Support optional toolchains with find_cpp_toolchain

Rules that want to use optional toolchains but still support builds that
don't use C++ toolchain resolution should point their `_cc_toolchain`
attribute to the new
`@bazel_tools//tools/cpp:optional_current_cc_toolchain` target.

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

@@ -32,7 +32,11 @@
 import com.google.devtools.build.lib.analysis.RunfilesProvider;
 import com.google.devtools.build.lib.analysis.TemplateVariableInfo;
 import com.google.devtools.build.lib.analysis.platform.ToolchainInfo;
+import com.google.devtools.build.lib.collect.nestedset.NestedSet;
+import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
+import com.google.devtools.build.lib.collect.nestedset.Order;
 import com.google.devtools.build.lib.packages.RuleClass;
+import com.google.devtools.build.lib.packages.Type;
 import javax.annotation.Nullable;
 
 /** Implementation of the {@code cc_toolchain_alias} rule. */
@@ -50,6 +54,7 @@ public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env)
         .add(
             attr(CcToolchain.CC_TOOLCHAIN_TYPE_ATTRIBUTE_NAME, NODEP_LABEL)
                 .value(CppRuleClasses.ccToolchainTypeAttribute(env)))
+        .add(attr("mandatory", Type.BOOLEAN).value(true))
         .requiresConfigurationFragments(PlatformConfiguration.class)
         .addToolchainTypes(CppRuleClasses.ccToolchainTypeRequirement(env))
         .build();
@@ -72,7 +77,14 @@ public ConfiguredTarget create(RuleContext ruleContext)
         throws InterruptedException, RuleErrorException, ActionConflictException {
 
       CcToolchainProvider ccToolchainProvider =
-          CppHelper.getToolchainUsingDefaultCcToolchainAttribute(ruleContext);
+          CppHelper.getToolchainUsingDefaultCcToolchainAttribute(ruleContext,
+              ruleContext.attributes().get("mandatory", Type.BOOLEAN));
+      if (ccToolchainProvider == null) {
+        return new RuleConfiguredTargetBuilder(ruleContext)
+            .addProvider(RunfilesProvider.simple(Runfiles.EMPTY))
+            .setFilesToBuild(NestedSetBuilder.emptySet(Order.STABLE_ORDER))
+            .build();
+      }
 
       TemplateVariableInfo templateVariableInfo =
           CcToolchain.createMakeVariableProvider(

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

@@ -249,13 +249,19 @@ public static NestedSet<Artifact> getGcovFilesIfNeeded(
   @Nullable
   public static CcToolchainProvider getToolchainUsingDefaultCcToolchainAttribute(
       RuleContext ruleContext) throws RuleErrorException {
+    return getToolchainUsingDefaultCcToolchainAttribute(ruleContext, true);
+  }
+
+  @Nullable
+  public static CcToolchainProvider getToolchainUsingDefaultCcToolchainAttribute(
+      RuleContext ruleContext, boolean mandatory) throws RuleErrorException {
     if (ruleContext.attributes().has(CcToolchain.CC_TOOLCHAIN_DEFAULT_ATTRIBUTE_NAME)) {
-      return getToolchain(ruleContext, CcToolchain.CC_TOOLCHAIN_DEFAULT_ATTRIBUTE_NAME);
+      return getToolchain(ruleContext, CcToolchain.CC_TOOLCHAIN_DEFAULT_ATTRIBUTE_NAME, mandatory);
     } else if (ruleContext
         .attributes()
         .has(CcToolchain.CC_TOOLCHAIN_DEFAULT_ATTRIBUTE_NAME_FOR_STARLARK)) {
       return getToolchain(
-          ruleContext, CcToolchain.CC_TOOLCHAIN_DEFAULT_ATTRIBUTE_NAME_FOR_STARLARK);
+          ruleContext, CcToolchain.CC_TOOLCHAIN_DEFAULT_ATTRIBUTE_NAME_FOR_STARLARK, mandatory);
     }
     return null;
   }
@@ -322,8 +328,9 @@ public static NestedSet<Artifact> getDefaultCcToolchainStaticRuntimeInputs(
    * Makes sure that the given info collection has a {@link CcToolchainProvider} (gives an error
    * otherwise), and returns a reference to that {@link CcToolchainProvider}.
    */
-  public static CcToolchainProvider getToolchain(RuleContext ruleContext, String toolchainAttribute)
-      throws RuleErrorException {
+  @Nullable
+  public static CcToolchainProvider getToolchain(RuleContext ruleContext, String toolchainAttribute,
+      boolean mandatory) throws RuleErrorException {
     if (!ruleContext.isAttrDefined(toolchainAttribute, LABEL)) {
       throw ruleContext.throwWithRuleError(
           String.format(
@@ -332,7 +339,7 @@ public static CcToolchainProvider getToolchain(RuleContext ruleContext, String t
               toolchainAttribute));
     }
     TransitiveInfoCollection dep = ruleContext.getPrerequisite(toolchainAttribute);
-    return getToolchain(ruleContext, dep);
+    return getToolchain(ruleContext, dep, mandatory);
   }
 
   /**
@@ -342,15 +349,21 @@ public static CcToolchainProvider getToolchain(RuleContext ruleContext, String t
    */
   public static CcToolchainProvider getToolchain(
       RuleContext ruleContext, TransitiveInfoCollection dep) throws RuleErrorException {
+    return getToolchain(ruleContext, dep, true);
+  }
+
+  public static CcToolchainProvider getToolchain(
+      RuleContext ruleContext, TransitiveInfoCollection dep, boolean mandatory)
+      throws RuleErrorException {
     Label toolchainType = getToolchainTypeFromRuleClass(ruleContext);
-    return getToolchain(ruleContext, dep, toolchainType);
+    return getToolchain(ruleContext, dep, toolchainType, mandatory);
   }
 
   public static CcToolchainProvider getToolchain(
-      RuleContext ruleContext, TransitiveInfoCollection dep, Label toolchainType)
+      RuleContext ruleContext, TransitiveInfoCollection dep, Label toolchainType, boolean mandatory)
       throws RuleErrorException {
     if (toolchainType != null && useToolchainResolution(ruleContext)) {
-      return getToolchainFromPlatformConstraints(ruleContext, toolchainType);
+      return getToolchainFromPlatformConstraints(ruleContext, toolchainType, mandatory);
     }
     return getToolchainFromLegacyToolchain(ruleContext, dep);
   }
@@ -372,12 +385,15 @@ public static Label getToolchainTypeFromRuleClass(RuleContext ruleContext) {
   }
 
   private static CcToolchainProvider getToolchainFromPlatformConstraints(
-      RuleContext ruleContext, Label toolchainType) throws RuleErrorException {
+      RuleContext ruleContext, Label toolchainType, boolean mandatory) throws RuleErrorException {
     ToolchainInfo toolchainInfo = ruleContext.getToolchainContext().forToolchainType(toolchainType);
     if (toolchainInfo == null) {
-      throw ruleContext.throwWithRuleError(
-          "Unable to find a CC toolchain using toolchain resolution. Did you properly set"
-              + " --platforms?");
+      if (mandatory) {
+        throw ruleContext.throwWithRuleError(
+            "Unable to find a CC toolchain using toolchain resolution. Did you properly set"
+                + " --platforms?");
+      }
+      return null;
     }
     try {
       return (CcToolchainProvider) toolchainInfo.getValue("cc");

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

@@ -399,7 +399,8 @@ private CcToolchainProvider ccToolchain(RuleContext ruleContext) throws RuleErro
       return CppHelper.getToolchain(
           ruleContext,
           ruleContext.getPrerequisite(CcToolchain.CC_TOOLCHAIN_DEFAULT_ATTRIBUTE_NAME),
-          ccToolchainType);
+          ccToolchainType,
+          /*mandatory=*/true);
     }
 
     private ImmutableSet<Artifact> getOutputFiles(Iterable<String> suffixes)

src/main/java/com/google/devtools/build/lib/rules/objc/J2ObjcAspect.java

@@ -270,7 +270,7 @@ private ConfiguredAspect buildAspect(
 
       try {
         CcToolchainProvider ccToolchain =
-            CppHelper.getToolchain(ruleContext, ":j2objc_cc_toolchain");
+            CppHelper.getToolchain(ruleContext, ":j2objc_cc_toolchain", /*mandatory*/true);
         ImmutableList<String> extraCompileArgs =
             j2objcCompileWithARC(ruleContext)
                 ? ImmutableList.of("-fno-strict-overflow", "-fobjc-arc-exceptions")

tools/cpp/BUILD.tools

@@ -57,6 +57,11 @@ constraint_value(
 
 cc_toolchain_alias(name = "current_cc_toolchain")
 
+cc_toolchain_alias(
+    name = "optional_current_cc_toolchain",
+    mandatory = False,
+)
+
 cc_host_toolchain_alias(name = "current_cc_host_toolchain")
 
 cc_libc_top_alias(name = "current_libc_top")

tools/cpp/toolchain_utils.bzl

@@ -31,7 +31,8 @@ def find_cpp_toolchain(ctx):
       ctx: The rule context for which to find a toolchain.
 
     Returns:
-      A CcToolchainProvider.
+      A CcToolchainProvider, or None if the c++ toolchain was declared as
+      optional and not found.
     """
 
     # Check the incompatible flag for toolchain resolution.
@@ -40,9 +41,9 @@ def find_cpp_toolchain(ctx):
             fail("In order to use find_cpp_toolchain, you must include the '%s' in the toolchains argument to your rule." % CPP_TOOLCHAIN_TYPE)
         toolchain_info = ctx.toolchains[CPP_TOOLCHAIN_TYPE]
         if toolchain_info == None:
-            # No cpp toolchain was found, so report an error.
-            fail("Unable to find a CC toolchain using toolchain resolution. Target: %s, Platform: %s, Exec platform: %s" %
-                 (ctx.label, ctx.fragments.platform.platform, ctx.fragments.platform.host_platform))
+            # No cpp toolchain was found, which is only possible when the rule declares it as
+            # optional explicitly.
+            return None
         if hasattr(toolchain_info, "cc_provider_in_toolchain") and hasattr(toolchain_info, "cc"):
             return toolchain_info.cc
         return toolchain_info