Add "toolchains" attribute to Skylark's rule definitions.

Part of bazelbuild#2219.

Change-Id: I4d749dd9981fe33f75310acb0ec3927cff6f28fe

src/main/java/com/google/devtools/build/lib/packages/RuleClass.java

@@ -508,6 +508,7 @@ public Transition buildTransitionFor(Rule rule) {
     private boolean supportsConstraintChecking = true;
 
     private final Map<String, Attribute> attributes = new LinkedHashMap<>();
+    private final List<SkylarkExportable> requiredToolchains = new ArrayList<>();
 
     /**
      * Constructs a new {@code RuleClassBuilder} using all attributes from all
@@ -618,6 +619,7 @@ public RuleClass build(String name) {
           ruleDefinitionEnvironmentHashCode,
           configurationFragmentPolicy.build(),
           supportsConstraintChecking,
+          requiredToolchains,
           attributes.values().toArray(new Attribute[0]));
     }
 
@@ -998,6 +1000,11 @@ public Builder setOptionReferenceFunctionForConfigSettingOnly(
       return this;
     }
 
+    public Builder addRequiredToolchain(SkylarkExportable toolchain) {
+      this.requiredToolchains.add(toolchain);
+      return this;
+    }
+
     /**
      * Returns an Attribute.Builder object which contains a replica of the
      * same attribute in the parent rule if exists.
@@ -1118,6 +1125,8 @@ public Attribute.Builder<?> copy(String name) {
    */
   private final boolean supportsConstraintChecking;
 
+  private final ImmutableList<SkylarkExportable> requiredToolchains;
+
   /**
    * Constructs an instance of RuleClass whose name is 'name', attributes
    * are 'attributes'. The {@code srcsAllowedFiles} determines which types of
@@ -1165,6 +1174,7 @@ public Attribute.Builder<?> copy(String name) {
       String ruleDefinitionEnvironmentHashCode,
       ConfigurationFragmentPolicy configurationFragmentPolicy,
       boolean supportsConstraintChecking,
+      List<SkylarkExportable> requiredToolchains,
       Attribute... attributes) {
     this.name = name;
     this.isSkylark = isSkylark;
@@ -1193,6 +1203,7 @@ public Attribute.Builder<?> copy(String name) {
     this.outputsDefaultExecutable = outputsDefaultExecutable;
     this.configurationFragmentPolicy = configurationFragmentPolicy;
     this.supportsConstraintChecking = supportsConstraintChecking;
+    this.requiredToolchains = ImmutableList.copyOf(requiredToolchains);
 
     // Create the index and collect non-configurable attributes.
     int index = 0;
@@ -2002,6 +2013,10 @@ public boolean outputsDefaultExecutable() {
     return outputsDefaultExecutable;
   }
 
+  public ImmutableList<SkylarkExportable> getRequiredToolchains() {
+    return requiredToolchains;
+  }
+
   public static boolean isThirdPartyPackage(PackageIdentifier packageIdentifier) {
     if (!packageIdentifier.getRepository().isMain()) {
       return false;

src/main/java/com/google/devtools/build/lib/rules/SkylarkRuleClassFunctions.java

@@ -357,6 +357,17 @@ public ClassObjectConstructor invoke(Location location) {
                       + "<br/><br/>"
                       + "This should only be used for testing the analysis-time behavior of "
                       + "Skylark rules. This flag may be removed in the future."
+          ),
+          @Param(
+              name = "toolchains",
+              type = SkylarkList.class,
+              generic1 = SkylarkExportable.class,
+              defaultValue = "[]",
+              doc =
+                  "<i>(Experimental)</i><br/><br/>"
+                      + "If set, the set of toolchains this rule requires. Toolchains will be "
+                      + "found by checking the current platform, and provided to the rule "
+                      + "implementation via <code>ctx.toolchain</code>."
           )
       },
       useAst = true,
@@ -376,6 +387,7 @@ public BaseFunction invoke(
             SkylarkList fragments,
             SkylarkList hostFragments,
             Boolean skylarkTestable,
+            SkylarkList<SkylarkExportable> toolchains,
             FuncallExpression ast,
             Environment funcallEnv)
             throws EvalException, ConversionException {
@@ -434,6 +446,11 @@ public BaseFunction invoke(
               hostFragments.getContents(String.class, "host_fragments"));
           builder.setConfiguredTargetFunction(implementation);
           builder.setRuleDefinitionEnvironment(funcallEnv);
+
+          for (SkylarkExportable toolchain : toolchains) {
+            builder.addRequiredToolchain(toolchain);
+          }
+
           return new RuleFunction(builder, type, attributes, ast.getLocation());
         }
       };

src/test/java/com/google/devtools/build/lib/packages/RuleClassTest.java

@@ -908,6 +908,7 @@ private static RuleClass newRuleClass(
             .setMissingFragmentPolicy(missingFragmentPolicy)
             .build(),
         supportsConstraintChecking,
+        ImmutableList.<SkylarkExportable>of(),
         attributes);
   }