Update native CC rules to use an optional CC toolchain.

If the toolchain is needed and can't be found, CppHelper will fail with a more useful error message.

Part of #14727.

PiperOrigin-RevId: 459785208
Change-Id: If191c0a5dc7cf5a1759fa0af63bd53096e9deda8

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

@@ -374,6 +374,11 @@ public static Label getToolchainTypeFromRuleClass(RuleContext ruleContext) {
   private static CcToolchainProvider getToolchainFromPlatformConstraints(
       RuleContext ruleContext, Label toolchainType) 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?");
+    }
     try {
       return (CcToolchainProvider) toolchainInfo.getValue("cc");
     } catch (EvalException e) {

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

@@ -78,9 +78,9 @@ public static Label ccToolchainTypeAttribute(RuleDefinitionEnvironment env) {
   }
 
   public static ToolchainTypeRequirement ccToolchainTypeRequirement(Label ccToolchainType) {
-    // TODO(https://github.com/bazelbuild/bazel/issues/14727): Evaluate whether this can be
-    // optional.
-    return ToolchainTypeRequirement.builder(ccToolchainType).mandatory(true).build();
+    // This is an optional dependency: if a toolchain cannot be found, CppHelper will give an
+    // appropriate error.
+    return ToolchainTypeRequirement.builder(ccToolchainType).mandatory(false).build();
   }
 
   public static ToolchainTypeRequirement ccToolchainTypeRequirement(RuleDefinitionEnvironment env) {