Allow CustomCommandLine's mapFn to expand each object into multiple items.

This is needed to migrate JavaCompileAction away from CustomMultiArgv.

PiperOrigin-RevId: 184136486

12 files changed, 155 insertions, 94 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/actions/CommandLineItem.java b/src/main/java/com/google/devtools/build/lib/analysis/actions/CommandLineItem.java
index f1418d9c10..6564a5b97d 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/actions/CommandLineItem.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/actions/CommandLineItem.java
@@ -13,15 +13,19 @@
 // limitations under the License.
 package com.google.devtools.build.lib.analysis.actions;
 
+import java.util.function.Consumer;
+
 /** An interface for an object that customizes how it is expanded into a command line. */
 public interface CommandLineItem {
   /**
    * A map function that allows caller customization how a type is expanded into the command line.
    */
   interface MapFn<T> {
-    MapFn<Object> DEFAULT = CommandLineItem::expandToCommandLine;
+    MapFn<Object> DEFAULT =
+        (Object object, Consumer<String> args) ->
+            args.accept(CommandLineItem.expandToCommandLine(object));
 
-    String expandToCommandLine(T object);
+    void expandToCommandLine(T object, Consumer<String> args);
   }
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/actions/CustomCommandLine.java b/src/main/java/com/google/devtools/build/lib/analysis/actions/CustomCommandLine.java
index eb0c1c72c4..a772b3ad9f 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/actions/CustomCommandLine.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/actions/CustomCommandLine.java
@@ -22,7 +22,6 @@ import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Interner;
 import com.google.common.collect.Iterables;
-import com.google.common.collect.Lists;
 import com.google.devtools.build.lib.actions.ActionKeyContext;
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.actions.Artifact.ArtifactExpander;
@@ -45,6 +44,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;
 import java.util.UUID;
+import java.util.function.Consumer;
 import javax.annotation.Nullable;
 
 /** A customizable, serializable class for building memory efficient command lines. */
@@ -308,6 +308,9 @@ public final class CustomCommandLine extends CommandLine {
       }
       vectorArgFragment = VectorArgFragment.interner.intern(vectorArgFragment);
       arguments.add(vectorArgFragment);
+      if (vectorArgFragment.hasMapEach) {
+        arguments.add(mapFn);
+      }
       if (vectorArgFragment.isNestedSet) {
         arguments.add(values);
       } else {
@@ -315,9 +318,6 @@ public final class CustomCommandLine extends CommandLine {
         arguments.add(vectorArg.count);
         Iterables.addAll(arguments, values);
       }
-      if (vectorArgFragment.hasMapEach) {
-        arguments.add(mapFn);
-      }
       if (vectorArgFragment.hasFormatEach) {
         arguments.add(vectorArg.formatEach);
       }
@@ -360,26 +360,38 @@ public final class CustomCommandLine extends CommandLine {
       @SuppressWarnings("unchecked")
       @Override
       public int eval(List<Object> arguments, int argi, ImmutableList.Builder<String> builder) {
-        final List<Object> mutatedValues;
-        final int count;
+        final List<String> mutatedValues;
+        CommandLineItem.MapFn<Object> mapFn =
+            hasMapEach ? (CommandLineItem.MapFn<Object>) arguments.get(argi++) : null;
         if (isNestedSet) {
-          Iterable<Object> values = (Iterable<Object>) arguments.get(argi++);
-          mutatedValues = Lists.newArrayList(values);
-          count = mutatedValues.size();
+          NestedSet<Object> values = (NestedSet<Object>) arguments.get(argi++);
+          Collection<Object> collection = values.toCollection();
+          mutatedValues = new ArrayList<>(collection.size());
+          if (mapFn != null) {
+            Consumer<String> args = mutatedValues::add; // Hoist out of loop to reduce GC
+            for (Object object : collection) {
+              mapFn.expandToCommandLine(object, args);
+            }
+          } else {
+            for (Object object : collection) {
+              mutatedValues.add(CommandLineItem.expandToCommandLine(object));
+            }
+          }
         } else {
-          count = (Integer) arguments.get(argi++);
+          int count = (Integer) arguments.get(argi++);
           mutatedValues = new ArrayList<>(count);
-          for (int i = 0; i < count; ++i) {
-            mutatedValues.add(arguments.get(argi++));
+          if (mapFn != null) {
+            Consumer<String> args = mutatedValues::add; // Hoist out of loop to reduce GC
+            for (int i = 0; i < count; ++i) {
+              mapFn.expandToCommandLine(arguments.get(argi++), args);
+            }
+          } else {
+            for (int i = 0; i < count; ++i) {
+              mutatedValues.add(CommandLineItem.expandToCommandLine(arguments.get(argi++)));
+            }
           }
         }
-        CommandLineItem.MapFn<Object> mapFn =
-            hasMapEach
-                ? (CommandLineItem.MapFn<Object>) arguments.get(argi++)
-                : CommandLineItem.MapFn.DEFAULT;
-        for (int i = 0; i < count; ++i) {
-          mutatedValues.set(i, mapFn.expandToCommandLine(mutatedValues.get(i)));
-        }
+        final int count = mutatedValues.size();
         if (hasFormatEach) {
           String formatStr = (String) arguments.get(argi++);
           for (int i = 0; i < count; ++i) {
@@ -390,14 +402,14 @@ public final class CustomCommandLine extends CommandLine {
           String beforeEach = (String) arguments.get(argi++);
           for (int i = 0; i < count; ++i) {
             builder.add(beforeEach);
-            builder.add((String) mutatedValues.get(i));
+            builder.add(mutatedValues.get(i));
           }
         } else if (hasJoinWith) {
           String joinWith = (String) arguments.get(argi++);
           builder.add(Joiner.on(joinWith).join(mutatedValues));
         } else {
           for (int i = 0; i < count; ++i) {
-            builder.add((String) mutatedValues.get(i));
+            builder.add(mutatedValues.get(i));
           }
         }
         return argi;
@@ -410,25 +422,25 @@ public final class CustomCommandLine extends CommandLine {
           int argi,
           ActionKeyContext actionKeyContext,
           Fingerprint fingerprint) {
+        CommandLineItem.MapFn<Object> mapFn =
+            hasMapEach ? (CommandLineItem.MapFn<Object>) arguments.get(argi++) : null;
         if (isNestedSet) {
           NestedSet<Object> values = (NestedSet<Object>) arguments.get(argi++);
-          CommandLineItem.MapFn<Object> mapFn =
-              hasMapEach
-                  ? (CommandLineItem.MapFn<Object>) arguments.get(argi++)
-                  : CommandLineItem.MapFn.DEFAULT;
-          actionKeyContext.addNestedSetToFingerprint(mapFn, fingerprint, values);
+          if (mapFn != null) {
+            actionKeyContext.addNestedSetToFingerprint(mapFn, fingerprint, values);
+          } else {
+            actionKeyContext.addNestedSetToFingerprint(fingerprint, values);
+          }
         } else {
           int count = (Integer) arguments.get(argi++);
-          CommandLineItem.MapFn<Object> mapFn =
-              hasMapEach
-                  ? (CommandLineItem.MapFn<Object>)
-                      arguments.get(argi + count) // Peek ahead to mapFn
-                  : CommandLineItem.MapFn.DEFAULT;
-          for (int i = 0; i < count; ++i) {
-            fingerprint.addString(mapFn.expandToCommandLine(arguments.get(argi++)));
-          }
-          if (hasMapEach) {
-            ++argi; // Consume mapFn
+          if (mapFn != null) {
+            for (int i = 0; i < count; ++i) {
+              mapFn.expandToCommandLine(arguments.get(argi++), fingerprint);
+            }
+          } else {
+            for (int i = 0; i < count; ++i) {
+              fingerprint.addString(CommandLineItem.expandToCommandLine(arguments.get(argi++)));
+            }
           }
         }
         if (hasFormatEach) {
diff --git a/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCache.java b/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCache.java
index 083ef06269..cd23964909 100644
--- a/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCache.java
+++ b/src/main/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCache.java
@@ -60,7 +60,7 @@ public class NestedSetFingerprintCache {
   private <T> void addNestedSetToFingerprintSlow(
       MapFn<? super T> mapFn, Fingerprint fingerprint, NestedSet<T> nestedSet) {
     for (T object : nestedSet) {
-      fingerprint.addString(mapFn.expandToCommandLine(object));
+      mapFn.expandToCommandLine(object, fingerprint);
     }
   }
 
@@ -92,7 +92,7 @@ public class NestedSetFingerprintCache {
   @VisibleForTesting
   <T> void addToFingerprint(
       CommandLineItem.MapFn<? super T> mapFn, Fingerprint fingerprint, T object) {
-    fingerprint.addString(mapFn.expandToCommandLine(object));
+    mapFn.expandToCommandLine(object, fingerprint);
   }
 
   private static Map<CommandLineItem.MapFn<?>, DigestMap> createMap() {
diff --git a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourceMergingActionBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourceMergingActionBuilder.java
index 71612341af..e8199ba4e6 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourceMergingActionBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourceMergingActionBuilder.java
@@ -167,8 +167,7 @@ public class AndroidResourceMergingActionBuilder {
     NestedSetBuilder<Artifact> inputs = createInputsForBuilder(builder);
 
     Preconditions.checkNotNull(primary);
-    builder.add(
-        "--primaryData", RESOURCE_CONTAINER_TO_ARG_FOR_COMPILED.expandToCommandLine(primary));
+    builder.add("--primaryData", RESOURCE_CONTAINER_TO_ARG_FOR_COMPILED.map(primary));
     inputs.addAll(primary.getArtifacts());
     inputs.add(primary.getCompiledSymbols());
 
@@ -204,7 +203,7 @@ public class AndroidResourceMergingActionBuilder {
     NestedSetBuilder<Artifact> inputs = createInputsForBuilder(builder);
 
     Preconditions.checkNotNull(primary);
-    builder.add("--primaryData", RESOURCE_CONTAINER_TO_ARG.expandToCommandLine(primary));
+    builder.add("--primaryData", RESOURCE_CONTAINER_TO_ARG.map(primary));
     inputs.addAll(primary.getArtifacts());
     inputs.add(primary.getSymbols());
 
diff --git a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourcesProcessorBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourcesProcessorBuilder.java
index 2f97a4b2a5..a79a9ba0c1 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourcesProcessorBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidResourcesProcessorBuilder.java
@@ -442,7 +442,7 @@ public class AndroidResourcesProcessorBuilder {
       List<Artifact> outs, NestedSetBuilder<Artifact> inputs, Builder builder) {
 
     // Add data
-    builder.add("--primaryData", RESOURCE_CONTAINER_TO_ARG.expandToCommandLine(primary));
+    builder.add("--primaryData", RESOURCE_CONTAINER_TO_ARG.map(primary));
     inputs.addAll(primary.getArtifacts());
     inputs.add(primary.getManifest());
 
diff --git a/src/main/java/com/google/devtools/build/lib/rules/android/ResourceContainerConverter.java b/src/main/java/com/google/devtools/build/lib/rules/android/ResourceContainerConverter.java
index e653fb6015..077b15fe8e 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/android/ResourceContainerConverter.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/android/ResourceContainerConverter.java
@@ -29,6 +29,7 @@ import com.google.devtools.build.lib.rules.android.ResourceContainer.ResourceTyp
 import com.google.devtools.build.lib.rules.android.ResourceContainerConverter.ToArg.Includes;
 import java.util.HashSet;
 import java.util.Set;
+import java.util.function.Consumer;
 
 /**
  * Factory for functions to convert a {@link ResourceContainer} to a commandline argument, or a
@@ -91,7 +92,11 @@ public class ResourceContainerConverter {
     }
 
     @Override
-    public String expandToCommandLine(ResourceContainer container) {
+    public void expandToCommandLine(ResourceContainer container, Consumer<String> args) {
+      args.accept(map(container));
+    }
+
+    String map(ResourceContainer container) {
       ImmutableList.Builder<String> cmdPieces = ImmutableList.builder();
       if (includes.contains(Includes.ResourceRoots)) {
         cmdPieces.add(convertRoots(container, ResourceType.RESOURCES));
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/OneVersionCheckActionBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/java/OneVersionCheckActionBuilder.java
index 3595728475..4a24106972 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/OneVersionCheckActionBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/OneVersionCheckActionBuilder.java
@@ -27,6 +27,7 @@ import com.google.devtools.build.lib.analysis.actions.SpawnAction;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.rules.java.JavaConfiguration.OneVersionEnforcementLevel;
+import java.util.function.Consumer;
 
 /** Utility for generating a call to the one-version binary. */
 public final class OneVersionCheckActionBuilder {
@@ -110,12 +111,12 @@ public final class OneVersionCheckActionBuilder {
     return outputArtifact;
   }
 
-  public static VectorArg<String> jarAndTargetVectorArg(NestedSet<Artifact> jarsToCheck) {
-    return VectorArg.of(jarsToCheck).mapped(OneVersionCheckActionBuilder::jarAndTargetArg);
+  static VectorArg<String> jarAndTargetVectorArg(NestedSet<Artifact> jarsToCheck) {
+    return VectorArg.of(jarsToCheck).mapped(OneVersionCheckActionBuilder::expandToJarAndTarget);
   }
 
-  private static String jarAndTargetArg(Artifact jar) {
-    return jar.getExecPathString() + "," + getArtifactOwnerGeneralizedLabel(jar);
+  private static void expandToJarAndTarget(Artifact jar, Consumer<String> args) {
+    args.accept(jar.getExecPathString() + "," + getArtifactOwnerGeneralizedLabel(jar));
   }
 
   private static String getArtifactOwnerGeneralizedLabel(Artifact artifact) {
diff --git a/src/main/java/com/google/devtools/build/lib/rules/java/SingleJarActionBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/java/SingleJarActionBuilder.java
index 29470d125f..0b819fc311 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/java/SingleJarActionBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/java/SingleJarActionBuilder.java
@@ -30,6 +30,7 @@ import com.google.devtools.build.lib.analysis.actions.ParamFileInfo;
 import com.google.devtools.build.lib.analysis.actions.SpawnAction;
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
+import java.util.function.Consumer;
 
 /**
  * Helper class to create singlejar actions - singlejar can merge multiple zip files without
@@ -175,13 +176,13 @@ public final class SingleJarActionBuilder {
     }
 
     @Override
-    public String expandToCommandLine(Artifact resource) {
+    public void expandToCommandLine(Artifact resource, Consumer<String> args) {
       String execPath = resource.getExecPathString();
       String resourcePath =
           semantics.getDefaultJavaResourcePath(resource.getRootRelativePath()).getPathString();
       StringBuilder sb = new StringBuilder(execPath.length() + resourcePath.length() + 1);
       sb.append(execPath).append(":").append(resourcePath);
-      return sb.toString();
+      args.accept(sb.toString());
     }
 
     @Override
diff --git a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
index 849265738e..8b496160ce 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
@@ -49,6 +49,7 @@ import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Consumer;
 import javax.annotation.Nullable;
 
 /** Constructs actions to run the protocol compiler to generate sources from .proto files. */
@@ -578,14 +579,15 @@ public class ProtoCompileActionBuilder {
       @Nullable NestedSet<Artifact> protosInDirectDependencies,
       NestedSet<Artifact> transitiveImports) {
     commandLine.addAll(
-        VectorArg.of(transitiveImports).mapped(ProtoCompileActionBuilder::transitiveImportArg));
+        VectorArg.of(transitiveImports)
+            .mapped(ProtoCompileActionBuilder::expandTransitiveImportArg));
     if (protosInDirectDependencies != null) {
       if (!protosInDirectDependencies.isEmpty()) {
         commandLine.addAll(
             "--direct_dependencies",
             VectorArg.join(":")
                 .each(protosInDirectDependencies)
-                .mapped(ProtoCompileActionBuilder::getPathIgnoringRepository));
+                .mapped(ProtoCompileActionBuilder::expandToPathIgnoringRepository));
       } else {
         // The proto compiler requires an empty list to turn on strict deps checking
         commandLine.add("--direct_dependencies=");
@@ -593,8 +595,12 @@ public class ProtoCompileActionBuilder {
     }
   }
 
-  private static String transitiveImportArg(Artifact artifact) {
-    return "-I" + getPathIgnoringRepository(artifact) + "=" + artifact.getExecPathString();
+  private static void expandTransitiveImportArg(Artifact artifact, Consumer<String> args) {
+    args.accept("-I" + getPathIgnoringRepository(artifact) + "=" + artifact.getExecPathString());
+  }
+
+  private static void expandToPathIgnoringRepository(Artifact artifact, Consumer<String> args) {
+    args.accept(getPathIgnoringRepository(artifact));
   }
 
   /**
diff --git a/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java b/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java
index ddb71b4aaf..a2bafa6f6a 100644
--- a/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java
+++ b/src/main/java/com/google/devtools/build/lib/util/Fingerprint.java
@@ -26,6 +26,7 @@ import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.util.Map;
 import java.util.UUID;
+import java.util.function.Consumer;
 import javax.annotation.Nullable;
 
 /**
@@ -33,7 +34,7 @@ import javax.annotation.Nullable;
  *
  * @see java.security.MessageDigest
  */
-public final class Fingerprint {
+public final class Fingerprint implements Consumer<String> {
 
   private static final MessageDigest MD5_PROTOTYPE;
   private static final boolean MD5_PROTOTYPE_SUPPORTS_CLONE;
@@ -299,4 +300,9 @@ public final class Fingerprint {
   public static String md5Digest(String input) {
     return hexDigest(cloneOrCreateMd5().digest(input.getBytes(StandardCharsets.UTF_8)));
   }
+
+  @Override
+  public void accept(String s) {
+    addString(s);
+  }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/actions/CustomCommandLineTest.java b/src/test/java/com/google/devtools/build/lib/actions/CustomCommandLineTest.java
index 7e514bd37f..9e4b9389bc 100644
--- a/src/test/java/com/google/devtools/build/lib/actions/CustomCommandLineTest.java
+++ b/src/test/java/com/google/devtools/build/lib/actions/CustomCommandLineTest.java
@@ -37,6 +37,7 @@ import com.google.devtools.build.lib.vfs.Root;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.function.Consumer;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -173,14 +174,14 @@ public class CustomCommandLineTest {
         .inOrder();
     assertThat(
             builder()
-                .addAll(VectorArg.of(list(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll(VectorArg.of(list(foo("1"), foo("2"))).mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("1", "2")
         .inOrder();
     assertThat(
             builder()
-                .addAll(VectorArg.of(nestedSet(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll(VectorArg.of(nestedSet(foo("1"), foo("2"))).mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("1", "2")
@@ -215,14 +216,15 @@ public class CustomCommandLineTest {
         .inOrder();
     assertThat(
             builder()
-                .addAll("--arg", VectorArg.of(list(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll("--arg", VectorArg.of(list(foo("1"), foo("2"))).mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "1", "2")
         .inOrder();
     assertThat(
             builder()
-                .addAll("--arg", VectorArg.of(nestedSet(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll(
+                    "--arg", VectorArg.of(nestedSet(foo("1"), foo("2"))).mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "1", "2")
@@ -276,14 +278,17 @@ public class CustomCommandLineTest {
         .inOrder();
     assertThat(
             builder()
-                .addAll(VectorArg.join(":").each(list(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll(VectorArg.join(":").each(list(foo("1"), foo("2"))).mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("1:2")
         .inOrder();
     assertThat(
             builder()
-                .addAll(VectorArg.join(":").each(nestedSet(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll(
+                    VectorArg.join(":")
+                        .each(nestedSet(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("1:2")
@@ -341,7 +346,8 @@ public class CustomCommandLineTest {
     assertThat(
             builder()
                 .addAll(
-                    "--arg", VectorArg.join(":").each(list(foo("1"), foo("2"))).mapped(Foo::str))
+                    "--arg",
+                    VectorArg.join(":").each(list(foo("1"), foo("2"))).mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "1:2")
@@ -350,7 +356,9 @@ public class CustomCommandLineTest {
             builder()
                 .addAll(
                     "--arg",
-                    VectorArg.join(":").each(nestedSet(foo("1"), foo("2"))).mapped(Foo::str))
+                    VectorArg.join(":")
+                        .each(nestedSet(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "1:2")
@@ -408,7 +416,10 @@ public class CustomCommandLineTest {
         .inOrder();
     assertThat(
             builder()
-                .addAll(VectorArg.format("-D%s").each(list(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll(
+                    VectorArg.format("-D%s")
+                        .each(list(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D1", "-D2")
@@ -416,7 +427,9 @@ public class CustomCommandLineTest {
     assertThat(
             builder()
                 .addAll(
-                    VectorArg.format("-D%s").each(nestedSet(foo("1"), foo("2"))).mapped(Foo::str))
+                    VectorArg.format("-D%s")
+                        .each(nestedSet(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D1", "-D2")
@@ -476,7 +489,9 @@ public class CustomCommandLineTest {
             builder()
                 .addAll(
                     "--arg",
-                    VectorArg.format("-D%s").each(list(foo("1"), foo("2"))).mapped(Foo::str))
+                    VectorArg.format("-D%s")
+                        .each(list(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "-D1", "-D2")
@@ -485,7 +500,9 @@ public class CustomCommandLineTest {
             builder()
                 .addAll(
                     "--arg",
-                    VectorArg.format("-D%s").each(nestedSet(foo("1"), foo("2"))).mapped(Foo::str))
+                    VectorArg.format("-D%s")
+                        .each(nestedSet(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "-D1", "-D2")
@@ -550,7 +567,7 @@ public class CustomCommandLineTest {
                     VectorArg.format("-D%s")
                         .join(":")
                         .each(list(foo("1"), foo("2")))
-                        .mapped(Foo::str))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D1:-D2")
@@ -561,7 +578,7 @@ public class CustomCommandLineTest {
                     VectorArg.format("-D%s")
                         .join(":")
                         .each(nestedSet(foo("1"), foo("2")))
-                        .mapped(Foo::str))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D1:-D2")
@@ -628,7 +645,7 @@ public class CustomCommandLineTest {
                     VectorArg.format("-D%s")
                         .join(":")
                         .each(list(foo("1"), foo("2")))
-                        .mapped(Foo::str))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "-D1:-D2")
@@ -640,7 +657,7 @@ public class CustomCommandLineTest {
                     VectorArg.format("-D%s")
                         .join(":")
                         .each(nestedSet(foo("1"), foo("2")))
-                        .mapped(Foo::str))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("--arg", "-D1:-D2")
@@ -698,7 +715,10 @@ public class CustomCommandLineTest {
         .inOrder();
     assertThat(
             builder()
-                .addAll(VectorArg.addBefore("-D").each(list(foo("1"), foo("2"))).mapped(Foo::str))
+                .addAll(
+                    VectorArg.addBefore("-D")
+                        .each(list(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D", "1", "-D", "2")
@@ -706,7 +726,9 @@ public class CustomCommandLineTest {
     assertThat(
             builder()
                 .addAll(
-                    VectorArg.addBefore("-D").each(nestedSet(foo("1"), foo("2"))).mapped(Foo::str))
+                    VectorArg.addBefore("-D")
+                        .each(nestedSet(foo("1"), foo("2")))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D", "1", "-D", "2")
@@ -772,7 +794,7 @@ public class CustomCommandLineTest {
                     VectorArg.addBefore("-D")
                         .format("D%s")
                         .each(list(foo("1"), foo("2")))
-                        .mapped(Foo::str))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D", "D1", "-D", "D2")
@@ -783,7 +805,7 @@ public class CustomCommandLineTest {
                     VectorArg.addBefore("-D")
                         .format("D%s")
                         .each(nestedSet(foo("1"), foo("2")))
-                        .mapped(Foo::str))
+                        .mapped(Foo::expandToStr))
                 .build()
                 .arguments())
         .containsExactly("-D", "D1", "-D", "D2")
@@ -915,7 +937,10 @@ public class CustomCommandLineTest {
             .add(builder().addAll(VectorArg.addBefore("--foo=%s").each(values)).build())
             .add(builder().addAll(VectorArg.join("--foo=%s").each(values)).build())
             .add(builder().addAll(VectorArg.format("--foo=%s").each(values)).build())
-            .add(builder().addAll(VectorArg.of(values).mapped(s -> s + "_mapped")).build())
+            .add(
+                builder()
+                    .addAll(VectorArg.of(values).mapped((s, args) -> args.accept(s + "_mapped")))
+                    .build())
             .build();
 
     // Ensure all these command lines have distinct keys
@@ -992,8 +1017,8 @@ public class CustomCommandLineTest {
       this.str = str;
     }
 
-    static String str(Foo foo) {
-      return foo.str;
+    static void expandToStr(Foo foo, Consumer<String> args) {
+      args.accept(foo.str);
     }
   }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCacheTest.java b/src/test/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCacheTest.java
index 99bc4bbd9c..feba06b6bd 100644
--- a/src/test/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCacheTest.java
+++ b/src/test/java/com/google/devtools/build/lib/collect/nestedset/NestedSetFingerprintCacheTest.java
@@ -23,6 +23,7 @@ import com.google.devtools.build.lib.analysis.actions.CommandLineItem.CapturingM
 import com.google.devtools.build.lib.analysis.actions.CommandLineItem.MapFn;
 import com.google.devtools.build.lib.testutil.MoreAsserts;
 import com.google.devtools.build.lib.util.Fingerprint;
+import java.util.function.Consumer;
 import org.junit.Before;
 import org.junit.Test;
 import org.junit.runner.RunWith;
@@ -102,7 +103,7 @@ public class NestedSetFingerprintCacheTest {
     cache.addNestedSetToFingerprint(
         CommandLineItem.MapFn.DEFAULT, explicitDefaultMapFnFingerprint, a);
     Fingerprint mappedFingerprint = new Fingerprint();
-    cache.addNestedSetToFingerprint(s -> s + "_mapped", mappedFingerprint, a);
+    cache.addNestedSetToFingerprint((s, args) -> args.accept(s + "_mapped"), mappedFingerprint, a);
 
     String defaultMapFnDigest = defaultMapFnFingerprint.hexDigestAndReset();
     String explicitDefaultMapFnDigest = explicitDefaultMapFnFingerprint.hexDigestAndReset();
@@ -124,24 +125,25 @@ public class NestedSetFingerprintCacheTest {
     // Make sure a normal method reference doesn't get blacklisted.
     for (int i = 0; i < 2; ++i) {
       cache.addNestedSetToFingerprint(
-          NestedSetFingerprintCacheTest::simpleMapFn, new Fingerprint(), nestedSet);
+          NestedSetFingerprintCacheTest::simpleExpand, new Fingerprint(), nestedSet);
     }
 
     // Try again to make sure Java synthesizes a new class for a second method reference.
     for (int i = 0; i < 2; ++i) {
       cache.addNestedSetToFingerprint(
-          NestedSetFingerprintCacheTest::simpleMapFn2, new Fingerprint(), nestedSet);
+          NestedSetFingerprintCacheTest::simpleExpand2, new Fingerprint(), nestedSet);
     }
 
     // Make sure a non-capturing lambda doesn't get blacklisted
     for (int i = 0; i < 2; ++i) {
-      cache.addNestedSetToFingerprint(s -> s + "_mapped", new Fingerprint(), nestedSet);
+      cache.addNestedSetToFingerprint(
+          (s, args) -> args.accept(s + "_mapped"), new Fingerprint(), nestedSet);
     }
 
     // Make sure a CapturingMapFn doesn't get blacklisted
     for (int i = 0; i < 2; ++i) {
       cache.addNestedSetToFingerprint(
-          (CapturingMapFn<String>) object -> object, new Fingerprint(), nestedSet);
+          (CapturingMapFn<String>) (s, args) -> args.accept(s + 1), new Fingerprint(), nestedSet);
     }
 
     // Make sure a ParametrizedMapFn doesn't get blacklisted until it exceeds its instance count
@@ -159,7 +161,7 @@ public class NestedSetFingerprintCacheTest {
         () -> {
           for (int i = 0; i < 2; ++i) {
             StringJoiner str = new StringJoiner("hello");
-            cache.addNestedSetToFingerprint(str::join, new Fingerprint(), nestedSet);
+            cache.addNestedSetToFingerprint(str::expand, new Fingerprint(), nestedSet);
           }
         });
 
@@ -170,7 +172,7 @@ public class NestedSetFingerprintCacheTest {
           for (int i = 0; i < 2; ++i) {
             final int capturedVariable = i;
             cache.addNestedSetToFingerprint(
-                s -> s + capturedVariable, new Fingerprint(), nestedSet);
+                (s, args) -> args.accept(s + capturedVariable), new Fingerprint(), nestedSet);
           }
         });
   }
@@ -183,8 +185,8 @@ public class NestedSetFingerprintCacheTest {
     }
 
     @Override
-    public String expandToCommandLine(String object) {
-      return object + i;
+    public void expandToCommandLine(String object, Consumer<String> args) {
+      args.accept(object + i);
     }
 
     @Override
@@ -217,16 +219,16 @@ public class NestedSetFingerprintCacheTest {
       this.str = str;
     }
 
-    private String join(String other) {
-      return str + other;
+    private void expand(String other, Consumer<String> args) {
+      args.accept(str + other);
     }
   }
 
-  private static String simpleMapFn(String o) {
-    return o + "_mapped";
+  private static void simpleExpand(String o, Consumer<String> args) {
+    args.accept(o + "_mapped");
   }
 
-  private static String simpleMapFn2(String o) {
-    return o + "_mapped2";
+  private static void simpleExpand2(String o, Consumer<String> args) {
+    args.accept(o + "_mapped2");
   }
 }