2 files changed, 36 insertions, 1 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/packages/AggregatingAttributeMapper.java b/src/main/java/com/google/devtools/build/lib/packages/AggregatingAttributeMapper.java
index 73d786bf5a..256cf93e6b 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/AggregatingAttributeMapper.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/AggregatingAttributeMapper.java
@@ -13,7 +13,6 @@
 // limitations under the License.
 package com.google.devtools.build.lib.packages;
 
-
 import com.google.common.base.Verify;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
@@ -26,6 +25,7 @@ import com.google.devtools.build.lib.packages.BuildType.Selector;
 import com.google.devtools.build.lib.packages.BuildType.SelectorList;
 import com.google.devtools.build.lib.syntax.Type;
 import com.google.devtools.build.lib.syntax.Type.LabelVisitor;
+import com.google.devtools.build.lib.syntax.Type.ListType;
 import com.google.devtools.build.lib.util.Preconditions;
 import java.util.ArrayList;
 import java.util.HashMap;
@@ -209,6 +209,33 @@ public class AggregatingAttributeMapper extends AbstractAttributeMapper {
   }
 
   /**
+   * If the attribute is a selector list of list type, then this method returns a list with number
+   * of elements equal to the number of select statements in the selector list. Each element of this
+   * list is equal to concatenating every possible attribute value in a single select statement.
+   * The conditions themselves in the select statements are completely ignored. Returns {@code null}
+   * if the attribute isn't of the desired format.
+   *
+   * As an example, if we have select({a: ["a"], b: ["a", "b"]}) + select({a: ["c", "d"], c: ["e"])
+   * The output will be [["a", "a", "b"], ["c", "d", "e"]]. The idea behind this structure is that
+   * at least some of the structure in the original selector list is preserved and we know any
+   * possible attribute value is the result of concatenating some sublist of each element.
+   */
+  @Nullable
+  public <T> Iterable<T> getConcatenatedSelectorListsOfListType(
+      String attributeName, Type<T> type) {
+    SelectorList<T> selectorList = getSelectorList(attributeName, type);
+    if (selectorList != null && type instanceof ListType) {
+      List<T> selectList = new ArrayList<>();
+
+      for (Selector<T> selector : selectorList.getSelectors()) {
+        selectList.add(type.concat(selector.getEntries().values()));
+      }
+      return ImmutableList.copyOf(selectList);
+    }
+    return null;
+  }
+
+  /**
    * Returns a list of all possible values an attribute can take for this rule.
    *
    * <p>Note that when an attribute uses multiple selects, or is a {@link Attribute.ComputedDefault}
diff --git a/src/main/java/com/google/devtools/build/lib/query2/output/OutputFormatter.java b/src/main/java/com/google/devtools/build/lib/query2/output/OutputFormatter.java
index 48923a3e83..4b732e1c15 100644
--- a/src/main/java/com/google/devtools/build/lib/query2/output/OutputFormatter.java
+++ b/src/main/java/com/google/devtools/build/lib/query2/output/OutputFormatter.java
@@ -18,6 +18,7 @@ import com.google.common.base.Function;
 import com.google.common.base.Joiner;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
 import com.google.common.collect.Sets;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.collect.CompactHashSet;
@@ -809,6 +810,7 @@ public abstract class OutputFormatter implements Serializable {
     }
 
     AggregatingAttributeMapper attributeMap = AggregatingAttributeMapper.of(rule);
+    Iterable<?> list;
     if (attr.getType().equals(BuildType.LABEL_LIST)
         && attributeMap.isConfigurable(attr.getName())) {
       // TODO(gregce): Expand this to all collection types (we don't do this for scalars because
@@ -819,7 +821,13 @@ public abstract class OutputFormatter implements Serializable {
           ImmutableList.<Object>of(
               attributeMap.getReachableLabels(attr.getName(), /*includeSelectKeys=*/false)),
           source);
+    } else if ((list =
+            attributeMap.getConcatenatedSelectorListsOfListType(
+                attr.getName(), attr.getType()))
+        != null) {
+      return new PossibleAttributeValues(Lists.newArrayList(list), source);
     } else {
+      // The call to getPossibleAttributeValues below is especially slow with selector lists.
       return new PossibleAttributeValues(attributeMap.getPossibleAttributeValues(rule, attr),
           source);
     }