src/main/java/com/google/devtools/build/lib/analysis/LocationExpander.java - bazel - Git at Google

 // 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.analysis;

 import com.google.common.base.Joiner;
 import com.google.common.collect.ImmutableCollection;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Maps;
 import com.google.common.collect.Sets;
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.analysis.RuleConfiguredTarget.Mode;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
 import com.google.devtools.build.lib.packages.BuildType;
 import com.google.devtools.build.lib.packages.OutputFile;
 import com.google.devtools.build.lib.rules.AliasProvider;
 import com.google.devtools.build.lib.vfs.PathFragment;

 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;

 /**
  * Expands $(location) tags inside target attributes.
  * You can specify something like this in the BUILD file:
  *
  * somerule(name='some name',
  *          someopt = [ '$(location //mypackage:myhelper)' ],
  *          ...)
  *
  * and location will be substituted with //mypackage:myhelper executable output.
  * Note that //mypackage:myhelper should have just one output.
  */
 public class LocationExpander {

   /**
    * List of options to tweak the LocationExpander.
    */
   public static enum Options {
     /** output the execPath instead of the relative path */
     EXEC_PATHS,
     /** Allow to take label from the data attribute */
     ALLOW_DATA,
   }

   private static final int MAX_PATHS_SHOWN = 5;
   private static final String LOCATION = "$(location";
   private final RuleContext ruleContext;
   private final ImmutableSet<Options> options;

   /**
    * This is a Map, not a Multimap, because we need to distinguish between the cases of "empty
    * value" and "absent key."
    */
   private Map<Label, Collection<Artifact>> locationMap;
   private ImmutableMap<Label, ImmutableCollection<Artifact>> labelMap;

   /**
    * Creates location expander helper bound to specific target and with default location map.
    *
    * @param ruleContext BUILD rule
    * @param labelMap A mapping of labels to build artifacts.
    * @param allowDataAttributeEntriesInLabel set to true if the <code>data</code> attribute should
    *        be used too.
    */
   public LocationExpander(
       RuleContext ruleContext, ImmutableMap<Label, ImmutableCollection<Artifact>> labelMap,
       boolean allowDataAttributeEntriesInLabel) {
     this.ruleContext = ruleContext;
     ImmutableSet.Builder<Options> builder = ImmutableSet.builder();
     builder.add(Options.EXEC_PATHS);
     if (allowDataAttributeEntriesInLabel) {
       builder.add(Options.ALLOW_DATA);
     }
     this.options = builder.build();
     this.labelMap = labelMap;
   }

   /**
    * Creates location expander helper bound to specific target.
    *
    * @param ruleContext the BUILD rule's context
    * @param options the list of options, see {@link Options}.
    */
   public LocationExpander(RuleContext ruleContext, ImmutableSet<Options> options) {
     this.ruleContext = ruleContext;
     this.options = options;
   }

   /**
    * Creates location expander helper bound to specific target.
    *
    * @param ruleContext the BUILD rule's context
    * @param options the list of options, see {@link Options}.
    */
   public LocationExpander(RuleContext ruleContext, Options... options) {
     this.ruleContext = ruleContext;
     this.options = ImmutableSet.copyOf(options);
   }

   private Map<Label, Collection<Artifact>> getLocationMap() {
     if (locationMap == null) {
       locationMap = buildLocationMap(ruleContext, labelMap, options.contains(Options.ALLOW_DATA));
     }
     return locationMap;
   }

   public String expand(String input) {
     return expand(input, new RuleErrorReporter());
   }

   /**
    * Expands attribute's location and locations tags based on the target and
    * location map.
    *
    * @param attrName  name of the attribute
    * @param attrValue initial value of the attribute
    * @return attribute value with expanded location tags or original value in
    *         case of errors
    */
   public String expandAttribute(String attrName, String attrValue) {
     return expand(attrValue, new AttributeErrorReporter(attrName));
   }

   private String expand(String value, ErrorReporter reporter) {
     int restart = 0;

     int attrLength = value.length();
     StringBuilder result = new StringBuilder(value.length());

     while (true) {
       // (1) find '$(location ' or '$(locations '
       String message = "$(location)";
       boolean multiple = false;
       int start = value.indexOf(LOCATION, restart);
       int scannedLength = LOCATION.length();
       if (start == -1 || start + scannedLength == attrLength) {
         result.append(value.substring(restart));
         break;
       }

       if (value.charAt(start + scannedLength) == 's') {
         scannedLength++;
         if (start + scannedLength == attrLength) {
           result.append(value.substring(restart));
           break;
         }
         message = "$(locations)";
         multiple = true;
       }

       if (value.charAt(start + scannedLength) != ' ') {
         result.append(value, restart, start + scannedLength);
         restart = start + scannedLength;
         continue;
       }
       scannedLength++;

       int end = value.indexOf(')', start + scannedLength);
       if (end == -1) {
         reporter.report(ruleContext, "unterminated " + message + " expression");
         return value;
       }

       message = String.format(" in %s expression", message);

       // (2) parse label
       String labelText = value.substring(start + scannedLength, end);
       Label label = parseLabel(labelText, message, reporter);

       if (label == null) {
         // Error was already reported in parseLabel()
         return value;
       }

       // (3) expand label; stop this operation if there is an error
       try {
         Collection<String> paths = resolveLabel(label, message, multiple);
         result.append(value, restart, start);

         if (multiple) {
           Joiner.on(' ').appendTo(result, paths);
         } else {
           result.append(Iterables.getOnlyElement(paths));
         }
       } catch (IllegalStateException ise) {
         reporter.report(ruleContext, ise.getMessage());
         return value;
       }

       restart = end + 1;
     }

     return result.toString();
   }

   private Label parseLabel(String labelText, String message, ErrorReporter reporter) {
     try {
       return ruleContext.getLabel().getRelative(labelText);
     } catch (LabelSyntaxException e) {
       reporter.report(ruleContext, String.format("invalid label%s: %s", message, e.getMessage()));
       return null;
     }
   }

   /**
    * Returns all possible target location(s) of the given label
    * @param message Original message, for error reporting purposes only
    * @param hasMultipleTargets Describes whether the label has multiple target locations
    * @return The collection of all path strings
    */
   private Collection<String> resolveLabel(
       Label unresolved, String message, boolean hasMultipleTargets) throws IllegalStateException {
     // replace with singleton artifact, iff unique.
     Collection<Artifact> artifacts = getLocationMap().get(unresolved);

     if (artifacts == null) {
       throw new IllegalStateException(
           "label '" + unresolved + "'" + message + " is not a declared prerequisite of this rule");
     }

     Set<String> paths = getPaths(artifacts, options.contains(Options.EXEC_PATHS));

     if (paths.isEmpty()) {
       throw new IllegalStateException(
           "label '" + unresolved + "'" + message + " expression expands to no files");
     }

     if (!hasMultipleTargets && paths.size() > 1) {
       throw new IllegalStateException(
           String.format(
               "label '%s'%s expands to more than one file, "
                   + "please use $(locations %s) instead.  Files (at most %d shown) are: %s",
               unresolved,
               message,
               unresolved,
               MAX_PATHS_SHOWN,
               Iterables.limit(paths, MAX_PATHS_SHOWN)));
     }

     return paths;
   }

   /**
    * Extracts all possible target locations from target specification.
    *
    * @param ruleContext BUILD target object
    * @param labelMap map of labels to build artifacts
    * @return map of all possible target locations
    */
   private static Map<Label, Collection<Artifact>> buildLocationMap(
       RuleContext ruleContext,
       Map<Label, ? extends Collection<Artifact>> labelMap,
       boolean allowDataAttributeEntriesInLabel) {
     Map<Label, Collection<Artifact>> locationMap = Maps.newHashMap();
     if (labelMap != null) {
       for (Map.Entry<Label, ? extends Collection<Artifact>> entry : labelMap.entrySet()) {
         mapGet(locationMap, entry.getKey()).addAll(entry.getValue());
       }
     }

     // Add all destination locations.
     for (OutputFile out : ruleContext.getRule().getOutputFiles()) {
       mapGet(locationMap, out.getLabel()).add(ruleContext.createOutputArtifact(out));
     }

     if (ruleContext.getRule().isAttrDefined("srcs", BuildType.LABEL_LIST)) {
       for (TransitiveInfoCollection src : ruleContext
           .getPrerequisitesIf("srcs", Mode.TARGET, FileProvider.class)) {
         Iterables.addAll(mapGet(locationMap, AliasProvider.getDependencyLabel(src)),
             src.getProvider(FileProvider.class).getFilesToBuild());
       }
     }

     // Add all locations associated with dependencies and tools
     List<TransitiveInfoCollection> depsDataAndTools = new ArrayList<>();
     if (ruleContext.getRule().isAttrDefined("deps", BuildType.LABEL_LIST)) {
       Iterables.addAll(depsDataAndTools,
           ruleContext.getPrerequisitesIf("deps", Mode.DONT_CHECK, FilesToRunProvider.class));
     }
     if (allowDataAttributeEntriesInLabel
         && ruleContext.getRule().isAttrDefined("data", BuildType.LABEL_LIST)) {
       Iterables.addAll(depsDataAndTools,
           ruleContext.getPrerequisitesIf("data", Mode.DATA, FilesToRunProvider.class));
     }
     if (ruleContext.getRule().isAttrDefined("tools", BuildType.LABEL_LIST)) {
       Iterables.addAll(depsDataAndTools,
           ruleContext.getPrerequisitesIf("tools", Mode.HOST, FilesToRunProvider.class));
     }

     for (TransitiveInfoCollection dep : depsDataAndTools) {
       Label label = AliasProvider.getDependencyLabel(dep);
       FilesToRunProvider filesToRun = dep.getProvider(FilesToRunProvider.class);
       Artifact executableArtifact = filesToRun.getExecutable();

       // If the label has an executable artifact add that to the multimaps.
       if (executableArtifact != null) {
         mapGet(locationMap, label).add(executableArtifact);
       } else {
         mapGet(locationMap, label).addAll(filesToRun.getFilesToRun());
       }
     }
     return locationMap;
   }

   /**
    * Extracts list of all executables associated with given collection of label
    * artifacts.
    *
    * @param artifacts to get the paths of
    * @param takeExecPath if false, the root relative path will be taken
    * @return all associated executable paths
    */
   private static Set<String> getPaths(Collection<Artifact> artifacts, boolean takeExecPath) {
     TreeSet<String> paths = Sets.newTreeSet();

     for (Artifact artifact : artifacts) {
       PathFragment execPath =
           takeExecPath ? artifact.getExecPath() : artifact.getRootRelativePath();
       if (execPath != null) {  // omit middlemen etc
         paths.add(execPath.getPathString());
       }
     }
     return paths;
   }

   /**
    * Returns the value in the specified map corresponding to 'key', creating and
    * inserting an empty container if absent. We use Map not Multimap because
    * we need to distinguish the cases of "empty value" and "absent key".
    *
    * @return the value in the specified map corresponding to 'key'
    */
   private static <K, V> Collection<V> mapGet(Map<K, Collection<V>> map, K key) {
     Collection<V> values = map.get(key);
     if (values == null) {
       // We use sets not lists, because it's conceivable that the same label
       // could appear twice, in "srcs" and "deps".
       values = Sets.newHashSet();
       map.put(key, values);
     }
     return values;
   }

   private static interface ErrorReporter {
     void report(RuleContext ctx, String error);
   }

   private static final class AttributeErrorReporter implements ErrorReporter {
     private final String attrName;

     public AttributeErrorReporter(String attrName) {
       this.attrName = attrName;
     }

     @Override
     public void report(RuleContext ctx, String error) {
       ctx.attributeError(attrName, error);
     }
   }

   private static final class RuleErrorReporter implements ErrorReporter {
     @Override
     public void report(RuleContext ctx, String error) {
       ctx.ruleError(error);
     }
   }
 }
	// 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.analysis;

	import com.google.common.base.Joiner;
	import com.google.common.collect.ImmutableCollection;
	import com.google.common.collect.ImmutableMap;
	import com.google.common.collect.ImmutableSet;
	import com.google.common.collect.Iterables;
	import com.google.common.collect.Maps;
	import com.google.common.collect.Sets;
	import com.google.devtools.build.lib.actions.Artifact;
	import com.google.devtools.build.lib.analysis.RuleConfiguredTarget.Mode;
	import com.google.devtools.build.lib.cmdline.Label;
	import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
	import com.google.devtools.build.lib.packages.BuildType;
	import com.google.devtools.build.lib.packages.OutputFile;
	import com.google.devtools.build.lib.rules.AliasProvider;
	import com.google.devtools.build.lib.vfs.PathFragment;

	import java.util.ArrayList;
	import java.util.Collection;
	import java.util.List;
	import java.util.Map;
	import java.util.Set;
	import java.util.TreeSet;

	/**
	* Expands $(location) tags inside target attributes.
	* You can specify something like this in the BUILD file:
	*
	* somerule(name='some name',
	* someopt = [ '$(location //mypackage:myhelper)' ],
	* ...)
	*
	* and location will be substituted with //mypackage:myhelper executable output.
	* Note that //mypackage:myhelper should have just one output.
	*/
	public class LocationExpander {

	/**
	* List of options to tweak the LocationExpander.
	*/
	public static enum Options {
	/** output the execPath instead of the relative path */
	EXEC_PATHS,
	/** Allow to take label from the data attribute */
	ALLOW_DATA,
	}

	private static final int MAX_PATHS_SHOWN = 5;
	private static final String LOCATION = "$(location";
	private final RuleContext ruleContext;
	private final ImmutableSet<Options> options;

	/**
	* This is a Map, not a Multimap, because we need to distinguish between the cases of "empty
	* value" and "absent key."
	*/
	private Map<Label, Collection<Artifact>> locationMap;
	private ImmutableMap<Label, ImmutableCollection<Artifact>> labelMap;

	/**
	* Creates location expander helper bound to specific target and with default location map.
	*
	* @param ruleContext BUILD rule
	* @param labelMap A mapping of labels to build artifacts.
	* @param allowDataAttributeEntriesInLabel set to true if the <code>data</code> attribute should
	* be used too.
	*/
	public LocationExpander(
	RuleContext ruleContext, ImmutableMap<Label, ImmutableCollection<Artifact>> labelMap,
	boolean allowDataAttributeEntriesInLabel) {
	this.ruleContext = ruleContext;
	ImmutableSet.Builder<Options> builder = ImmutableSet.builder();
	builder.add(Options.EXEC_PATHS);
	if (allowDataAttributeEntriesInLabel) {
	builder.add(Options.ALLOW_DATA);
	}
	this.options = builder.build();
	this.labelMap = labelMap;
	}

	/**
	* Creates location expander helper bound to specific target.
	*
	* @param ruleContext the BUILD rule's context
	* @param options the list of options, see {@link Options}.
	*/
	public LocationExpander(RuleContext ruleContext, ImmutableSet<Options> options) {
	this.ruleContext = ruleContext;
	this.options = options;
	}

	/**
	* Creates location expander helper bound to specific target.
	*
	* @param ruleContext the BUILD rule's context
	* @param options the list of options, see {@link Options}.
	*/
	public LocationExpander(RuleContext ruleContext, Options... options) {
	this.ruleContext = ruleContext;
	this.options = ImmutableSet.copyOf(options);
	}

	private Map<Label, Collection<Artifact>> getLocationMap() {
	if (locationMap == null) {
	locationMap = buildLocationMap(ruleContext, labelMap, options.contains(Options.ALLOW_DATA));
	}
	return locationMap;
	}

	public String expand(String input) {
	return expand(input, new RuleErrorReporter());
	}

	/**
	* Expands attribute's location and locations tags based on the target and
	* location map.
	*
	* @param attrName name of the attribute
	* @param attrValue initial value of the attribute
	* @return attribute value with expanded location tags or original value in
	* case of errors
	*/
	public String expandAttribute(String attrName, String attrValue) {
	return expand(attrValue, new AttributeErrorReporter(attrName));
	}

	private String expand(String value, ErrorReporter reporter) {
	int restart = 0;

	int attrLength = value.length();
	StringBuilder result = new StringBuilder(value.length());

	while (true) {
	// (1) find '$(location ' or '$(locations '
	String message = "$(location)";
	boolean multiple = false;
	int start = value.indexOf(LOCATION, restart);
	int scannedLength = LOCATION.length();
	if (start == -1 \|\| start + scannedLength == attrLength) {
	result.append(value.substring(restart));
	break;
	}

	if (value.charAt(start + scannedLength) == 's') {
	scannedLength++;
	if (start + scannedLength == attrLength) {
	result.append(value.substring(restart));
	break;
	}
	message = "$(locations)";
	multiple = true;
	}

	if (value.charAt(start + scannedLength) != ' ') {
	result.append(value, restart, start + scannedLength);
	restart = start + scannedLength;
	continue;
	}
	scannedLength++;

	int end = value.indexOf(')', start + scannedLength);
	if (end == -1) {
	reporter.report(ruleContext, "unterminated " + message + " expression");
	return value;
	}

	message = String.format(" in %s expression", message);

	// (2) parse label
	String labelText = value.substring(start + scannedLength, end);
	Label label = parseLabel(labelText, message, reporter);

	if (label == null) {
	// Error was already reported in parseLabel()
	return value;
	}

	// (3) expand label; stop this operation if there is an error
	try {
	Collection<String> paths = resolveLabel(label, message, multiple);
	result.append(value, restart, start);

	if (multiple) {
	Joiner.on(' ').appendTo(result, paths);
	} else {
	result.append(Iterables.getOnlyElement(paths));
	}
	} catch (IllegalStateException ise) {
	reporter.report(ruleContext, ise.getMessage());
	return value;
	}

	restart = end + 1;
	}

	return result.toString();
	}

	private Label parseLabel(String labelText, String message, ErrorReporter reporter) {
	try {
	return ruleContext.getLabel().getRelative(labelText);
	} catch (LabelSyntaxException e) {
	reporter.report(ruleContext, String.format("invalid label%s: %s", message, e.getMessage()));
	return null;
	}
	}

	/**
	* Returns all possible target location(s) of the given label
	* @param message Original message, for error reporting purposes only
	* @param hasMultipleTargets Describes whether the label has multiple target locations
	* @return The collection of all path strings
	*/
	private Collection<String> resolveLabel(
	Label unresolved, String message, boolean hasMultipleTargets) throws IllegalStateException {
	// replace with singleton artifact, iff unique.
	Collection<Artifact> artifacts = getLocationMap().get(unresolved);

	if (artifacts == null) {
	throw new IllegalStateException(
	"label '" + unresolved + "'" + message + " is not a declared prerequisite of this rule");
	}

	Set<String> paths = getPaths(artifacts, options.contains(Options.EXEC_PATHS));

	if (paths.isEmpty()) {
	throw new IllegalStateException(
	"label '" + unresolved + "'" + message + " expression expands to no files");
	}

	if (!hasMultipleTargets && paths.size() > 1) {
	throw new IllegalStateException(
	String.format(
	"label '%s'%s expands to more than one file, "
	+ "please use $(locations %s) instead. Files (at most %d shown) are: %s",
	unresolved,
	message,
	unresolved,
	MAX_PATHS_SHOWN,
	Iterables.limit(paths, MAX_PATHS_SHOWN)));
	}

	return paths;
	}

	/**
	* Extracts all possible target locations from target specification.
	*
	* @param ruleContext BUILD target object
	* @param labelMap map of labels to build artifacts
	* @return map of all possible target locations
	*/
	private static Map<Label, Collection<Artifact>> buildLocationMap(
	RuleContext ruleContext,
	Map<Label, ? extends Collection<Artifact>> labelMap,
	boolean allowDataAttributeEntriesInLabel) {
	Map<Label, Collection<Artifact>> locationMap = Maps.newHashMap();
	if (labelMap != null) {
	for (Map.Entry<Label, ? extends Collection<Artifact>> entry : labelMap.entrySet()) {
	mapGet(locationMap, entry.getKey()).addAll(entry.getValue());
	}
	}

	// Add all destination locations.
	for (OutputFile out : ruleContext.getRule().getOutputFiles()) {
	mapGet(locationMap, out.getLabel()).add(ruleContext.createOutputArtifact(out));
	}

	if (ruleContext.getRule().isAttrDefined("srcs", BuildType.LABEL_LIST)) {
	for (TransitiveInfoCollection src : ruleContext
	.getPrerequisitesIf("srcs", Mode.TARGET, FileProvider.class)) {
	Iterables.addAll(mapGet(locationMap, AliasProvider.getDependencyLabel(src)),
	src.getProvider(FileProvider.class).getFilesToBuild());
	}
	}

	// Add all locations associated with dependencies and tools
	List<TransitiveInfoCollection> depsDataAndTools = new ArrayList<>();
	if (ruleContext.getRule().isAttrDefined("deps", BuildType.LABEL_LIST)) {
	Iterables.addAll(depsDataAndTools,
	ruleContext.getPrerequisitesIf("deps", Mode.DONT_CHECK, FilesToRunProvider.class));
	}
	if (allowDataAttributeEntriesInLabel
	&& ruleContext.getRule().isAttrDefined("data", BuildType.LABEL_LIST)) {
	Iterables.addAll(depsDataAndTools,
	ruleContext.getPrerequisitesIf("data", Mode.DATA, FilesToRunProvider.class));
	}
	if (ruleContext.getRule().isAttrDefined("tools", BuildType.LABEL_LIST)) {
	Iterables.addAll(depsDataAndTools,
	ruleContext.getPrerequisitesIf("tools", Mode.HOST, FilesToRunProvider.class));
	}

	for (TransitiveInfoCollection dep : depsDataAndTools) {
	Label label = AliasProvider.getDependencyLabel(dep);
	FilesToRunProvider filesToRun = dep.getProvider(FilesToRunProvider.class);
	Artifact executableArtifact = filesToRun.getExecutable();

	// If the label has an executable artifact add that to the multimaps.
	if (executableArtifact != null) {
	mapGet(locationMap, label).add(executableArtifact);
	} else {
	mapGet(locationMap, label).addAll(filesToRun.getFilesToRun());
	}
	}
	return locationMap;
	}

	/**
	* Extracts list of all executables associated with given collection of label
	* artifacts.
	*
	* @param artifacts to get the paths of
	* @param takeExecPath if false, the root relative path will be taken
	* @return all associated executable paths
	*/
	private static Set<String> getPaths(Collection<Artifact> artifacts, boolean takeExecPath) {
	TreeSet<String> paths = Sets.newTreeSet();

	for (Artifact artifact : artifacts) {
	PathFragment execPath =
	takeExecPath ? artifact.getExecPath() : artifact.getRootRelativePath();
	if (execPath != null) { // omit middlemen etc
	paths.add(execPath.getPathString());
	}
	}
	return paths;
	}

	/**
	* Returns the value in the specified map corresponding to 'key', creating and
	* inserting an empty container if absent. We use Map not Multimap because
	* we need to distinguish the cases of "empty value" and "absent key".
	*
	* @return the value in the specified map corresponding to 'key'
	*/
	private static <K, V> Collection<V> mapGet(Map<K, Collection<V>> map, K key) {
	Collection<V> values = map.get(key);
	if (values == null) {
	// We use sets not lists, because it's conceivable that the same label
	// could appear twice, in "srcs" and "deps".
	values = Sets.newHashSet();
	map.put(key, values);
	}
	return values;
	}

	private static interface ErrorReporter {
	void report(RuleContext ctx, String error);
	}

	private static final class AttributeErrorReporter implements ErrorReporter {
	private final String attrName;

	public AttributeErrorReporter(String attrName) {
	this.attrName = attrName;
	}

	@Override
	public void report(RuleContext ctx, String error) {
	ctx.attributeError(attrName, error);
	}
	}

	private static final class RuleErrorReporter implements ErrorReporter {
	@Override
	public void report(RuleContext ctx, String error) {
	ctx.ruleError(error);
	}
	}
	}