src/main/java/com/google/devtools/build/lib/packages/Attribute.java

// 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.packages;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.base.Verify;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Iterables;
import com.google.common.collect.Ordering;
import com.google.common.collect.Sets;
import com.google.devtools.build.lib.analysis.TransitiveInfoProvider;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.events.EventHandler;
import com.google.devtools.build.lib.events.Location;
import com.google.devtools.build.lib.syntax.ClassObject;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.syntax.EvalUtils;
import com.google.devtools.build.lib.syntax.Runtime;
import com.google.devtools.build.lib.syntax.SkylarkCallbackFunction;
import com.google.devtools.build.lib.syntax.Type;
import com.google.devtools.build.lib.syntax.Type.ConversionException;
import com.google.devtools.build.lib.util.FileType;
import com.google.devtools.build.lib.util.FileTypeSet;
import com.google.devtools.build.lib.util.Preconditions;
import com.google.devtools.build.lib.util.StringUtil;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.atomic.AtomicReference;
import javax.annotation.concurrent.Immutable;

/**
 * Metadata of a rule attribute. Contains the attribute name and type, and an
 * default value to be used if none is provided in a rule declaration in a BUILD
 * file. Attributes are immutable, and may be shared by more than one rule (for
 * example, <code>foo_binary</code> and <code>foo_library</code> may share many
 * attributes in common).
 */
@Immutable
public final class Attribute implements Comparable<Attribute> {

  public static final Predicate<RuleClass> ANY_RULE = Predicates.alwaysTrue();

  public static final Predicate<RuleClass> NO_RULE = Predicates.alwaysFalse();

  /**
   * Wraps the information necessary to construct an Aspect.
   */
  private abstract static class RuleAspect<C extends AspectClass> {
    protected final C aspectClass;
    protected final Function<Rule, AspectParameters> parametersExtractor;

    protected RuleAspect(C aspectClass, Function<Rule, AspectParameters> parametersExtractor) {
      this.aspectClass = aspectClass;
      this.parametersExtractor = parametersExtractor;
    }

    public String getName() {
      return this.aspectClass.getName();
    }

    public ImmutableSet<String> getRequiredParameters() {
      return ImmutableSet.<String>of();
    }

    public abstract Aspect getAspect(Rule rule);

    public C getAspectClass() {
      return aspectClass;
    }
  }

  private static class NativeRuleAspect extends RuleAspect<NativeAspectClass> {
    public NativeRuleAspect(NativeAspectClass aspectClass,
        Function<Rule, AspectParameters> parametersExtractor) {
      super(aspectClass, parametersExtractor);
    }

    @Override
    public Aspect getAspect(Rule rule) {
      return Aspect.forNative(aspectClass, parametersExtractor.apply(rule));
    }
  }

  private static class SkylarkRuleAspect extends RuleAspect<SkylarkAspectClass> {
    private final SkylarkAspect aspect;

    public SkylarkRuleAspect(SkylarkAspect aspect) {
      super(aspect.getAspectClass(), aspect.getDefaultParametersExtractor());
      this.aspect = aspect;
    }

    @Override
    public ImmutableSet<String> getRequiredParameters() {
      return aspect.getParamAttributes();
    }

    @Override
    public Aspect getAspect(Rule rule) {
      AspectParameters parameters = parametersExtractor.apply(rule);
      return Aspect.forSkylark(aspectClass, aspect.getDefinition(parameters), parameters);
    }
  }

  /** A RuleAspect that just wraps a pre-existing Aspect that doesn't vary with the Rule. */
  private static class PredefinedRuleAspect extends RuleAspect<AspectClass> {
    private Aspect aspect;

    public PredefinedRuleAspect(Aspect aspect) {
      super(aspect.getAspectClass(), null);
      this.aspect = aspect;
    }

    @Override
    public Aspect getAspect(Rule rule) {
      return aspect;
    }
  }

  /**
   * A configuration transition.
   */
  public interface Transition {
    /**
     * Usually, a non-existent entry in the configuration transition table indicates an error.
     * Unfortunately, that means that we need to always build the full table. This method allows a
     * transition to indicate that a non-existent entry indicates a self transition, i.e., that the
     * resulting configuration is the same as the current configuration. This can simplify the code
     * needed to set up the transition table.
     */
    boolean defaultsToSelf();
  }

  /**
   * A configuration split transition; this should be used to transition to multiple configurations
   * simultaneously. Note that the corresponding rule implementations must have special support to
   * handle this.
   *
   * <p>{@code T} must always be {@code BuildOptions}, but it can't be defined that way because
   * the symbol isn't available here.
   */
  // TODO(bazel-team): Serializability constraints?
  public interface SplitTransition<T> extends Transition {
    /**
     * Return the list of {@code BuildOptions} after splitting; empty if not applicable.
     */
    List<T> split(T buildOptions);
  }

  /**
   * Declaration how the configuration should change when following a label or label list attribute.
   */
  public enum ConfigurationTransition implements Transition {
    /** No transition, i.e., the same configuration as the current. */
    NONE,

    /** Transition to the host configuration. */
    HOST,

    /** Transition to a null configuration (applies to, e.g., input files). */
    NULL,

    /** Transition from the target configuration to the data configuration. */
    // TODO(bazel-team): Move this elsewhere.
    DATA,

    /**
     * Transition to one or more configurations. To obtain the actual child configurations,
     * invoke {@link Attribute#getSplitTransition(Rule)}. See {@link SplitTransition}.
     **/
    SPLIT(true);

    private boolean defaultsToSelf;

    ConfigurationTransition() {
      this(false);
    }

    ConfigurationTransition(boolean defaultsToSelf) {
      this.defaultsToSelf = defaultsToSelf;
    }

    @Override
    public boolean defaultsToSelf() {
      return defaultsToSelf;
    }
  }

  private enum PropertyFlag {
    MANDATORY,
    EXECUTABLE,
    UNDOCUMENTED,
    TAGGABLE,

    /**
     * Whether the list attribute is order-independent and can be sorted.
     */
    ORDER_INDEPENDENT,

    /**
     * Whether the allowedRuleClassesForLabels or allowedFileTypesForLabels are
     * set to custom values. If so, and the attribute is called "deps", the
     * legacy deps checking is skipped, and the new stricter checks are used
     * instead. For non-"deps" attributes, this allows skipping the check if it
     * would pass anyway, as the default setting allows any rule classes and
     * file types.
     */
    STRICT_LABEL_CHECKING,

    /**
     * Set for things that would cause the a compile or lint-like action to
     * be executed when the input changes.  Used by compile_one_dependency.
     * Set for attributes like hdrs and srcs on cc_ rules or srcs on java_
     * or py_rules.  Generally not set on data/resource attributes.
     */
    DIRECT_COMPILE_TIME_INPUT,

    /**
     * Whether the value of the list type attribute must not be an empty list.
     */
    NON_EMPTY,

    /**
     * Verifies that the referenced rule produces a single artifact. Note that this check happens
     * on a per label basis, i.e. the check happens separately for every label in a label list.
     */
    SINGLE_ARTIFACT,

    /**
     * Whether we perform silent ruleclass filtering of the dependencies of the label type
     * attribute according to their rule classes. I.e. elements of the list which don't match the
     * allowedRuleClasses predicate or not rules will be filtered out without throwing any errors.
     * This flag is introduced to handle plugins, do not use it in other cases.
     */
    SILENT_RULECLASS_FILTER,

    // TODO(bazel-team): This is a hack introduced because of the bad design of the original rules.
    // Depot cleanup would be too expensive, but don't migrate this to Skylark.
    /**
     * Whether to perform analysis time filetype check on this label-type attribute or not.
     * If the flag is set, we skip the check that applies the allowedFileTypes filter
     * to generated files. Do not use this if avoidable.
     */
    SKIP_ANALYSIS_TIME_FILETYPE_CHECK,

    /**
     * Whether the value of the attribute should come from a given set of values.
     */
    CHECK_ALLOWED_VALUES,

    /**
     * Whether this attribute is opted out of "configurability", i.e. the ability to determine
     * its value based on properties of the build configuration.
     */
    NONCONFIGURABLE,

    /**
     * Whether we should skip dependency validation checks done by
     * {@link com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider.PrerequisiteValidator}
     * (for visibility, etc.).
     */
    SKIP_PREREQ_VALIDATOR_CHECKS,

    /**
     * Whether we should check constraints on this attribute even if default enforcement policy
     * would skip it. See
     * {@link com.google.devtools.build.lib.analysis.constraints.ConstraintSemantics} for more on
     * constraints.
     */
    CHECK_CONSTRAINTS_OVERRIDE,

    /**
     * Whether we should skip constraints checking on this attribute even if default enforcement
     * policy would check it.
     */
    SKIP_CONSTRAINTS_OVERRIDE,

    /**
     * Whether we should use output_licenses to check the licences on this attribute.
     */
    OUTPUT_LICENSES,
  }

  // TODO(bazel-team): modify this interface to extend Predicate and have an extra error
  // message function like AllowedValues does
  /**
   * A predicate-like class that determines whether an edge between two rules is valid or not.
   */
  public interface ValidityPredicate {
    /**
     * This method should return null if the edge is valid, or a suitable error message
     * if it is not. Note that warnings are not supported.
     */
    String checkValid(Rule from, Rule to);
  }

  public static final ValidityPredicate ANY_EDGE =
      new ValidityPredicate() {
        @Override
        public String checkValid(Rule from, Rule to) {
          return null;
        }
      };

  /**
   * Using this callback function, rules can set the configuration of their dependencies during the
   * analysis phase.
   */
  public interface Configurator<TConfig, TRule> {
    TConfig apply(TRule fromRule, TConfig fromConfiguration, Attribute attribute, Target toTarget);
  }

  /**
   * Provides a {@link SplitTransition} given the originating target {@link Rule}. The split
   * transition may be constant for all instances of the originating rule, or it may differ
   * based on attributes of that rule. For instance, a split transition on a rule's deps may differ
   * depending on the 'platform' attribute of the rule.
   */
  public interface SplitTransitionProvider {
    /**
     * Returns the {@link SplitTransition} given the originating rule.
     */
    SplitTransition<?> apply(Rule fromRule);
  }

  /**
   * Implementation of {@link SplitTransitionProvider} that returns a single {@link SplitTransition}
   * regardless of the originating rule.
   */
  private static class BasicSplitTransitionProvider implements SplitTransitionProvider {

    private final SplitTransition<?> splitTransition;

    BasicSplitTransitionProvider(SplitTransition<?> splitTransition) {
      this.splitTransition = splitTransition;
    }

    @Override
    public SplitTransition<?> apply(Rule fromRule) {
      return splitTransition;
    }
  }

  /**
   * A predicate class to check if the value of the attribute comes from a predefined set.
   */
  public static class AllowedValueSet implements PredicateWithMessage<Object> {

    private final Set<Object> allowedValues;

    public <T> AllowedValueSet(T... values) {
      this(Arrays.asList(values));
    }

    public AllowedValueSet(Iterable<?> values) {
      Preconditions.checkNotNull(values);
      Preconditions.checkArgument(!Iterables.isEmpty(values));
      // Do not remove <Object>: workaround for Java 7 type inference.
      allowedValues = ImmutableSet.<Object>copyOf(values);
    }

    @Override
    public boolean apply(Object input) {
      return allowedValues.contains(input);
    }

    @Override
    public String getErrorReason(Object value) {
      return String.format("has to be one of %s instead of '%s'",
          StringUtil.joinEnglishList(allowedValues, "or", "'"), value);
    }

    @VisibleForTesting
    public Collection<Object> getAllowedValues() {
      return allowedValues;
    }
  }

  public ImmutableMap<String, ImmutableSet<String>> getRequiredAspectParameters() {
    ImmutableMap.Builder<String, ImmutableSet<String>> paramBuilder = ImmutableMap.builder();
    for (RuleAspect<?> aspect : aspects) {
      paramBuilder.put(aspect.getName(), aspect.getRequiredParameters());
    }
    return paramBuilder.build();
  }

  /**
   * Creates a new attribute builder.
   *
   * @param name attribute name
   * @param type attribute type
   * @return attribute builder
   *
   * @param <TYPE> attribute type class
   */
  public static <TYPE> Attribute.Builder<TYPE> attr(String name, Type<TYPE> type) {
    return new Builder<>(name, type);
  }

  /**
   * A fluent builder for the {@code Attribute} instances.
   *
   * <p>All methods could be called only once per builder. The attribute
   * already undocumented based on its name cannot be marked as undocumented.
   */
  public static class Builder <TYPE> {
    private String name;
    private final Type<TYPE> type;
    private Transition configTransition = ConfigurationTransition.NONE;
    private Predicate<RuleClass> allowedRuleClassesForLabels = Predicates.alwaysTrue();
    private Predicate<RuleClass> allowedRuleClassesForLabelsWarning = Predicates.alwaysFalse();
    private Configurator<?, ?> configurator;
    private SplitTransitionProvider splitTransitionProvider;
    private FileTypeSet allowedFileTypesForLabels;
    private ValidityPredicate validityPredicate = ANY_EDGE;
    private Object value;
    private AttributeValueSource valueSource = AttributeValueSource.DIRECT;
    private boolean valueSet;
    private Predicate<AttributeMap> condition;
    private Set<PropertyFlag> propertyFlags = EnumSet.noneOf(PropertyFlag.class);
    private PredicateWithMessage<Object> allowedValues = null;
    private ImmutableList<ImmutableSet<SkylarkProviderIdentifier>> mandatoryProvidersList =
        ImmutableList.<ImmutableSet<SkylarkProviderIdentifier>>of();
    private ImmutableList<ImmutableList<Class<? extends TransitiveInfoProvider>>>
        mandatoryNativeProvidersList = ImmutableList.of();
    private HashMap<String, RuleAspect<?>> aspects = new LinkedHashMap<>();

    /**
     * Creates an attribute builder with given name and type. This attribute is optional, uses
     * target configuration and has a default value the same as its type default value. This
     * attribute will be marked as undocumented if its name starts with the dollar sign ({@code $})
     * or colon ({@code :}).
     *
     * @param name attribute name
     * @param type attribute type
     */
    public Builder(String name, Type<TYPE> type) {
      this.name = Preconditions.checkNotNull(name);
      this.type = Preconditions.checkNotNull(type);
      if (isImplicit(name) || isLateBound(name)) {
        setPropertyFlag(PropertyFlag.UNDOCUMENTED, "undocumented");
      }
    }

    private Builder<TYPE> setPropertyFlag(PropertyFlag flag, String propertyName) {
      Preconditions.checkState(
          !propertyFlags.contains(flag), "%s flag is already set", propertyName);
      propertyFlags.add(flag);
      return this;
    }

    /**
     * Sets the property flag of the corresponding name if exists, otherwise throws an Exception.
     * Only meant to use from Skylark, do not use from Java.
     */
    public Builder<TYPE> setPropertyFlag(String propertyName) {
      PropertyFlag flag = null;
      try {
        flag = PropertyFlag.valueOf(propertyName);
      } catch (IllegalArgumentException e) {
        throw new IllegalArgumentException("unknown attribute flag " + propertyName);
      }
      setPropertyFlag(flag, propertyName);
      return this;
    }

    /**
     * Makes the built attribute mandatory.
     */
    public Builder<TYPE> mandatory() {
      return setPropertyFlag(PropertyFlag.MANDATORY, "mandatory");
    }

    /**
     * Makes the built attribute non empty, meaning the attribute cannot have an empty list value.
     * Only applicable for list type attributes.
     */
    public Builder<TYPE> nonEmpty() {
      Preconditions.checkNotNull(type.getListElementType(), "attribute '%s' must be a list", name);
      return setPropertyFlag(PropertyFlag.NON_EMPTY, "non_empty");
    }

    /**
     * Makes the built attribute producing a single artifact.
     */
    public Builder<TYPE> singleArtifact() {
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "attribute '%s' must be a label-valued type", name);
      return setPropertyFlag(PropertyFlag.SINGLE_ARTIFACT, "single_artifact");
    }

    /**
     * Forces silent ruleclass filtering on the label type attribute.
     * This flag is introduced to handle plugins, do not use it in other cases.
     */
    public Builder<TYPE> silentRuleClassFilter() {
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "must be a label-valued type");
      return setPropertyFlag(PropertyFlag.SILENT_RULECLASS_FILTER, "silent_ruleclass_filter");
    }

    /**
     * Skip analysis time filetype check. Don't use it if avoidable.
     */
    public Builder<TYPE> skipAnalysisTimeFileTypeCheck() {
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "must be a label-valued type");
      return setPropertyFlag(PropertyFlag.SKIP_ANALYSIS_TIME_FILETYPE_CHECK,
          "skip_analysis_time_filetype_check");
    }

    /**
     * Mark the built attribute as order-independent.
     */
    public Builder<TYPE> orderIndependent() {
      Preconditions.checkNotNull(type.getListElementType(), "attribute '%s' must be a list", name);
      return setPropertyFlag(PropertyFlag.ORDER_INDEPENDENT, "order-independent");
    }

    /**
     * Mark the built attribute as to use output_licenses for license checking.
     */
    public Builder<TYPE> useOutputLicenses() {
      Preconditions.checkState(BuildType.isLabelType(type), "must be a label type");
      return setPropertyFlag(PropertyFlag.OUTPUT_LICENSES, "output_license");
    }

    /**
     * Defines the configuration transition for this attribute.
     */
    public Builder<TYPE> cfg(SplitTransitionProvider splitTransitionProvider) {
      Preconditions.checkState(this.configTransition == ConfigurationTransition.NONE,
          "the configuration transition is already set");

      this.splitTransitionProvider = Preconditions.checkNotNull(splitTransitionProvider);
      this.configTransition = ConfigurationTransition.SPLIT;
      return this;
    }

    /**
     * Defines the configuration transition for this attribute. Defaults to
     * {@code NONE}.
     */
    public Builder<TYPE> cfg(SplitTransition<?> configTransition) {
      return cfg(new BasicSplitTransitionProvider(Preconditions.checkNotNull(configTransition)));
    }

    /**
     * Defines the configuration transition for this attribute. Defaults to
     * {@code NONE}.
     */
    public Builder<TYPE> cfg(Transition configTransition) {
      Preconditions.checkState(this.configTransition == ConfigurationTransition.NONE,
          "the configuration transition is already set");
      Preconditions.checkArgument(configTransition != ConfigurationTransition.SPLIT,
          "split transitions must be defined using the SplitTransition object");
      if (configTransition instanceof SplitTransition) {
        return cfg((SplitTransition<?>) configTransition);
      } else {
        this.configTransition = configTransition;
        return this;
      }
    }

    public Builder<TYPE> cfg(Configurator<?, ?> configurator) {
      this.configurator = configurator;
      return this;
    }

    /**
     * Requires the attribute target to be executable; only for label or label
     * list attributes. Defaults to {@code false}.
     */
    public Builder<TYPE> exec() {
      return setPropertyFlag(PropertyFlag.EXECUTABLE, "executable");
    }

    /**
     * Indicates that the attribute (like srcs or hdrs) should be used as an input when calculating
     * compile_one_dependency.
     */
    public Builder<TYPE> direct_compile_time_input() {
      return setPropertyFlag(PropertyFlag.DIRECT_COMPILE_TIME_INPUT,
                             "direct_compile_time_input");
    }

    /**
     * Makes the built attribute undocumented.
     *
     * @param reason explanation why the attribute is undocumented. This is not
     *        used but required for documentation
     */
    public Builder<TYPE> undocumented(String reason) {
      return setPropertyFlag(PropertyFlag.UNDOCUMENTED, "undocumented");
    }

    /**
     * Sets the attribute default value. The type of the default value must
     * match the type parameter. (e.g. list=[], integer=0, string="",
     * label=null). The {@code defaultValue} must be immutable.
     *
     * <p>If defaultValue is of type Label and is a target, that target will
     * become an implicit dependency of the Rule; we will load the target
     * (and its dependencies) if it encounters the Rule and build the target
     * if needs to apply the Rule.
     */
    public Builder<TYPE> value(TYPE defaultValue) {
      Preconditions.checkState(!valueSet, "the default value is already set");
      value = defaultValue;
      valueSet = true;
      return this;
    }

    /**
     * See value(TYPE) above. This method is only meant for Skylark usage.
     *
     * <p>The parameter {@code context} is relevant iff the default value is a Label string.
     * In this case, {@code context} must point to the parent Label in order to be able to convert
     * the default value string to a proper Label.
     */
    public Builder<TYPE> defaultValue(Object defaultValue, Object context)
        throws ConversionException {
      Preconditions.checkState(!valueSet, "the default value is already set");
      value = type.convert(defaultValue, "attribute " + name, context);
      valueSet = true;
      return this;
    }

    /** See value(TYPE) above. This method is only meant for Skylark usage. */
    public Builder<TYPE> defaultValue(Object defaultValue) throws ConversionException {
      return defaultValue(defaultValue, null);
    }

    public boolean isValueSet() {
      return valueSet;
    }

    /**
     * Sets the attribute default value to a computed default value - use this when the default
     * value is a function of other attributes of the Rule. The type of the computed default value
     * for a mandatory attribute must match the type parameter: (e.g. list=[], integer=0, string="",
     * label=null). The {@code defaultValue} implementation must be immutable.
     *
     * <p>If the computed default returns a Label that is a target, that target will become an
     * implicit dependency of this Rule; we will load the target (and its dependencies) if it
     * encounters the Rule and build the target if needs to apply the Rule.
     */
    public Builder<TYPE> value(ComputedDefault defaultValue) {
      Preconditions.checkState(!valueSet, "the default value is already set");
      value = defaultValue;
      valueSource = AttributeValueSource.COMPUTED_DEFAULT;
      valueSet = true;
      return this;
    }

    /**
     * Sets the attribute default value to a Skylark computed default template. Like a native
     * Computed Default, this allows a Skylark-defined Rule Class to specify that the default value
     * of an attribute is a function of other attributes of the Rule.
     *
     * <p>During the loading phase, the computed default template will be specialized for each rule
     * it applies to. Those rules' attribute values will not be references to {@link
     * SkylarkComputedDefaultTemplate}s, but instead will be references to {@link
     * SkylarkComputedDefault}s.
     *
     * <p>If the computed default returns a Label that is a target, that target will become an
     * implicit dependency of this Rule; we will load the target (and its dependencies) if it
     * encounters the Rule and build the target if needs to apply the Rule.
     */
    public Builder<TYPE> value(SkylarkComputedDefaultTemplate skylarkComputedDefaultTemplate) {
      Preconditions.checkState(!valueSet, "the default value is already set");
      value = skylarkComputedDefaultTemplate;
      valueSource = AttributeValueSource.COMPUTED_DEFAULT;
      valueSet = true;
      return this;
    }

    /**
     * Sets the attribute default value to be late-bound, i.e., it is derived from the build
     * configuration.
     */
    public Builder<TYPE> value(LateBoundDefault<?> defaultValue) {
      Preconditions.checkState(!valueSet, "the default value is already set");
      Preconditions.checkState(name.isEmpty() || isLateBound(name));
      value = defaultValue;
      valueSource = AttributeValueSource.LATE_BOUND;
      valueSet = true;
      return this;
    }

    /**
     * Returns where the value of this attribute comes from. Useful only for Skylark.
     */
    public AttributeValueSource getValueSource() {
      return valueSource;
    }

    /**
     * Sets a condition predicate. The default value of the attribute only applies if the condition
     * evaluates to true. If the value is explicitly provided, then this condition is ignored.
     *
     * <p>The condition is only evaluated if the attribute is not explicitly set, and after all
     * explicit attributes have been set. It can generally not access default values of other
     * attributes.
     */
    public Builder<TYPE> condition(Predicate<AttributeMap> condition) {
      Preconditions.checkState(this.condition == null, "the condition is already set");
      this.condition = condition;
      return this;
    }

    /**
     * Switches on the capability of an attribute to be published to the rule's
     * tag set.
     */
    public Builder<TYPE> taggable() {
      return setPropertyFlag(PropertyFlag.TAGGABLE, "taggable");
    }

    /**
     * Disables dependency checks done by
     * {@link com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider.PrerequisiteValidator}.
     */
    public Builder<TYPE> skipPrereqValidatorCheck() {
      return setPropertyFlag(PropertyFlag.SKIP_PREREQ_VALIDATOR_CHECKS,
          "skip_prereq_validator_checks");
    }

    /**
     * Enforces constraint checking on this attribute even if default enforcement policy would skip
     * it. If default policy checks the attribute, this is a no-op.
     *
     * <p>Most attributes are enforced by default, so in the common case this call is unnecessary.
     *
     * <p>See {@link com.google.devtools.build.lib.analysis.constraints.ConstraintSemantics#getConstraintCheckedDependencies}
     * for enforcement policy details.
     */
    public Builder<TYPE> checkConstraints() {
      Verify.verify(!propertyFlags.contains(PropertyFlag.SKIP_CONSTRAINTS_OVERRIDE),
          "constraint checking is already overridden to be skipped");
      return setPropertyFlag(PropertyFlag.CHECK_CONSTRAINTS_OVERRIDE, "check_constraints");
    }

    /**
     * Skips constraint checking on this attribute even if default enforcement policy would check
     * it. If default policy skips the attribute, this is a no-op.
     *
     * <p>See {@link com.google.devtools.build.lib.analysis.constraints.ConstraintSemantics#getConstraintCheckedDependencies}
     * for enforcement policy details.
     */
    public Builder<TYPE> dontCheckConstraints() {
      Verify.verify(!propertyFlags.contains(PropertyFlag.CHECK_CONSTRAINTS_OVERRIDE),
          "constraint checking is already overridden to be checked");
      return setPropertyFlag(PropertyFlag.SKIP_CONSTRAINTS_OVERRIDE, "dont_check_constraints");
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * rule types for the labels occurring in the attribute. If the attribute
     * contains Labels of any other rule type, then an error is produced during
     * the analysis phase. Defaults to allow any types.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedRuleClasses(Iterable<String> allowedRuleClasses) {
      return allowedRuleClasses(
          new RuleClass.Builder.RuleClassNamePredicate(allowedRuleClasses));
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * rule types for the labels occurring in the attribute. If the attribute
     * contains Labels of any other rule type, then an error is produced during
     * the analysis phase. Defaults to allow any types.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedRuleClasses(Predicate<RuleClass> allowedRuleClasses) {
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "must be a label-valued type");
      propertyFlags.add(PropertyFlag.STRICT_LABEL_CHECKING);
      allowedRuleClassesForLabels = allowedRuleClasses;
      return this;
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * rule types for the labels occurring in the attribute. If the attribute
     * contains Labels of any other rule type, then an error is produced during
     * the analysis phase. Defaults to allow any types.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedRuleClasses(String... allowedRuleClasses) {
      return allowedRuleClasses(ImmutableSet.copyOf(allowedRuleClasses));
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * file types for file labels occurring in the attribute. If the attribute
     * contains labels that correspond to files of any other type, then an error
     * is produced during the analysis phase.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedFileTypes(FileTypeSet allowedFileTypes) {
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "must be a label-valued type");
      propertyFlags.add(PropertyFlag.STRICT_LABEL_CHECKING);
      allowedFileTypesForLabels = Preconditions.checkNotNull(allowedFileTypes);
      return this;
    }

    /**
     * Allow all files for legacy compatibility. All uses of this method should be audited and then
     * removed. In some cases, it's correct to allow any file, but mostly the set of files should be
     * restricted to a reasonable set.
     */
    public Builder<TYPE> legacyAllowAnyFileType() {
      return allowedFileTypes(FileTypeSet.ANY_FILE);
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * file types for file labels occurring in the attribute. If the attribute
     * contains labels that correspond to files of any other type, then an error
     * is produced during the analysis phase.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedFileTypes(FileType... allowedFileTypes) {
      return allowedFileTypes(FileTypeSet.of(allowedFileTypes));
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * rule types with warning for the labels occurring in the attribute. If the attribute
     * contains Labels of any other rule type (other than this or those set in
     * allowedRuleClasses()), then a warning is produced during
     * the analysis phase. Defaults to deny any types.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedRuleClassesWithWarning(Collection<String> allowedRuleClasses) {
      return allowedRuleClassesWithWarning(
          new RuleClass.Builder.RuleClassNamePredicate(allowedRuleClasses));
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * rule types for the labels occurring in the attribute. If the attribute
     * contains Labels of any other rule type (other than this or those set in
     * allowedRuleClasses()), then a warning is produced during
     * the analysis phase. Defaults to deny any types.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedRuleClassesWithWarning(Predicate<RuleClass> allowedRuleClasses) {
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "must be a label-valued type");
      propertyFlags.add(PropertyFlag.STRICT_LABEL_CHECKING);
      allowedRuleClassesForLabelsWarning = allowedRuleClasses;
      return this;
    }

    /**
     * If this is a label or label-list attribute, then this sets the allowed
     * rule types for the labels occurring in the attribute. If the attribute
     * contains Labels of any other rule type (other than this or those set in
     * allowedRuleClasses()), then a warning is produced during
     * the analysis phase. Defaults to deny any types.
     *
     * <p>This only works on a per-target basis, not on a per-file basis; with
     * other words, it works for 'deps' attributes, but not 'srcs' attributes.
     */
    public Builder<TYPE> allowedRuleClassesWithWarning(String... allowedRuleClasses) {
      return allowedRuleClassesWithWarning(ImmutableSet.copyOf(allowedRuleClasses));
    }

    /**
     * Sets a list of lists of mandatory native providers. Every configured target occurring in this
     * label type attribute has to provide all the providers from one of those lists, otherwise an
     * error is produced during the analysis phase.
     */
    public final Builder<TYPE> mandatoryNativeProvidersList(
        Iterable<? extends Iterable<Class<? extends TransitiveInfoProvider>>> providersList) {
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "must be a label-valued type");
      ImmutableList.Builder<ImmutableList<Class<? extends TransitiveInfoProvider>>> listBuilder
          = ImmutableList.builder();
      for (Iterable<Class<? extends TransitiveInfoProvider>> providers : providersList) {
        listBuilder.add(ImmutableList.<Class<? extends TransitiveInfoProvider>>copyOf(providers));
      }
      this.mandatoryNativeProvidersList = listBuilder.build();
      return this;
    }

    public Builder<TYPE> mandatoryNativeProviders(
        Iterable<Class<? extends TransitiveInfoProvider>> providers) {
      if (providers.iterator().hasNext()) {
        mandatoryNativeProvidersList(ImmutableList.of(providers));
      }
      return this;
    }

    /**
     * Sets a list of sets of mandatory Skylark providers. Every configured target occurring in
     * this label type attribute has to provide all the providers from one of those sets,
     * or be one of {@link #allowedRuleClasses}, otherwise an error is produced during
     * the analysis phase.
     */
    public Builder<TYPE> mandatoryProvidersList(
        Iterable<? extends Iterable<SkylarkProviderIdentifier>> providersList){
      Preconditions.checkState((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST),
          "must be a label-valued type");
      ImmutableList.Builder<ImmutableSet<SkylarkProviderIdentifier>> listBuilder
          = ImmutableList.builder();
      for (Iterable<SkylarkProviderIdentifier> providers : providersList) {
        listBuilder.add(ImmutableSet.copyOf(providers));
      }
      this.mandatoryProvidersList = listBuilder.build();
      return this;
    }

    public Builder<TYPE> legacyMandatoryProviders(String... ids) {
      return mandatoryProviders(
          Iterables.transform(Arrays.asList(ids),
              new Function<String, SkylarkProviderIdentifier>() {
        @Override
        public SkylarkProviderIdentifier apply(String s) {
          Preconditions.checkNotNull(s);
          return SkylarkProviderIdentifier.forLegacy(s);
        }
      }));
    }

    public Builder<TYPE> mandatoryProviders(Iterable<SkylarkProviderIdentifier> providers) {
      if (providers.iterator().hasNext()) {
        mandatoryProvidersList(ImmutableList.of(providers));
      }
      return this;
    }

    /**
     * Asserts that a particular parameterized aspect probably needs to be computed for all direct
     * dependencies through this attribute.
     *
     * @param evaluator function that extracts aspect parameters from rule.
     */
    public Builder<TYPE> aspect(
        NativeAspectClass aspect, Function<Rule, AspectParameters> evaluator) {
      NativeRuleAspect nativeRuleAspect = new NativeRuleAspect(aspect, evaluator);
      RuleAspect<?> oldAspect = this.aspects.put(nativeRuleAspect.getName(), nativeRuleAspect);
      if (oldAspect != null) {
        throw new AssertionError(
            String.format("Aspect %s has already been added", oldAspect.getName()));
      }
      return this;
    }

    /**
     * Asserts that a particular parameterized aspect probably needs to be computed for all direct
     * dependencies through this attribute.
     */
    public Builder<TYPE> aspect(NativeAspectClass aspect) {
      Function<Rule, AspectParameters> noParameters =
          new Function<Rule, AspectParameters>() {
            @Override
            public AspectParameters apply(Rule input) {
              return AspectParameters.EMPTY;
            }
          };
      return this.aspect(aspect, noParameters);
    }

    public Builder<TYPE> aspect(
        SkylarkAspect skylarkAspect, Location location) throws EvalException {
      SkylarkRuleAspect skylarkRuleAspect = new SkylarkRuleAspect(skylarkAspect);
      RuleAspect<?> oldAspect = this.aspects.put(skylarkAspect.getName(), skylarkRuleAspect);
      if (oldAspect != null) {
        throw new EvalException(
            location, String.format("aspect %s added more than once", skylarkAspect.getName()));
      }
      return this;
    }

    /**
     * Should only be used for deserialization.
     */
    public Builder<TYPE> aspect(final Aspect aspect) {
      PredefinedRuleAspect predefinedRuleAspect = new PredefinedRuleAspect(aspect);
      RuleAspect<?> oldAspect =
          this.aspects.put(predefinedRuleAspect.getName(), predefinedRuleAspect);
      if (oldAspect != null) {
        throw new AssertionError(
            String.format("Aspect %s has already been added", oldAspect.getName()));
      }
      return this;
    }

    /**
     * Sets the predicate-like edge validity checker.
     */
    public Builder<TYPE> validityPredicate(ValidityPredicate validityPredicate) {
      propertyFlags.add(PropertyFlag.STRICT_LABEL_CHECKING);
      this.validityPredicate = validityPredicate;
      return this;
    }

    /**
     * The value of the attribute must be one of allowedValues.
     */
    public Builder<TYPE> allowedValues(PredicateWithMessage<Object> allowedValues) {
      this.allowedValues = allowedValues;
      propertyFlags.add(PropertyFlag.CHECK_ALLOWED_VALUES);
      return this;
    }

    /**
     * Makes the built attribute "non-configurable", i.e. its value cannot be influenced by
     * the build configuration. Attributes are "configurable" unless explicitly opted out here.
     *
     * <p>Non-configurability indicates an exceptional state: there exists Blaze logic that needs
     * the attribute's value, has no access to configurations, and can't apply a workaround
     * through an appropriate {@link AbstractAttributeMapper} implementation. Scenarios like
     * this should be as uncommon as possible, so it's important we maintain clear documentation
     * on what causes them and why users consequently can't configure certain attributes.
     *
     * @param reason why this attribute can't be configurable. This isn't used by Blaze - it's
     *    solely a documentation mechanism.
     */
    public Builder<TYPE> nonconfigurable(String reason) {
      Preconditions.checkState(!reason.isEmpty());
      return setPropertyFlag(PropertyFlag.NONCONFIGURABLE, "nonconfigurable");
    }

    /**
     * Creates the attribute. Uses name, type, optionality, configuration type
     * and the default value configured by the builder.
     */
    public Attribute build() {
      return build(this.name);
    }

    /**
     * Creates the attribute. Uses type, optionality, configuration type
     * and the default value configured by the builder. Use the name
     * passed as an argument. This function is used by Skylark where the
     * name is provided only when we build. We don't want to modify the
     * builder, as it is shared in a multithreaded environment.
     */
    public Attribute build(String name) {
      Preconditions.checkState(!name.isEmpty(), "name has not been set");
      // TODO(bazel-team): Set the default to be no file type, then remove this check, and also
      // remove all allowedFileTypes() calls without parameters.

      // do not modify this.allowedFileTypesForLabels, instead create a copy.
      FileTypeSet allowedFileTypesForLabels = this.allowedFileTypesForLabels;
      if ((type == BuildType.LABEL) || (type == BuildType.LABEL_LIST)) {
        if (isPrivateAttribute(name) && allowedFileTypesForLabels == null) {
          allowedFileTypesForLabels = FileTypeSet.ANY_FILE;
        }
        Preconditions.checkNotNull(
            allowedFileTypesForLabels, "allowedFileTypesForLabels not set for %s", name);
      } else if ((type == BuildType.OUTPUT) || (type == BuildType.OUTPUT_LIST)) {
        // TODO(bazel-team): Set the default to no file type and make explicit calls instead.
        if (allowedFileTypesForLabels == null) {
          allowedFileTypesForLabels = FileTypeSet.ANY_FILE;
        }
      }
      return new Attribute(
          name,
          type,
          Sets.immutableEnumSet(propertyFlags),
          valueSet ? value : type.getDefaultValue(),
          configTransition,
          configurator,
          splitTransitionProvider,
          allowedRuleClassesForLabels,
          allowedRuleClassesForLabelsWarning,
          allowedFileTypesForLabels,
          validityPredicate,
          condition,
          allowedValues,
          mandatoryProvidersList,
          mandatoryNativeProvidersList,
          ImmutableList.copyOf(aspects.values()));
    }
  }

  /**
   * A strategy for dealing with too many computations, used when creating lookup tables for {@link
   * ComputedDefault}s.
   *
   * @param <TException> The type of exception this strategy throws if too many computations are
   *     attempted.
   */
  interface ComputationLimiter<TException extends Exception> {
    void onComputationCount(int count) throws TException;
  }

  /**
   * An implementation of {@link ComputationLimiter} that never throws. For use with
   * natively-defined {@link ComputedDefault}s, which are limited in the number of configurable
   * attributes they depend on, not on the number of different combinations of possible inputs.
   */
  private static final ComputationLimiter<RuntimeException> NULL_COMPUTATION_LIMITER =
      new ComputationLimiter<RuntimeException>() {
        @Override
        public void onComputationCount(int count) throws RuntimeException {}
      };

  /** Exception for computed default attributes that depend on too many configurable attributes. */
  private static class TooManyConfigurableAttributesException extends Exception {
    TooManyConfigurableAttributesException(int max) {
      super(
          String.format(
              "Too many configurable attributes to compute all possible values: "
                  + "Found more than %d possible values.",
              max));
    }
  }

  private static class FixedComputationLimiter
      implements ComputationLimiter<TooManyConfigurableAttributesException> {

    /** Upper bound of the number of combinations of values for a computed default attribute. */
    private static final int COMPUTED_DEFAULT_MAX_COMBINATIONS = 64;

    private static final FixedComputationLimiter INSTANCE = new FixedComputationLimiter();

    @Override
    public void onComputationCount(int count) throws TooManyConfigurableAttributesException {
      if (count > COMPUTED_DEFAULT_MAX_COMBINATIONS) {
        throw new TooManyConfigurableAttributesException(COMPUTED_DEFAULT_MAX_COMBINATIONS);
      }
    }
  }

  /**
   * Specifies how values of {@link ComputedDefault} attributes are computed based on the values of
   * other attributes.
   *
   * <p>The {@code TComputeException} type parameter allows the two specializations of this class to
   * describe whether and how their computations throw. For natively defined computed defaults,
   * computation does not throw, but for Skylark-defined computed defaults, computation may throw
   * {@link InterruptedException}.
   */
  private abstract static class ComputationStrategy<TComputeException extends Exception> {
    abstract Object compute(AttributeMap map) throws TComputeException;

    /**
     * Returns a lookup table mapping from:
     *
     * <ul>
     * <li>tuples of values that may be assigned by {@code rule} to attributes with names in {@code
     *     dependencies} (note that there may be more than one such tuple for any given rule, if any
     *     of the dependencies are configurable)
     * </ul>
     *
     * <p>to:
     *
     * <ul>
     * <li>the value {@link #compute(AttributeMap)} evaluates to when the provided {@link
     *     AttributeMap} contains the values specified by that assignment, or {@code null} if the
     *     {@link ComputationStrategy} failed to evaluate.
     * </ul>
     *
     * <p>The lookup table contains a tuple for each possible assignment to the {@code dependencies}
     * attributes. The meaning of each tuple is well-defined because {@code dependencies} is
     * ordered.
     *
     * <p>This is useful because configurable attributes may have many possible values. During the
     * loading phase a configurable attribute can't be resolved to a single value. Configuration
     * information, needed to resolve such an attribute, is only available during analysis. However,
     * any labels that a ComputedDefault attribute may evaluate to must be loaded during the loading
     * phase.
     */
    <T, TLimitException extends Exception> Map<List<Object>, T> computeValuesForAllCombinations(
        List<String> dependencies,
        Type<T> type,
        Rule rule,
        ComputationLimiter<TLimitException> limiter)
        throws TComputeException, TLimitException {
      // This will hold every (value1, value2, ..) combination of the declared dependencies.
      // Collect those combinations.
      AggregatingAttributeMapper mapper = AggregatingAttributeMapper.of(rule);
      List<Map<String, Object>> depMaps = mapper.visitAttributes(dependencies, limiter);
      // For each combination, call compute() on a specialized AttributeMap providing those
      // values.
      Map<List<Object>, T> valueMap = new HashMap<>(depMaps.size());
      for (Map<String, Object> depMap : depMaps) {
        AttributeMap attrMap = mapper.createMapBackedAttributeMap(depMap);
        Object value = compute(attrMap);
        List<Object> key = createDependencyAssignmentTuple(dependencies, attrMap);
        valueMap.put(key, type.cast(value));
      }
      return valueMap;
    }

    /**
     * Given an {@link AttributeMap}, containing an assignment to each attribute in {@code
     * dependencies}, this returns a list of the assigned values, ordered as {@code dependencies} is
     * ordered.
     */
    static List<Object> createDependencyAssignmentTuple(
        List<String> dependencies, AttributeMap attrMap) {
      ArrayList<Object> tuple = new ArrayList<>(dependencies.size());
      for (String attrName : dependencies) {
        Type<?> attrType = attrMap.getAttributeType(attrName);
        tuple.add(attrMap.get(attrName, attrType));
      }
      return tuple;
    }
  }

  /**
   * A computed default is a default value for a Rule attribute that is a function of other
   * attributes of the rule.
   *
   * <p>Attributes whose defaults are computed are first initialized to the default for their type,
   * and then the computed defaults are evaluated after all non-computed defaults have been
   * initialized. There is no defined order among computed defaults, so they must not depend on each
   * other.
   *
   * <p>If a computed default reads the value of another attribute, at least one of the following
   * must be true:
   *
   * <ol>
   * <li>The other attribute must be declared in the computed default's constructor
   * <li>The other attribute must be non-configurable ({@link Builder#nonconfigurable}
   * </ol>
   *
   * <p>The reason for enforced declarations is that, since attribute values might be configurable,
   * a computed default that depends on them may itself take multiple values. Since we have no
   * access to a target's configuration at the time these values are computed, we need the ability
   * to probe the default's *complete* dependency space. Declared dependencies allow us to do so
   * sanely. Non-configurable attributes don't have this problem because their value is fixed and
   * known even without configuration information.
   *
   * <p>Implementations of this interface must be immutable.
   */
  public abstract static class ComputedDefault {
    private final ImmutableList<String> dependencies;

    /**
     * Create a computed default that can read all non-configurable attribute values and no
     * configurable attribute values.
     */
    public ComputedDefault() {
      this(ImmutableList.<String>of());
    }

    /**
     * Create a computed default that can read all non-configurable attributes values and one
     * explicitly specified configurable attribute value
     */
    public ComputedDefault(String depAttribute) {
      this(ImmutableList.of(depAttribute));
    }

    /**
     * Create a computed default that can read all non-configurable attributes values and two
     * explicitly specified configurable attribute values.
     */
    public ComputedDefault(String depAttribute1, String depAttribute2) {
      this(ImmutableList.of(depAttribute1, depAttribute2));
    }

    /**
     * Creates a computed default that can read all non-configurable attributes and some explicitly
     * specified configurable attribute values.
     *
     * <p>This constructor should not be used by native {@link ComputedDefault} functions. The limit
     * of at-most-two depended-on configurable attributes is intended, to limit the exponential
     * growth of possible values. {@link SkylarkComputedDefault} uses this, but is limited by {@link
     * FixedComputationLimiter#COMPUTED_DEFAULT_MAX_COMBINATIONS}.
     */
    protected ComputedDefault(ImmutableList<String> dependencies) {
      // Order is important for #createDependencyAssignmentTuple.
      this.dependencies = Ordering.natural().immutableSortedCopy(dependencies);
    }

    <T> Iterable<T> getPossibleValues(Type<T> type, Rule rule) {
      final ComputedDefault owner = ComputedDefault.this;
      ComputationStrategy<RuntimeException> strategy =
          new ComputationStrategy<RuntimeException>() {
            @Override
            public Object compute(AttributeMap map) {
              return owner.getDefault(map);
            }
          };
      // Note that this uses ArrayList instead of something like ImmutableList because some
      // values may be null.
      return new ArrayList<>(
          strategy
              .computeValuesForAllCombinations(dependencies, type, rule, NULL_COMPUTATION_LIMITER)
              .values());
    }

    /** The list of configurable attributes this ComputedDefault declares it may read. */
    public ImmutableList<String> dependencies() {
      return dependencies;
    }

    /**
     * Returns the value this {@link ComputedDefault} evaluates to, given the inputs contained in
     * {@code rule}.
     */
    public abstract Object getDefault(AttributeMap rule);
  }

  /**
   * A Skylark-defined computed default, which can be precomputed for a specific {@link Rule} by
   * calling {@link #computePossibleValues}, which returns a {@link SkylarkComputedDefault} that
   * contains a lookup table.
   */
  public static final class SkylarkComputedDefaultTemplate {
    private final Type<?> type;
    private final SkylarkCallbackFunction callback;
    private final Location location;
    private final ImmutableList<String> dependencies;

    /**
     * Creates a new SkylarkComputedDefaultTemplate that allows the computation of attribute values
     * via a callback function during loading phase.
     *
     * @param type The type of the value of this attribute.
     * @param dependencies A list of all names of other attributes that are accessed by this
     *     attribute.
     * @param callback A function to compute the actual attribute value.
     * @param location The location of the Skylark function.
     */
    public SkylarkComputedDefaultTemplate(
        Type<?> type,
        ImmutableList<String> dependencies,
        SkylarkCallbackFunction callback,
        Location location) {
      this.type = Preconditions.checkNotNull(type);
      // Order is important for #createDependencyAssignmentTuple.
      this.dependencies =
          Ordering.natural().immutableSortedCopy(Preconditions.checkNotNull(dependencies));
      this.callback = Preconditions.checkNotNull(callback);
      this.location = Preconditions.checkNotNull(location);
    }

    /**
     * Returns a {@link SkylarkComputedDefault} containing a lookup table specifying the output of
     * this {@link SkylarkComputedDefaultTemplate}'s callback given each possible assignment {@code
     * rule} might make to the attributes specified by {@link #dependencies}.
     *
     * <p>If the rule is missing an attribute specified by {@link #dependencies}, or if there are
     * too many possible assignments, or if any evaluation fails, this throws {@link
     * CannotPrecomputeDefaultsException}.
     *
     * <p>May only be called after all non-{@link ComputedDefault} attributes have been set on the
     * {@code rule}.
     */
    SkylarkComputedDefault computePossibleValues(
        Attribute attr, final Rule rule, final EventHandler eventHandler)
        throws InterruptedException, CannotPrecomputeDefaultsException {

      final SkylarkComputedDefaultTemplate owner = SkylarkComputedDefaultTemplate.this;
      final String msg =
          String.format(
              "Cannot compute default value of attribute '%s' in rule '%s': ",
              attr.getPublicName(), rule.getLabel());
      final AtomicReference<EvalException> caughtEvalExceptionIfAny = new AtomicReference<>();
      ComputationStrategy<InterruptedException> strategy =
          new ComputationStrategy<InterruptedException>() {
            @Override
            public Object compute(AttributeMap map) throws InterruptedException {
              try {
                return owner.computeValue(map);
              } catch (EvalException ex) {
                caughtEvalExceptionIfAny.compareAndSet(null, ex);
                return null;
              }
            }
          };

      ImmutableList.Builder<Type<?>> dependencyTypesBuilder = ImmutableList.builder();
      Map<List<Object>, Object> lookupTable = new HashMap<>();
      try {
        for (String dependency : dependencies) {
          Attribute attribute = rule.getRuleClassObject().getAttributeByNameMaybe(dependency);
          if (attribute == null) {
            throw new AttributeNotFoundException(
                String.format("No such attribute %s in rule %s", dependency, rule.getLabel()));
          }
          dependencyTypesBuilder.add(attribute.getType());
        }
        lookupTable.putAll(
            strategy.computeValuesForAllCombinations(
                dependencies, attr.getType(), rule, FixedComputationLimiter.INSTANCE));
        if (caughtEvalExceptionIfAny.get() != null) {
          throw caughtEvalExceptionIfAny.get();
        }
      } catch (AttributeNotFoundException
          | TooManyConfigurableAttributesException
          | EvalException ex) {
        String error = msg + ex.getMessage();
        rule.reportError(error, eventHandler);
        throw new CannotPrecomputeDefaultsException(error);
      }
      return new SkylarkComputedDefault(dependencies, dependencyTypesBuilder.build(), lookupTable);
    }

    private Object computeValue(AttributeMap rule) throws EvalException, InterruptedException {
      Map<String, Object> attrValues = new HashMap<>();
      for (String attrName : rule.getAttributeNames()) {
        Attribute attr = rule.getAttributeDefinition(attrName);
        if (!attr.hasComputedDefault()) {
          Object value = rule.get(attrName, attr.getType());
          if (!EvalUtils.isNullOrNone(value)) {
            attrValues.put(attr.getName(), value);
          }
        }
      }
      return invokeCallback(attrValues);
    }

    private Object invokeCallback(Map<String, Object> attrValues)
        throws EvalException, InterruptedException {
      ClassObject attrs =
          SkylarkClassObjectConstructor.STRUCT.create(
              attrValues, "No such regular (non computed) attribute '%s'.");
      Object result = callback.call(attrs);
      try {
        return type.cast((result == Runtime.NONE) ? type.getDefaultValue() : result);
      } catch (ClassCastException ex) {
        throw new EvalException(
            location,
            String.format(
                "expected '%s', but got '%s'", type, EvalUtils.getDataTypeName(result, true)));
      }
    }

    private static class AttributeNotFoundException extends Exception {
      private AttributeNotFoundException(String message) {
        super(message);
      }
    }

    static class CannotPrecomputeDefaultsException extends Exception {
      private CannotPrecomputeDefaultsException(String message) {
        super(message);
      }
    }
  }

  /**
   * A class for computed attributes defined in Skylark.
   *
   * <p>Unlike {@link ComputedDefault}, instances of this class contain a pre-computed table of all
   * possible assignments of depended-on attributes and what the Skylark function evaluates to, and
   * {@link #getPossibleValues(Type, Rule)} and {@link #getDefault(AttributeMap)} do lookups in that
   * table.
   */
  static final class SkylarkComputedDefault extends ComputedDefault {

    private final List<Type<?>> dependencyTypes;
    private final Map<List<Object>, Object> lookupTable;

    /**
     * Creates a new SkylarkComputedDefault containing a lookup table.
     *
     * @param requiredAttributes A list of all names of other attributes that are accessed by this
     *     attribute.
     * @param dependencyTypes A list of requiredAttributes' types.
     * @param lookupTable An exhaustive mapping from requiredAttributes assignments to values this
     *     computed default evaluates to.
     */
    SkylarkComputedDefault(
        ImmutableList<String> requiredAttributes,
        ImmutableList<Type<?>> dependencyTypes,
        Map<List<Object>, Object> lookupTable) {
      super(Preconditions.checkNotNull(requiredAttributes));
      this.dependencyTypes = Preconditions.checkNotNull(dependencyTypes);
      this.lookupTable = Preconditions.checkNotNull(lookupTable);
    }

    List<Type<?>> getDependencyTypes() {
      return dependencyTypes;
    }

    Map<List<Object>, Object> getLookupTable() {
      return lookupTable;
    }

    @Override
    public Object getDefault(AttributeMap rule) {
      List<Object> key = ComputationStrategy.createDependencyAssignmentTuple(dependencies(), rule);
      Preconditions.checkState(
          lookupTable.containsKey(key),
          "Error in rule '%s': precomputed value missing for dependencies: %s. Available keys: %s.",
          rule.getLabel(),
          Iterables.toString(key),
          Iterables.toString(lookupTable.keySet()));
      return lookupTable.get(key);
    }

    @Override
    <T> Iterable<T> getPossibleValues(Type<T> type, Rule rule) {
      List<T> result = new ArrayList<>(lookupTable.size());
      for (Object obj : lookupTable.values()) {
        result.add(type.cast(obj));
      }
      return result;
    }
  }

  /**
   * Marker interface for late-bound values. Unfortunately, we can't refer to BuildConfiguration
   * right now, since that is in a separate compilation unit.
   *
   * <p>Implementations of this interface must be immutable.
   *
   * <p>Use sparingly - having different values for attributes during loading and analysis can
   * confuse users.
   */
  public interface LateBoundDefault<T> {
    /**
     * Whether to look up the label in the host configuration. This is only here for host
     * compilation tools - we usually need to look up labels in the target configuration.
     *
     * <p>This method only sets the configuration passed to {@link #resolve}. If you want the
     * dependency to also be analyzed in the host configuration, use
     * {@link ConfigurationTransition#HOST}.
     */
    boolean useHostConfiguration();

    /**
     * Returns the set of required configuration fragments, i.e., fragments that will be accessed by
     * the code.
     */
    Set<Class<?>> getRequiredConfigurationFragments();

    /**
     * The default value for the attribute that is set during the loading phase.
     */
    Object getDefault();

    /**
     * The actual value for the attribute for the analysis phase, which depends on the build
     * configuration. Note that configurations transitions are applied after the late-bound
     * attribute was evaluated.
     *
     * @param rule the rule being evaluated
     * @param attributes interface for retrieving the values of the rule's other attributes
     * @param o the configuration to evaluate with
     */
    Object resolve(Rule rule, AttributeMap attributes, T o)
        throws EvalException, InterruptedException;
  }

  /**
   * Abstract super class for label-typed {@link LateBoundDefault} implementations that simplifies
   * the client code a little and makes it a bit more type-safe.
   */
  public abstract static class LateBoundLabel<T> implements LateBoundDefault<T> {
    private final Label label;
    private final ImmutableSet<Class<?>> requiredConfigurationFragments;

    public LateBoundLabel() {
      this((Label) null);
    }

    public LateBoundLabel(Class<?>... requiredConfigurationFragments) {
      this((Label) null, requiredConfigurationFragments);
    }

    public LateBoundLabel(Label label) {
      this.label = label;
      this.requiredConfigurationFragments = ImmutableSet.of();
    }

    public LateBoundLabel(Label label, Class<?>... requiredConfigurationFragments) {
      this.label = label;
      this.requiredConfigurationFragments = ImmutableSet.copyOf(requiredConfigurationFragments);
    }

    public LateBoundLabel(String label) {
      this(Label.parseAbsoluteUnchecked(label));
    }

    public LateBoundLabel(String label, Class<?>... requiredConfigurationFragments) {
      this(Label.parseAbsoluteUnchecked(label), requiredConfigurationFragments);
    }

    @Override
    public boolean useHostConfiguration() {
      return false;
    }

    @Override
    public ImmutableSet<Class<?>> getRequiredConfigurationFragments() {
      return requiredConfigurationFragments;
    }

    @Override
    public final Label getDefault() {
      return label;
    }

    @Override
    public abstract Label resolve(Rule rule, AttributeMap attributes, T configuration);
  }

  /**
   * Abstract super class for label-list-typed {@link LateBoundDefault} implementations that
   * simplifies the client code a little and makes it a bit more type-safe.
   */
  public abstract static class LateBoundLabelList<T> implements LateBoundDefault<T> {
    private final ImmutableList<Label> labels;
    private final ImmutableSet<Class<?>> requiredConfigurationFragments;

    public LateBoundLabelList() {
      this(ImmutableList.<Label>of());
    }

    public LateBoundLabelList(List<Label> labels, Class<?>... requiredConfigurationFragments) {
      this.labels = ImmutableList.copyOf(labels);
      this.requiredConfigurationFragments = ImmutableSet.copyOf(requiredConfigurationFragments);
    }

    @Override
    public boolean useHostConfiguration() {
      return false;
    }

    @Override
    public ImmutableSet<Class<?>> getRequiredConfigurationFragments() {
      return requiredConfigurationFragments;
    }

    @Override
    public final List<Label> getDefault() {
      return labels;
    }

    @Override
    public abstract List<Label> resolve(Rule rule, AttributeMap attributes, T configuration);
  }

  private final String name;

  private final Type<?> type;

  private final Set<PropertyFlag> propertyFlags;

  // Exactly one of these conditions is true:
  // 1. defaultValue == null.
  // 2. defaultValue instanceof ComputedDefault &&
  //    type.isValid(defaultValue.getDefault())
  // 3. defaultValue instanceof SkylarkComputedDefaultTemplate &&
  //    type.isValid(defaultValue.computePossibleValues().getDefault())
  // 4. type.isValid(defaultValue).
  // 5. defaultValue instanceof LateBoundDefault &&
  //    type.isValid(defaultValue.getDefault(configuration))
  // (We assume a hypothetical Type.isValid(Object) predicate.)
  private final Object defaultValue;

  private final Transition configTransition;

  private final Configurator<?, ?> configurator;
  private final SplitTransitionProvider splitTransitionProvider;

  /**
   * For label or label-list attributes, this predicate returns which rule
   * classes are allowed for the targets in the attribute.
   */
  private final Predicate<RuleClass> allowedRuleClassesForLabels;

  /**
   * For label or label-list attributes, this predicate returns which rule
   * classes are allowed for the targets in the attribute with warning.
   */
  private final Predicate<RuleClass> allowedRuleClassesForLabelsWarning;

  /**
   * For label or label-list attributes, this predicate returns which file
   * types are allowed for targets in the attribute that happen to be file
   * targets (rather than rules).
   */
  private final FileTypeSet allowedFileTypesForLabels;

  /**
   * This predicate-like object checks
   * if the edge between two rules using this attribute is valid
   * in the dependency graph. Returns null if valid, otherwise an error message.
   */
  private final ValidityPredicate validityPredicate;

  private final Predicate<AttributeMap> condition;

  private final PredicateWithMessage<Object> allowedValues;

  private final ImmutableList<ImmutableSet<SkylarkProviderIdentifier>> mandatoryProvidersList;

  private final ImmutableList<ImmutableList<Class<? extends TransitiveInfoProvider>>>
      mandatoryNativeProvidersList;

  private final ImmutableList<RuleAspect<?>> aspects;

  /**
   * Constructs a rule attribute with the specified name, type and default
   * value.
   *
   * @param name the name of the attribute
   * @param type the type of the attribute
   * @param defaultValue the default value to use for this attribute if none is
   *        specified in rule declaration in the BUILD file. Must be null, or of
   *        type "type". May be an instance of ComputedDefault, in which case
   *        its getDefault() method must return an instance of "type", or null.
   *        Must be immutable.
   * @param configTransition the configuration transition for this attribute
   *        (which must be of type LABEL, LABEL_LIST, NODEP_LABEL or
   *        NODEP_LABEL_LIST).
   */
  private Attribute(
      String name,
      Type<?> type,
      Set<PropertyFlag> propertyFlags,
      Object defaultValue,
      Transition configTransition,
      Configurator<?, ?> configurator,
      SplitTransitionProvider splitTransitionProvider,
      Predicate<RuleClass> allowedRuleClassesForLabels,
      Predicate<RuleClass> allowedRuleClassesForLabelsWarning,
      FileTypeSet allowedFileTypesForLabels,
      ValidityPredicate validityPredicate,
      Predicate<AttributeMap> condition,
      PredicateWithMessage<Object> allowedValues,
      ImmutableList<ImmutableSet<SkylarkProviderIdentifier>> mandatoryProvidersList,
      ImmutableList<ImmutableList<Class<? extends TransitiveInfoProvider>>>
          mandatoryNativeProvidersList,
      ImmutableList<RuleAspect<?>> aspects) {
    Preconditions.checkNotNull(configTransition);
    Preconditions.checkArgument(
        (configTransition == ConfigurationTransition.NONE && configurator == null)
        || type == BuildType.LABEL || type == BuildType.LABEL_LIST
        || type == BuildType.NODEP_LABEL || type == BuildType.NODEP_LABEL_LIST,
        "Configuration transitions can only be specified for label or label list attributes");
    Preconditions.checkArgument(
        isLateBound(name) == (defaultValue instanceof LateBoundDefault),
        "late bound attributes require a default value that is late bound (and vice versa): %s",
        name);
    if (isLateBound(name)) {
      LateBoundDefault<?> lateBoundDefault = (LateBoundDefault<?>) defaultValue;
      Preconditions.checkArgument((configurator == null),
          "a late bound attribute cannot specify a configurator");
      Preconditions.checkArgument(!lateBoundDefault.useHostConfiguration()
          || (configTransition == ConfigurationTransition.HOST),
          "a late bound default value using the host configuration must use the host transition");
    }

    this.name = name;
    this.type = type;
    this.propertyFlags = propertyFlags;
    this.defaultValue = defaultValue;
    this.configTransition = configTransition;
    this.configurator = configurator;
    this.splitTransitionProvider = splitTransitionProvider;
    this.allowedRuleClassesForLabels = allowedRuleClassesForLabels;
    this.allowedRuleClassesForLabelsWarning = allowedRuleClassesForLabelsWarning;
    this.allowedFileTypesForLabels = allowedFileTypesForLabels;
    this.validityPredicate = validityPredicate;
    this.condition = condition;
    this.allowedValues = allowedValues;
    this.mandatoryProvidersList = mandatoryProvidersList;
    this.mandatoryNativeProvidersList = mandatoryNativeProvidersList;
    this.aspects = aspects;
  }

  /**
   * Returns the name of this attribute.
   */
  public String getName() {
    return name;
  }

  /**
   * Returns the public name of this attribute. This is the name we use in Skylark code
   * and we can use it to display to the end-user.
   * Implicit and late-bound attributes start with '_' (instead of '$' or ':').
   */
  public String getPublicName() {
    return getSkylarkName(getName());
  }

  /**
   * Returns the logical type of this attribute. (May differ from the actual
   * representation as a value in the build interpreter; for example, an
   * attribute may logically be a list of labels, but be represented as a list
   * of strings.)
   */
  public Type<?> getType() {
    return type;
  }

  private boolean getPropertyFlag(PropertyFlag flag) {
    return propertyFlags.contains(flag);
  }

  /**
   *  Returns true if this parameter is mandatory.
   */
  public boolean isMandatory() {
    return getPropertyFlag(PropertyFlag.MANDATORY);
  }

  /**
   *  Returns true if this list parameter cannot have an empty list as a value.
   */
  public boolean isNonEmpty() {
    return getPropertyFlag(PropertyFlag.NON_EMPTY);
  }

  /**
   *  Returns true if this label parameter must produce a single artifact.
   */
  public boolean isSingleArtifact() {
    return getPropertyFlag(PropertyFlag.SINGLE_ARTIFACT);
  }

  /**
   *  Returns true if this label type parameter is checked by silent ruleclass filtering.
   */
  public boolean isSilentRuleClassFilter() {
    return getPropertyFlag(PropertyFlag.SILENT_RULECLASS_FILTER);
  }

  /**
   *  Returns true if this label type parameter skips the analysis time filetype check.
   */
  public boolean isSkipAnalysisTimeFileTypeCheck() {
    return getPropertyFlag(PropertyFlag.SKIP_ANALYSIS_TIME_FILETYPE_CHECK);
  }

  /**
   *  Returns true if this parameter is order-independent.
   */
  public boolean isOrderIndependent() {
    return getPropertyFlag(PropertyFlag.ORDER_INDEPENDENT);
  }

  /**
   *  Returns true if output_licenses should be used for checking licensing.
   */
  public boolean useOutputLicenses() {
    return getPropertyFlag(PropertyFlag.OUTPUT_LICENSES);
  }

  /**
   * Returns the configuration transition for this attribute for label or label
   * list attributes. For other attributes it will always return {@code NONE}.
   */
  public Transition getConfigurationTransition() {
    return configTransition;
  }

  /**
   * Returns the configurator instance for this attribute for label or label list attributes.
   * For other attributes it will always return {@code null}.
   */
  public Configurator<?, ?> getConfigurator() {
    return configurator;
  }

  /**
   * Returns the split configuration transition for this attribute.
   *
   * @param rule the originating {@link Rule} which owns this attribute
   * @return a SplitTransition<BuildOptions> object
   * @throws IllegalStateException if {@link #hasSplitConfigurationTransition} is not true
   */
  public SplitTransition<?> getSplitTransition(Rule rule) {
    Preconditions.checkState(hasSplitConfigurationTransition());
    return splitTransitionProvider.apply(rule);
  }

  /**
   * Returns true if this attribute transitions on a split transition.
   * See {@link SplitTransition}.
   */
  public boolean hasSplitConfigurationTransition() {
    return (splitTransitionProvider != null);
  }

  /**
   * Returns whether the target is required to be executable for label or label
   * list attributes. For other attributes it always returns {@code false}.
   */
  public boolean isExecutable() {
    return getPropertyFlag(PropertyFlag.EXECUTABLE);
  }

  /**
   * Returns {@code true} iff the rule is a direct input for an action.
   */
  public boolean isDirectCompileTimeInput() {
    return getPropertyFlag(PropertyFlag.DIRECT_COMPILE_TIME_INPUT);
  }

  /**
   * Returns {@code true} iff this attribute requires documentation.
   */
  public boolean isDocumented() {
    return !getPropertyFlag(PropertyFlag.UNDOCUMENTED);
  }

  /**
   * Returns {@code true} iff this attribute should be published to the rule's
   * tag set. Note that not all Type classes support tag conversion.
   */
  public boolean isTaggable() {
    return getPropertyFlag(PropertyFlag.TAGGABLE);
  }

  public boolean isStrictLabelCheckingEnabled() {
    return getPropertyFlag(PropertyFlag.STRICT_LABEL_CHECKING);
  }

  /**
   * Returns true if the value of this attribute should be a part of a given set.
   */
  public boolean checkAllowedValues() {
    return getPropertyFlag(PropertyFlag.CHECK_ALLOWED_VALUES);
  }

  public boolean performPrereqValidatorCheck() {
    return !getPropertyFlag(PropertyFlag.SKIP_PREREQ_VALIDATOR_CHECKS);
  }

  public boolean checkConstraintsOverride() {
    return getPropertyFlag(PropertyFlag.CHECK_CONSTRAINTS_OVERRIDE);
  }

  public boolean skipConstraintsOverride() {
    return getPropertyFlag(PropertyFlag.SKIP_CONSTRAINTS_OVERRIDE);
  }

  /**
   * Returns true if this attribute's value can be influenced by the build configuration.
   */
  public boolean isConfigurable() {
    return !(type == BuildType.OUTPUT      // Excluded because of Rule#populateExplicitOutputFiles.
        || type == BuildType.OUTPUT_LIST
        || getPropertyFlag(PropertyFlag.NONCONFIGURABLE));
  }

  /**
   * Returns a predicate that evaluates to true for rule classes that are
   * allowed labels in this attribute. If this is not a label or label-list
   * attribute, the returned predicate always evaluates to true.
   */
  public Predicate<RuleClass> getAllowedRuleClassesPredicate() {
    return allowedRuleClassesForLabels;
  }

  /**
   * Returns a predicate that evaluates to true for rule classes that are
   * allowed labels in this attribute with warning. If this is not a label or label-list
   * attribute, the returned predicate always evaluates to true.
   */
  public Predicate<RuleClass> getAllowedRuleClassesWarningPredicate() {
    return allowedRuleClassesForLabelsWarning;
  }

  /**
   * Returns the list of sets of mandatory Skylark providers.
   */
  public ImmutableList<ImmutableSet<SkylarkProviderIdentifier>> getMandatoryProvidersList() {
    return mandatoryProvidersList;
  }

  /** Returns the list of lists of mandatory native providers. */
  public ImmutableList<ImmutableList<Class<? extends TransitiveInfoProvider>>>
      getMandatoryNativeProvidersList() {
    return mandatoryNativeProvidersList;
  }

  public FileTypeSet getAllowedFileTypesPredicate() {
    return allowedFileTypesForLabels;
  }

  public ValidityPredicate getValidityPredicate() {
    return validityPredicate;
  }

  public Predicate<AttributeMap> getCondition() {
    return condition == null ? Predicates.<AttributeMap>alwaysTrue() : condition;
  }

  public PredicateWithMessage<Object> getAllowedValues() {
    return allowedValues;
  }

  /**
   * Returns the list of aspects required for dependencies through this attribute.
   */
  public ImmutableList<Aspect> getAspects(Rule rule) {
    ImmutableList.Builder<Aspect> builder = ImmutableList.builder();
    for (RuleAspect aspect : aspects) {
      builder.add(aspect.getAspect(rule));
    }
    return builder.build();
  }

  public ImmutableList<AspectClass> getAspectClasses() {
    ImmutableList.Builder<AspectClass> result = ImmutableList.builder();
    for (RuleAspect<?> aspect : aspects) {
      result.add(aspect.getAspectClass());
    }
    return result.build();
  }

  /**
   * Returns the default value of this attribute in the context of the
   * specified Rule.  For attributes with a computed default, i.e. {@code
   * hasComputedDefault()}, {@code rule} must be non-null since the result may
   * depend on the values of its other attributes.
   *
   * <p>The result may be null (although this is not a value in the build
   * language).
   *
   * <p>During population of the rule's attribute dictionary, all non-computed
   * defaults must be set before all computed ones.
   *
   * @param rule the rule to which this attribute belongs; non-null if
   *   {@code hasComputedDefault()}; ignored otherwise.
   */
  public Object getDefaultValue(Rule rule) {
    if (!getCondition().apply(rule == null ? null : NonconfigurableAttributeMapper.of(rule))) {
      return null;
    } else if (defaultValue instanceof LateBoundDefault<?>) {
      return ((LateBoundDefault<?>) defaultValue).getDefault();
    } else {
      return defaultValue;
    }
  }

  /**
   * Returns the default value of this attribute, even if it has a condition, is a computed default,
   * or a late-bound default.
   */
  @VisibleForTesting
  public Object getDefaultValueForTesting() {
    return defaultValue;
  }

  public LateBoundDefault<?> getLateBoundDefault() {
    Preconditions.checkState(isLateBound());
    return (LateBoundDefault<?>) defaultValue;
  }

  /**
   * Returns true iff this attribute has a computed default or a condition.
   *
   * @see #getDefaultValue(Rule)
   */
  boolean hasComputedDefault() {
    return (defaultValue instanceof ComputedDefault)
        || (defaultValue instanceof SkylarkComputedDefaultTemplate)
        || (condition != null);
  }

  /**
   * Returns if this attribute is an implicit dependency according to the naming policy that
   * designates implicit attributes.
   */
  public boolean isImplicit() {
    return isImplicit(getName());
  }

  /**
   * Returns if an attribute with the given name is an implicit dependency according to the
   * naming policy that designates implicit attributes.
   */
  public static boolean isImplicit(String name) {
    return name.startsWith("$");
  }

  /**
   * Returns if this attribute is late-bound according to the naming policy that designates
   * late-bound attributes.
   */
  public boolean isLateBound() {
    return isLateBound(getName());
  }

  /**
   * Returns if an attribute with the given name is late-bound according to the naming policy
   * that designates late-bound attributes.
   */
  public static boolean isLateBound(String name) {
    return name.startsWith(":");
  }

  /** Returns whether this attribute is considered private in Skylark. */
  private static boolean isPrivateAttribute(String nativeAttrName) {
    return isLateBound(nativeAttrName) || isImplicit(nativeAttrName);
  }

  /**
   * Returns the Skylark-usable name of this attribute.
   *
   * Implicit and late-bound attributes start with '_' (instead of '$' or ':').
   */
  public static String getSkylarkName(String nativeAttrName) {
    if (isPrivateAttribute(nativeAttrName)) {
      return "_" + nativeAttrName.substring(1);
    }
    return nativeAttrName;
  }

  @Override
  public String toString() {
    return "Attribute(" + name + ", " + type + ")";
  }

  @Override
  public int compareTo(Attribute other) {
    return name.compareTo(other.name);
  }

  /**
   * Returns a replica builder of this Attribute.
   */
  public <TYPE> Attribute.Builder<TYPE> cloneBuilder(Type<TYPE> tp) {
    Preconditions.checkArgument(tp == this.type);
    Builder<TYPE> builder = new Builder<>(name, tp);
    builder.allowedFileTypesForLabels = allowedFileTypesForLabels;
    builder.allowedRuleClassesForLabels = allowedRuleClassesForLabels;
    builder.allowedRuleClassesForLabelsWarning = allowedRuleClassesForLabelsWarning;
    builder.mandatoryNativeProvidersList = mandatoryNativeProvidersList;
    builder.mandatoryProvidersList = mandatoryProvidersList;
    builder.validityPredicate = validityPredicate;
    builder.condition = condition;
    builder.configTransition = configTransition;
    builder.splitTransitionProvider = splitTransitionProvider;
    builder.propertyFlags = propertyFlags.isEmpty() ?
        EnumSet.noneOf(PropertyFlag.class) : EnumSet.copyOf(propertyFlags);
    builder.value = defaultValue;
    builder.valueSet = false;
    builder.allowedValues = allowedValues;
    builder.aspects = new LinkedHashMap<>();
    for (RuleAspect<?> aspect : aspects) {
      builder.aspects.put(aspect.getName(), aspect);
    }

    return builder;
  }

  public Attribute.Builder<?> cloneBuilder() {
    return cloneBuilder(this.type);
  }
}