visibility</code>, <code>deprecation</code>, <code>tags</code>, "
                + "<code>testonly</code>, and <code>features</code> are implicitly added and "
                + "cannot be overriden."
      ),
      // TODO(bazel-team): need to give the types of these builtin attributes
      @Param(
        name = "outputs",
        type = SkylarkDict.class,
        callbackEnabled = true,
        noneable = true,
        defaultValue = "None",
        doc =
            "outputs of this rule. "
                + "It is a dictionary mapping from string to a template name. "
                + "For example: <code>{\"ext\": \"%{name}.ext\"}</code>. <br>"
                + "The dictionary key becomes an attribute in <code>ctx.outputs</code>. "
                + "Similar to computed dependency rule attributes, you can also specify the "
                + "name of a function that returns the dictionary. This function can access "
                + "all rule attributes that are listed as parameters in its function "
                + "signature. For example, <code>outputs = _my_func</code> with "
                + "<code>def _my_func(srcs, deps):</code> has access to the attributes "
                + "'srcs' and 'deps' (if defined)."
      ),
      @Param(
        name = "executable",
        type = Boolean.class,
        defaultValue = "False",
        doc =
            "whether this rule is marked as executable or not. If True, "
                + "there must be an action that generates "
                + "<code>ctx.outputs.executable</code>."
      ),
      @Param(
        name = "output_to_genfiles",
        type = Boolean.class,
        defaultValue = "False",
        doc =
            "If true, the files will be generated in the genfiles directory instead of the "
                + "bin directory. Unless you need it for compatibility with existing rules "
                + "(e.g. when generating header files for C++), do not set this flag."
      ),
      @Param(
        name = "fragments",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc =
            "List of names of configuration fragments that the rule requires "
                + "in target configuration."
      ),
      @Param(
        name = "host_fragments",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc =
            "List of names of configuration fragments that the rule requires "
                + "in host configuration."
      ),
      @Param(
        name = "_skylark_testable",
        type = Boolean.class,
        defaultValue = "False",
        doc =
            "<i>(Experimental)</i><br/><br/>"
                + "If true, this rule will expose its actions for inspection by rules that "
                + "depend on it via an <a href=\"globals.html#Actions\">Actions</a> "
                + "provider. The provider is also available to the rule itself by calling "
                + "<a href=\"ctx.html#created_actions\">ctx.created_actions()</a>."
                + "<br/><br/>"
                + "This should only be used for testing the analysis-time behavior of "
                + "Skylark rules. This flag may be removed in the future."
      ),
      @Param(
        name = "toolchains",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc =
            "<i>(Experimental)</i><br/><br/>"
                + "If set, the set of toolchains this rule requires. Toolchains will be "
                + "found by checking the current platform, and provided to the rule "
                + "implementation via <code>ctx.toolchain</code>."
      ),
      @Param(
        name = "doc",
        type = String.class,
        defaultValue = "''",
        doc = "A description of the rule that can be extracted by documentation generating tools."
      )
    },
    useAst = true,
    useEnvironment = true
  )
  private static final BuiltinFunction rule =
      new BuiltinFunction("rule") {
        @SuppressWarnings({"rawtypes", "unchecked"}) // castMap produces
        // an Attribute.Builder instead of a Attribute.Builder<?> but it's OK.
        public BaseFunction invoke(
            BaseFunction implementation,
            Boolean test,
            Object attrs,
            Object implicitOutputs,
            Boolean executable,
            Boolean outputToGenfiles,
            SkylarkList fragments,
            SkylarkList hostFragments,
            Boolean skylarkTestable,
            SkylarkList<String> toolchains,
            String doc,
            FuncallExpression ast,
            Environment funcallEnv)
            throws EvalException, ConversionException {
          funcallEnv.checkLoadingOrWorkspacePhase("rule", ast.getLocation());
          RuleClassType type = test ? RuleClassType.TEST : RuleClassType.NORMAL;
          RuleClass parent =
              test
                  ? getTestBaseRule(SkylarkUtils.getToolsRepository(funcallEnv))
                  : (executable ? binaryBaseRule : baseRule);

          // We'll set the name later, pass the empty string for now.
          RuleClass.Builder builder = new RuleClass.Builder("", type, true, parent);
          ImmutableList<Pair<String, SkylarkAttr.Descriptor>> attributes =
              attrObjectToAttributesList(attrs, ast);

          if (skylarkTestable) {
            builder.setSkylarkTestable();
          }

          if (executable || test) {
            addAttribute(
                ast.getLocation(),
                builder,
                attr("$is_executable", BOOLEAN)
                    .value(true)
                    .nonconfigurable("Called from RunCommand.isExecutable, which takes a Target")
                    .build());
            builder.setOutputsDefaultExecutable();
          }

          if (implicitOutputs != Runtime.NONE) {
            if (implicitOutputs instanceof BaseFunction) {
              BaseFunction func = (BaseFunction) implicitOutputs;
              SkylarkCallbackFunction callback = new SkylarkCallbackFunction(func, ast, funcallEnv);
              builder.setImplicitOutputsFunction(
                  new SkylarkImplicitOutputsFunctionWithCallback(callback, ast.getLocation()));
            } else {
              builder.setImplicitOutputsFunction(
                  new SkylarkImplicitOutputsFunctionWithMap(
                      ImmutableMap.copyOf(
                          castMap(
                              implicitOutputs,
                              String.class,
                              String.class,
                              "implicit outputs of the rule class"))));
            }
          }

          if (outputToGenfiles) {
            builder.setOutputToGenfiles();
          }

          builder.requiresConfigurationFragmentsBySkylarkModuleName(
              fragments.getContents(String.class, "fragments"));
          builder.requiresHostConfigurationFragmentsBySkylarkModuleName(
              hostFragments.getContents(String.class, "host_fragments"));
          builder.setConfiguredTargetFunction(implementation);
          builder.setRuleDefinitionEnvironment(funcallEnv);
          builder.addRequiredToolchains(collectToolchainLabels(toolchains, ast));

          return new RuleFunction(builder, type, attributes, ast.getLocation());
        }
      };

  protected static ImmutableList<Pair<String, Descriptor>> attrObjectToAttributesList(
      Object attrs, FuncallExpression ast) throws EvalException {
    ImmutableList.Builder<Pair<String, Descriptor>> attributes = ImmutableList.builder();

    if (attrs != Runtime.NONE) {
      for (Map.Entry<String, Descriptor> attr :
          castMap(attrs, String.class, Descriptor.class, "attrs").entrySet()) {
        Descriptor attrDescriptor = attr.getValue();
        AttributeValueSource source = attrDescriptor.getValueSource();
        String attrName = source.convertToNativeName(attr.getKey(), ast.getLocation());
        attributes.add(Pair.of(attrName, attrDescriptor));
      }
    }
    return attributes.build();
  }

  private static void addAttribute(
      Location location, RuleClass.Builder builder, Attribute attribute) throws EvalException {
    try {
      builder.addOrOverrideAttribute(attribute);
    } catch (IllegalArgumentException ex) {
      throw new EvalException(location, ex);
    }
  }

  private static ImmutableList<Label> collectToolchainLabels(
      Iterable<String> rawLabels, FuncallExpression ast) throws EvalException {
    ImmutableList.Builder<Label> requiredToolchains = new ImmutableList.Builder<>();
    for (String rawLabel : rawLabels) {
      try {
        Label toolchainLabel = Label.parseAbsolute(rawLabel);
        requiredToolchains.add(toolchainLabel);
      } catch (LabelSyntaxException e) {
        throw new EvalException(
            ast.getLocation(),
            String.format("Unable to parse toolchain %s: %s", rawLabel, e.getMessage()),
            e);
      }
    }

    return requiredToolchains.build();
  }

  @SkylarkSignature(
    name = "aspect",
    doc =
        "Creates a new aspect. The result of this function must be stored in a global value. "
            + "Please see the <a href=\"../aspects.md\">introduction to Aspects</a> for more "
            + "details.",
    returnType = SkylarkAspect.class,
    parameters = {
      @Param(
        name = "implementation",
        type = BaseFunction.class,
        doc =
            "the function implementing this aspect. Must have two parameters: "
                + "<a href=\"Target.html\">Target</a> (the target to which the aspect is "
                + "applied) and <a href=\"ctx.html\">ctx</a>. Attributes of the target are "
                + "available via ctx.rule field. The function is called during the analysis "
                + "phase for each application of an aspect to a target."
      ),
      @Param(
        name = "attr_aspects",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc =
            "List of attribute names.  The aspect propagates along dependencies specified "
                + "by attributes of a target with this name. The list can also contain a single "
                + "string '*': in that case aspect propagates along all dependencies of a target."
      ),
      @Param(
        name = "attrs",
        type = SkylarkDict.class,
        noneable = true,
        defaultValue = "None",
        doc =
            "dictionary to declare all the attributes of the aspect.  "
                + "It maps from an attribute name to an attribute object "
                + "(see <a href=\"attr.html\">attr</a> module). "
                + "Aspect attributes are available to implementation function as fields of ctx "
                + "parameter. Implicit attributes starting with <code>_</code> must have default "
                + "values, and have type <code>label</code> or <code>label_list</code>. "
                + "Explicit attributes must have type <code>string</code>, and must use the "
                + "<code>values</code> restriction. If explicit attributes are present, the "
                + "aspect can only be used with rules that have attributes of the same name and "
                + "type, with valid values."
      ),
      @Param(
        name = "required_aspect_providers",
        type = SkylarkList.class,
        defaultValue = "[]",
        // todo(dslomov): Document once it works.
        doc = "<not available>"
      ),
      @Param(
        name = "provides",
        type = SkylarkList.class,
        defaultValue = "[]",
        // todo(dslomov): Document once it works.
        doc = "<not available>"
      ),
      @Param(
        name = "fragments",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc =
            "List of names of configuration fragments that the aspect requires "
                + "in target configuration."
      ),
      @Param(
        name = "host_fragments",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc =
            "List of names of configuration fragments that the aspect requires "
                + "in host configuration."
      ),
      @Param(
        name = "toolchains",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc =
            "<i>(Experimental)</i><br/><br/>"
                + "If set, the set of toolchains this rule requires. Toolchains will be "
                + "found by checking the current platform, and provided to the rule "
                + "implementation via <code>ctx.toolchain</code>."
      ),
      @Param(
        name = "doc",
        type = String.class,
        defaultValue = "''",
        doc = "A description of the aspect that can be extracted by documentation generating tools."
      )
    },
    useEnvironment = true,
    useAst = true
  )
  private static final BuiltinFunction aspect =
      new BuiltinFunction("aspect") {
        public SkylarkAspect invoke(
            BaseFunction implementation,
            SkylarkList attributeAspects,
            Object attrs,
            SkylarkList requiredAspectProvidersArg,
            SkylarkList providesArg,
            SkylarkList fragments,
            SkylarkList hostFragments,
            SkylarkList<String> toolchains,
            String doc,
            FuncallExpression ast,
            Environment funcallEnv)
            throws EvalException {
          ImmutableList.Builder<String> attrAspects = ImmutableList.builder();
          for (Object attributeAspect : attributeAspects) {
            String attrName = STRING.convert(attributeAspect, "attr_aspects");

            if (attrName.equals("*") && attributeAspects.size() != 1) {
              throw new EvalException(
                  ast.getLocation(), "'*' must be the only string in 'attr_aspects' list");
            }

            if (!attrName.startsWith("_")) {
              attrAspects.add(attrName);
            } else {
              // Implicit attribute names mean either implicit or late-bound attributes
              // (``$attr`` or ``:attr``). Depend on both.
              attrAspects.add(
                  AttributeValueSource.COMPUTED_DEFAULT.convertToNativeName(attrName, location));
              attrAspects.add(
                  AttributeValueSource.LATE_BOUND.convertToNativeName(attrName, location));
            }
          }

          ImmutableList<Pair<String, SkylarkAttr.Descriptor>> descriptors =
              attrObjectToAttributesList(attrs, ast);
          ImmutableList.Builder<Attribute> attributes = ImmutableList.builder();
          ImmutableSet.Builder<String> requiredParams = ImmutableSet.builder();
          for (Pair<String, Descriptor> nameDescriptorPair : descriptors) {
            String nativeName = nameDescriptorPair.first;
            boolean hasDefault = nameDescriptorPair.second.hasDefault();
            Attribute attribute = nameDescriptorPair.second.build(nameDescriptorPair.first);
            if (attribute.getType() == Type.STRING
                && ((String) attribute.getDefaultValue(null)).isEmpty()) {
              hasDefault = false; // isValueSet() is always true for attr.string.
            }
            if (!Attribute.isImplicit(nativeName)) {
              if (!attribute.checkAllowedValues() || attribute.getType() != Type.STRING) {
                throw new EvalException(
                    ast.getLocation(),
                    String.format(
                        "Aspect parameter attribute '%s' must have type 'string' and use the "
                            + "'values' restriction.",
                        nativeName));
              }
              if (!hasDefault) {
                requiredParams.add(nativeName);
              } else {
                PredicateWithMessage<Object> allowed = attribute.getAllowedValues();
                Object defaultVal = attribute.getDefaultValue(null);
                if (!allowed.apply(defaultVal)) {
                  throw new EvalException(
                      ast.getLocation(),
                      String.format(
                          "Aspect parameter attribute '%s' has a bad default value: %s",
                          nativeName, allowed.getErrorReason(defaultVal)));
                }
              }
            } else if (!hasDefault) { // Implicit attribute
              String skylarkName = "_" + nativeName.substring(1);
              throw new EvalException(
                  ast.getLocation(),
                  String.format("Aspect attribute '%s' has no default value.", skylarkName));
            }
            attributes.add(attribute);
          }

          for (Object o : providesArg) {
            if (!SkylarkAttr.isProvider(o)) {
              throw new EvalException(
                  ast.getLocation(),
                  String.format(
                      "Illegal argument: element in 'provides' is of unexpected type. "
                          + "Should be list of providers, but got %s. ",
                      EvalUtils.getDataTypeName(o, true)));
            }
          }

          return new SkylarkAspect(
              implementation,
              attrAspects.build(),
              attributes.build(),
              SkylarkAttr.buildProviderPredicate(
                  requiredAspectProvidersArg, "required_aspect_providers", ast.getLocation()),
              SkylarkAttr.getSkylarkProviderIdentifiers(providesArg, ast.getLocation()),
              requiredParams.build(),
              ImmutableSet.copyOf(fragments.getContents(String.class, "fragments")),
              ImmutableSet.copyOf(hostFragments.getContents(String.class, "host_fragments")),
              collectToolchainLabels(toolchains, ast),
              funcallEnv);
        }
      };

  /** The implementation for the magic function "rule" that creates Skylark rule classes */
  public static final class RuleFunction extends BaseFunction implements SkylarkExportable {
    private RuleClass.Builder builder;

    private RuleClass ruleClass;
    private final RuleClassType type;
    private ImmutableList<Pair<String, SkylarkAttr.Descriptor>> attributes;
    private final Location definitionLocation;
    private Label skylarkLabel;

    public RuleFunction(Builder builder, RuleClassType type,
        ImmutableList<Pair<String, SkylarkAttr.Descriptor>> attributes,
        Location definitionLocation) {
      super("rule", FunctionSignature.KWARGS);
      this.builder = builder;
      this.type = type;
      this.attributes = attributes;
      this.definitionLocation = definitionLocation;
    }

    @Override
    @SuppressWarnings("unchecked") // the magic hidden $pkg_context variable is guaranteed
    // to be a PackageContext
    public Object call(Object[] args, FuncallExpression ast, Environment env)
        throws EvalException, InterruptedException, ConversionException {
      env.checkLoadingPhase(getName(), ast.getLocation());
      if (ruleClass == null) {
        throw new EvalException(ast.getLocation(),
            "Invalid rule class hasn't been exported by a Skylark file");
      }

      for (Attribute attribute : ruleClass.getAttributes()) {
        // TODO(dslomov): If a Skylark parameter extractor is specified for this aspect, its
        // attributes may not be required.
        for (Map.Entry<String, ImmutableSet<String>> attrRequirements :
            attribute.getRequiredAspectParameters().entrySet()) {
          for (String required : attrRequirements.getValue()) {
            if (!ruleClass.hasAttr(required, Type.STRING)) {
              throw new EvalException(definitionLocation, String.format(
                  "Aspect %s requires rule %s to specify attribute '%s' with type string.",
                  attrRequirements.getKey(),
                  ruleClass.getName(),
                  required));
            }
          }
        }
      }

      BuildLangTypedAttributeValuesMap attributeValues =
          new BuildLangTypedAttributeValuesMap((Map<String, Object>) args[0]);
      try {
        PackageContext pkgContext = (PackageContext) env.lookup(PackageFactory.PKG_CONTEXT);
        if (pkgContext == null) {
          throw new EvalException(ast.getLocation(),
              "Cannot instantiate a rule when loading a .bzl file. Rules can only be called from "
                  + "a BUILD file (possibly via a macro).");
        }
        RuleFactory.createAndAddRule(
            pkgContext,
            ruleClass,
            attributeValues,
            ast,
            env,
            pkgContext.getAttributeContainerFactory().apply(ruleClass));
        return Runtime.NONE;
      } catch (InvalidRuleException | NameConflictException e) {
        throw new EvalException(ast.getLocation(), e.getMessage());
      }
    }

    /**
     * Export a RuleFunction from a Skylark file with a given name.
     */
    public void export(Label skylarkLabel, String ruleClassName) throws EvalException {
      Preconditions.checkState(ruleClass == null && builder != null);
      this.skylarkLabel = skylarkLabel;
      if (type == RuleClassType.TEST != TargetUtils.isTestRuleName(ruleClassName)) {
        throw new EvalException(definitionLocation, "Invalid rule class name '" + ruleClassName
            + "', test rule class names must end with '_test' and other rule classes must not");
      }
      for (Pair<String, SkylarkAttr.Descriptor> attribute : attributes) {
        SkylarkAttr.Descriptor descriptor = attribute.getSecond();

        addAttribute(definitionLocation, builder,
            descriptor.build(attribute.getFirst()));
      }
      this.ruleClass = builder.build(ruleClassName);

      this.builder = null;
      this.attributes = null;
    }

    @VisibleForTesting
    public RuleClass getRuleClass() {
      Preconditions.checkState(ruleClass != null && builder == null);
      return ruleClass;
    }

    @Override
    public boolean isExported() {
      return skylarkLabel != null;
    }

    @Override
    public void repr(SkylarkPrinter printer) {
      printer.append("<rule>");
    }
  }

  /**
   * All classes of values that need special processing after they are exported from an extension
   * file.
   *
   * <p>Order in list is significant: all {@link SkylarkAspect}s need to be exported before {@link
   * RuleFunction}s etc.
   */
  private static final ImmutableList<Class<? extends SkylarkExportable>> EXPORTABLES =
      ImmutableList.of(
          SkylarkClassObjectConstructor.class, SkylarkAspect.class, RuleFunction.class);

  @SkylarkSignature(
    name = "Label",
    doc =
        "Creates a Label referring to a BUILD target. Use "
            + "this function only when you want to give a default value for the label attributes. "
            + "The argument must refer to an absolute label. "
            + "Example: <br><pre class=language-python>Label(\"//tools:default\")</pre>",
    returnType = Label.class,
    objectType = Label.class,
    parameters = {
      @Param(name = "label_string", type = String.class, doc = "the label string."),
      @Param(
        name = "relative_to_caller_repository",
        type = Boolean.class,
        defaultValue = "False",
        named = true,
        positional = false,
        doc =
            "whether the label should be resolved relative to the label of the file this "
                + "function is called from."
      )
    },
    useLocation = true,
    useEnvironment = true
  )
  private static final BuiltinFunction label =
      new BuiltinFunction("Label") {
        @SuppressWarnings({"unchecked", "unused"})
        public Label invoke(
            String labelString, Boolean relativeToCallerRepository, Location loc, Environment env)
            throws EvalException {
          Label parentLabel = null;
          if (relativeToCallerRepository) {
            parentLabel = env.getCallerLabel();
          } else {
            parentLabel = env.getGlobals().getTransitiveLabel();
          }
          try {
            if (parentLabel != null) {
              LabelValidator.parseAbsoluteLabel(labelString);
              labelString = parentLabel.getRelative(labelString).getUnambiguousCanonicalForm();
            }
            return labelCache.get(labelString);
          } catch (LabelValidator.BadLabelException | LabelSyntaxException | ExecutionException e) {
            throw new EvalException(loc, "Illegal absolute label syntax: " + labelString);
          }
        }
      };

  // We want the Label ctor to show up under the Label documentation, but to be a "global
  // function." Thus, we create a global Label object here, which just points to the Skylark
  // function above.
  @SkylarkSignature(name = "Label",
      documented = false)
  private static final BuiltinFunction globalLabel = label;

  @SkylarkSignature(
    name = "FileType",
    doc =
        "Deprecated. Creates a file filter from a list of strings. For example, to match "
            + "files ending with .cc or .cpp, use: "
            + "<pre class=language-python>FileType([\".cc\", \".cpp\"])</pre>",
    returnType = SkylarkFileType.class,
    objectType = SkylarkFileType.class,
    parameters = {
      @Param(
        name = "types",
        type = SkylarkList.class,
        generic1 = String.class,
        defaultValue = "[]",
        doc = "a list of the accepted file extensions."
      )
    }
  )
  private static final BuiltinFunction fileType =
      new BuiltinFunction("FileType") {
        public SkylarkFileType invoke(SkylarkList types) throws EvalException {
          return SkylarkFileType.of(types.getContents(String.class, "types"));
        }
      };

  // We want the FileType ctor to show up under the FileType documentation, but to be a "global
  // function." Thus, we create a global FileType object here, which just points to the Skylark
  // function above.
  @SkylarkSignature(name = "FileType",
      documented = false)
  private static final BuiltinFunction globalFileType = fileType;

  @SkylarkSignature(
    name = "to_proto",
    doc =
        "Creates a text message from the struct parameter. This method only works if all "
            + "struct elements (recursively) are strings, ints, booleans, other structs or a "
            + "list of these types. Quotes and new lines in strings are escaped. "
            + "Keys are iterated in the sorted order. "
            + "Examples:<br><pre class=language-python>"
            + "struct(key=123).to_proto()\n# key: 123\n\n"
            + "struct(key=True).to_proto()\n# key: true\n\n"
            + "struct(key=[1, 2, 3]).to_proto()\n# key: 1\n# key: 2\n# key: 3\n\n"
            + "struct(key='text').to_proto()\n# key: \"text\"\n\n"
            + "struct(key=struct(inner_key='text')).to_proto()\n"
            + "# key {\n#   inner_key: \"text\"\n# }\n\n"
            + "struct(key=[struct(inner_key=1), struct(inner_key=2)]).to_proto()\n"
            + "# key {\n#   inner_key: 1\n# }\n# key {\n#   inner_key: 2\n# }\n\n"
            + "struct(key=struct(inner_key=struct(inner_inner_key='text'))).to_proto()\n"
            + "# key {\n#    inner_key {\n#     inner_inner_key: \"text\"\n#   }\n# }\n</pre>",
    objectType = SkylarkClassObject.class,
    returnType = String.class,
    parameters = {
      // TODO(bazel-team): shouldn't we accept any ClassObject?
      @Param(name = "self", type = SkylarkClassObject.class, doc = "this struct.")
    },
    useLocation = true
  )
  private static final BuiltinFunction toProto =
      new BuiltinFunction("to_proto") {
        public String invoke(SkylarkClassObject self, Location loc) throws EvalException {
          StringBuilder sb = new StringBuilder();
          printProtoTextMessage(self, sb, 0, loc);
          return sb.toString();
        }

        private void printProtoTextMessage(
            ClassObject object, StringBuilder sb, int indent, Location loc) throws EvalException {
          // For determinism sort the keys alphabetically
          List<String> keys = new ArrayList<>(object.getKeys());
          Collections.sort(keys);
          for (String key : keys) {
            printProtoTextMessage(key, object.getValue(key), sb, indent, loc);
          }
        }

        private void printProtoTextMessage(
            String key, Object value, StringBuilder sb, int indent, Location loc, String container)
            throws EvalException {
          if (value instanceof ClassObject) {
            print(sb, key + " {", indent);
            printProtoTextMessage((ClassObject) value, sb, indent + 1, loc);
            print(sb, "}", indent);
          } else if (value instanceof String) {
            print(
                sb,
                key + ": \"" + escapeDoubleQuotesAndBackslashesAndNewlines((String) value) + "\"",
                indent);
          } else if (value instanceof Integer) {
            print(sb, key + ": " + value, indent);
          } else if (value instanceof Boolean) {
            // We're relying on the fact that Java converts Booleans to Strings in the same way
            // as the protocol buffers do.
            print(sb, key + ": " + value, indent);
          } else {
            throw new EvalException(
                loc,
                "Invalid text format, expected a struct, a string, a bool, or an int but got a "
                    + EvalUtils.getDataTypeName(value)
                    + " for "
                    + container
                    + " '"
                    + key
                    + "'");
          }
        }

        private void printProtoTextMessage(
            String key, Object value, StringBuilder sb, int indent, Location loc)
            throws EvalException {
          if (value instanceof SkylarkList) {
            for (Object item : ((SkylarkList) value)) {
              // TODO(bazel-team): There should be some constraint on the fields of the structs
              // in the same list but we ignore that for now.
              printProtoTextMessage(key, item, sb, indent, loc, "list element in struct field");
            }
          } else {
            printProtoTextMessage(key, value, sb, indent, loc, "struct field");
          }
        }

        private void print(StringBuilder sb, String text, int indent) {
          for (int i = 0; i < indent; i++) {
            sb.append("  ");
          }
          sb.append(text);
          sb.append("\n");
        }
      };

  /**
   * Escapes the given string for use in proto/JSON string.
   *
   * <p>This escapes double quotes, backslashes, and newlines.
   */
  private static String escapeDoubleQuotesAndBackslashesAndNewlines(String string) {
    return TextFormat.escapeDoubleQuotesAndBackslashes(string).replace("\n", "\\n");
  }

  @SkylarkSignature(
    name = "to_json",
    doc =
        "Creates a JSON string from the struct parameter. This method only works if all "
            + "struct elements (recursively) are strings, ints, booleans, other structs or a "
            + "list of these types. Quotes and new lines in strings are escaped. "
            + "Examples:<br><pre class=language-python>"
            + "struct(key=123).to_json()\n# {\"key\":123}\n\n"
            + "struct(key=True).to_json()\n# {\"key\":true}\n\n"
            + "struct(key=[1, 2, 3]).to_json()\n# {\"key\":[1,2,3]}\n\n"
            + "struct(key='text').to_json()\n# {\"key\":\"text\"}\n\n"
            + "struct(key=struct(inner_key='text')).to_json()\n"
            + "# {\"key\":{\"inner_key\":\"text\"}}\n\n"
            + "struct(key=[struct(inner_key=1), struct(inner_key=2)]).to_json()\n"
            + "# {\"key\":[{\"inner_key\":1},{\"inner_key\":2}]}\n\n"
            + "struct(key=struct(inner_key=struct(inner_inner_key='text'))).to_json()\n"
            + "# {\"key\":{\"inner_key\":{\"inner_inner_key\":\"text\"}}}\n</pre>",
    objectType = SkylarkClassObject.class,
    returnType = String.class,
    parameters = {
      // TODO(bazel-team): shouldn't we accept any ClassObject?
      @Param(name = "self", type = SkylarkClassObject.class, doc = "this struct.")
    },
    useLocation = true
  )
  private static final BuiltinFunction toJson =
      new BuiltinFunction("to_json") {
        public String invoke(SkylarkClassObject self, Location loc) throws EvalException {
          StringBuilder sb = new StringBuilder();
          printJson(self, sb, loc, "struct field", null);
          return sb.toString();
        }

        private void printJson(
            Object value, StringBuilder sb, Location loc, String container, String key)
            throws EvalException {
          if (value == Runtime.NONE) {
            sb.append("null");
          } else if (value instanceof ClassObject) {
            sb.append("{");

            String join = "";
            for (String subKey : ((ClassObject) value).getKeys()) {
              sb.append(join);
              join = ",";
              sb.append("\"");
              sb.append(subKey);
              sb.append("\":");
              printJson(((ClassObject) value).getValue(subKey), sb, loc, "struct field", subKey);
            }
            sb.append("}");
          } else if (value instanceof List) {
            sb.append("[");
            String join = "";
            for (Object item : ((List) value)) {
              sb.append(join);
              join = ",";
              printJson(item, sb, loc, "list element in struct field", key);
            }
            sb.append("]");
          } else if (value instanceof String) {
            sb.append("\"");
            sb.append(jsonEscapeString((String) value));
            sb.append("\"");
          } else if (value instanceof Integer || value instanceof Boolean) {
            sb.append(value);
          } else {
            String errorMessage =
                "Invalid text format, expected a struct, a string, a bool, or an int "
                    + "but got a "
                    + EvalUtils.getDataTypeName(value)
                    + " for "
                    + container;
            if (key != null) {
              errorMessage += " '" + key + "'";
            }
            throw new EvalException(loc, errorMessage);
          }
        }

        private String jsonEscapeString(String string) {
          return escapeDoubleQuotesAndBackslashesAndNewlines(string)
              .replace("\r", "\\r")
              .replace("\t", "\\t");
        }
      };

  @SkylarkSignature(name = "output_group",
      documented = false, //  TODO(dslomov): document.
      objectType =  TransitiveInfoCollection.class,
      returnType = SkylarkNestedSet.class,
      parameters = {
          @Param(name = "self", type = TransitiveInfoCollection.class, doc =
              "this target"
          ),
          @Param(name = "group_name", type = String.class, doc =
              "Output group name"
          )
      }
  )
  private static final BuiltinFunction output_group = new BuiltinFunction("output_group") {
    public SkylarkNestedSet invoke(TransitiveInfoCollection self, String group) {
      OutputGroupProvider provider = OutputGroupProvider.get(self);
      NestedSet<Artifact> result = provider != null
          ? provider.getOutputGroup(group)
          : NestedSetBuilder.<Artifact>emptySet(Order.STABLE_ORDER);
      return SkylarkNestedSet.of(Artifact.class, result);
    }
  };

  static {
    SkylarkSignatureProcessor.configureSkylarkFunctions(SkylarkRuleClassFunctions.class);
  }
}