Add platform and toolchain related flags to the BuildConfiguration.

Part of bazelbuild#2219.

Change-Id: Ifd8aacef04bfbe2aa25eec7551803dc6e6a59ce5

src/main/java/com/google/devtools/build/lib/analysis/config/BuildConfiguration.java

@@ -1046,6 +1046,35 @@ public DynamicConfigsConverter() {
     )
     public TriState buildPythonZip;
 
+    @Option(
+        name = "experimental_target_platform",
+        defaultValue = "@bazel_tools//platforms:host_platform",
+        converter = LabelConverter.class,
+        optionUsageRestrictions = OptionUsageRestrictions.HIDDEN,
+        help = "Declare the platform targeted by the current build action"
+    )
+    public Label targetPlatform;
+
+    @Option(
+        name = "experimental_host_platform",
+        defaultValue = "@bazel_tools//platforms:host_platform",
+        converter = LabelConverter.class,
+        optionUsageRestrictions = OptionUsageRestrictions.HIDDEN,
+        help = "Declare the platform the build is started from"
+    )
+    public Label hostPlatform;
+
+    // TODO(katre): Add available execution platforms.
+
+    @Option(
+        name = "experimental_toolchains",
+        defaultValue = "",
+        converter = LabelListConverter.class,
+        optionUsageRestrictions = OptionUsageRestrictions.HIDDEN,
+        help = "Declares which toolchains to consider during toolchain resolution"
+    )
+    public List<Label> toolchains;
+
     @Override
     public FragmentOptions getHost(boolean fallback) {
       Options host = (Options) getDefault();
@@ -1092,6 +1121,13 @@ public FragmentOptions getHost(boolean fallback) {
       // === Pass on C++ compiler features.
       host.defaultFeatures = ImmutableList.copyOf(defaultFeatures);
 
+      // == Set the target platform.
+      host.targetPlatform = hostPlatform;
+      host.hostPlatform = hostPlatform;
+
+      // == Pass on all toolchains.
+      host.toolchains = ImmutableList.copyOf(toolchains);
+
       return host;
     }
 
@@ -1102,7 +1138,11 @@ public void addAllLabels(Multimap<String, Label> labelMap) {
       if ((runUnder != null) && (runUnder.getLabel() != null)) {
         labelMap.put("RunUnder", runUnder.getLabel());
       }
+      labelMap.put("target_platform", targetPlatform);
+      labelMap.put("host_platform", hostPlatform);
+      labelMap.putAll("toolchains", toolchains);
     }
+
     @Override
     public Map<String, Set<Label>> getDefaultsLabels(BuildConfiguration.Options commonOptions) {
       return ImmutableMap.<String, Set<Label>>of(
@@ -2216,7 +2256,7 @@ public ImmutableMap<String, String> getLocalShellEnvironment() {
    * client's own environment, and are returned by this function.
    *
    * <p>The values of the "variable" variables are tracked in Skyframe via the {@link
-   * com.google.devtools.build.lib.skyframe.SkyFunctions.CLIENT_ENVIRONMENT_VARIABLE} skyfunction.
+   * com.google.devtools.build.lib.skyframe.SkyFunctions#CLIENT_ENVIRONMENT_VARIABLE} skyfunction.
    * This method only returns the names of those variables to be inherited, if set in the client's
    * environment. (Variables where the name is not returned in this set should not be taken from the
    * client environment.)
@@ -2594,4 +2634,16 @@ public Class<? extends Fragment> getSkylarkFragmentByName(String name) {
   public ImmutableCollection<String> getSkylarkFragmentNames() {
     return skylarkVisibleFragments.keySet();
   }
+
+  public Label getTargetPlatform() {
+    return options.targetPlatform;
+  }
+
+  public Label getHostPlatform() {
+    return options.hostPlatform;
+  }
+
+  public ImmutableList<Label> getToolchains() {
+    return ImmutableList.copyOf(options.toolchains);
+  }
 }