Expose object files in the C++ Sandwich API

With these it is now possible to:

1) implement --start-lib/--end-lib behavior in Starlark rules
2) do the right thing when ThinLTO is enabled and objects actually contain bitcode files

RELNOTES: None.
PiperOrigin-RevId: 274176867

src/main/java/com/google/devtools/build/lib/rules/cpp/LibraryToLink.java

@@ -20,6 +20,8 @@
 import com.google.common.collect.ImmutableSet;
 import com.google.devtools.build.lib.actions.Artifact;
 import com.google.devtools.build.lib.skylarkbuildapi.cpp.LibraryToLinkApi;
+import com.google.devtools.build.lib.syntax.SkylarkList;
+import com.google.devtools.build.lib.syntax.SkylarkList.MutableList;
 import java.util.List;
 import javax.annotation.Nullable;
 
@@ -58,6 +60,15 @@ public Artifact getDynamicLibraryForRuntimeOrNull(boolean linkingStatically) {
   @Nullable
   public abstract ImmutableList<Artifact> getObjectFiles();
 
+  @Nullable
+  @Override
+  public SkylarkList<Artifact> getObjectFilesForStarlark() {
+    if (getObjectFiles() == null) {
+      return MutableList.empty();
+    }
+    return SkylarkList.createImmutable(getObjectFiles());
+  }
+
   @Nullable
   public abstract ImmutableMap<Artifact, LtoBackendArtifacts> getSharedNonLtoBackends();
 
@@ -71,6 +82,15 @@ public Artifact getDynamicLibraryForRuntimeOrNull(boolean linkingStatically) {
   @Nullable
   public abstract ImmutableList<Artifact> getPicObjectFiles();
 
+  @Nullable
+  @Override
+  public SkylarkList<Artifact> getPicObjectFilesForStarlark() {
+    if (getPicObjectFiles() == null) {
+      return MutableList.empty();
+    }
+    return SkylarkList.createImmutable(getPicObjectFiles());
+  }
+
   @Nullable
   public abstract ImmutableMap<Artifact, LtoBackendArtifacts> getPicSharedNonLtoBackends();
 

src/main/java/com/google/devtools/build/lib/skylarkbuildapi/cpp/LibraryToLinkApi.java

@@ -18,6 +18,7 @@
 import com.google.devtools.build.lib.skylarkinterface.SkylarkCallable;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModule;
 import com.google.devtools.build.lib.skylarkinterface.SkylarkModuleCategory;
+import com.google.devtools.build.lib.syntax.SkylarkList;
 
 /**
  * A library the user can link to. This is different from a simple linker input in that it also has
@@ -28,6 +29,20 @@
     category = SkylarkModuleCategory.BUILTIN,
     doc = "A library the user can link against.")
 public interface LibraryToLinkApi<FileT extends FileApi> {
+  @SkylarkCallable(
+      name = "objects",
+      allowReturnNones = true,
+      doc = "<code>List</code> of object files in the library.",
+      structField = true)
+  SkylarkList<FileT> getObjectFilesForStarlark();
+
+  @SkylarkCallable(
+      name = "pic_objects",
+      allowReturnNones = true,
+      doc = "<code>List</code> of pic object files in the library.",
+      structField = true)
+  SkylarkList<FileT> getPicObjectFilesForStarlark();
+
   @SkylarkCallable(
       name = "static_library",
       allowReturnNones = true,

src/test/java/com/google/devtools/build/lib/rules/cpp/SkylarkCcCommonTest.java

@@ -32,6 +32,7 @@
 import com.google.devtools.build.lib.packages.Provider;
 import com.google.devtools.build.lib.packages.SkylarkInfo;
 import com.google.devtools.build.lib.packages.SkylarkProvider;
+import com.google.devtools.build.lib.packages.SkylarkProviderIdentifier;
 import com.google.devtools.build.lib.packages.StructImpl;
 import com.google.devtools.build.lib.packages.util.Crosstool.CcToolchainConfig;
 import com.google.devtools.build.lib.packages.util.MockCcSupport;
@@ -51,8 +52,10 @@
 import com.google.devtools.build.lib.rules.cpp.CcToolchainVariables.StringValueParser;
 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.SkylarkDict;
 import com.google.devtools.build.lib.syntax.SkylarkList;
+import com.google.devtools.build.lib.syntax.SkylarkList.MutableList;
 import com.google.devtools.build.lib.syntax.SkylarkNestedSet;
 import com.google.devtools.build.lib.testutil.Scratch;
 import com.google.devtools.build.lib.testutil.TestConstants;
@@ -5504,6 +5507,96 @@ public void testApiWithAspectsOnTargetsInExternalRepos() throws Exception {
     assertThat(getConfiguredTarget("//b:b")).isNotNull();
   }
 
+  @Test
+  public void testObjectsApi() throws Exception {
+    useConfiguration("--compilation_mode=opt");
+    AnalysisMock.get()
+        .ccSupport()
+        .setupCcToolchainConfig(
+            mockToolsConfig,
+            CcToolchainConfig.builder()
+                .withFeatures(CppRuleClasses.SUPPORTS_PIC, CppRuleClasses.PIC));
+
+    scratchObjectsProvidingRule();
+
+    Provider.Key key =
+        new SkylarkProvider.SkylarkKey(
+            Label.parseAbsolute("//foo:foo.bzl", ImmutableMap.of()), "FooInfo");
+    LibraryToLink fooLibrary =
+        Iterables.getOnlyElement(
+            getConfiguredTarget("//foo:dep")
+                .get(CcInfo.PROVIDER)
+                .getCcLinkingContext()
+                .getLibraries()
+                .toList());
+    SkylarkInfo fooInfo =
+        (SkylarkInfo) getConfiguredTarget("//foo:foo").get(SkylarkProviderIdentifier.forKey(key));
+
+    assertThat(fooLibrary.getObjectFiles()).isEqualTo(fooInfo.getValue("objects"));
+    assertThat(fooLibrary.getPicObjectFiles()).isEqualTo(fooInfo.getValue("pic_objects"));
+  }
+
+  @Test
+  public void testObjectsApiNeverReturningNones() throws Exception {
+    scratchObjectsProvidingRule();
+
+    Provider.Key key =
+        new SkylarkProvider.SkylarkKey(
+            Label.parseAbsolute("//foo:foo.bzl", ImmutableMap.of()), "FooInfo");
+
+    // Default toolchain is without PIC support, so pic_objects should be None
+    SkylarkInfo fooInfoForPic =
+        (SkylarkInfo) getConfiguredTarget("//foo:foo").get(SkylarkProviderIdentifier.forKey(key));
+
+    Object picObjects = fooInfoForPic.getValue("pic_objects");
+    assertThat(picObjects).isNotEqualTo(Runtime.NONE);
+    assertThat((MutableList) picObjects).isEmpty();
+
+    // With PIC and the default compilation_mode which is fastbuild C++ rules only produce PIC
+    // objects.
+    AnalysisMock.get()
+        .ccSupport()
+        .setupCcToolchainConfig(
+            mockToolsConfig,
+            CcToolchainConfig.builder()
+                .withFeatures(CppRuleClasses.SUPPORTS_PIC, CppRuleClasses.PIC));
+    invalidatePackages();
+    SkylarkInfo fooInfoForNoPic =
+        (SkylarkInfo) getConfiguredTarget("//foo:foo").get(SkylarkProviderIdentifier.forKey(key));
+
+    Object objects = fooInfoForNoPic.getValue("objects");
+    assertThat(objects).isNotEqualTo(Runtime.NONE);
+    assertThat((MutableList) objects).isEmpty();
+  }
+
+  private void scratchObjectsProvidingRule() throws IOException {
+    scratch.file(
+        "foo/BUILD",
+        "load(':foo.bzl', 'foo')",
+        "foo(",
+        "  name = 'foo',",
+        "  dep = ':dep',",
+        ")",
+        "cc_library(",
+        "  name = 'dep',",
+        "  srcs = ['dep.cc'],",
+        ")");
+    scratch.file(
+        "foo/foo.bzl",
+        "FooInfo = provider(fields=['objects', 'pic_objects'])",
+        "",
+        "def _foo_impl(ctx):",
+        "  lib = ctx.attr.dep[CcInfo].linking_context.libraries_to_link.to_list()[0]",
+        "  return [FooInfo(objects=lib.objects, pic_objects=lib.pic_objects)]",
+        "",
+        "foo = rule(",
+        "  implementation = _foo_impl,",
+        "  attrs = {",
+        "    'dep': attr.label(),",
+        "  }",
+        ")");
+  }
+
   private static void createFiles(
       Scratch scratch, String bzlFilePath, String compileProviderLines, String linkProviderLines)
       throws Exception {