Add non-Starlark optional toolchain API

src/main/java/com/google/devtools/build/lib/analysis/BUILD

@@ -1822,6 +1822,15 @@ java_library(
     ],
 )
 
+java_library(
+    name = "config/toolchain_type_requirement",
+    srcs = ["config/ToolchainTypeRequirement.java"],
+    deps = [
+        "//src/main/java/com/google/devtools/build/lib/cmdline",
+        "//third_party:auto_value",
+    ],
+)
+
 java_library(
     name = "config/transition_factories",
     srcs = ["config/TransitionFactories.java"],

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

@@ -0,0 +1,51 @@
+// Copyright 2022 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.analysis.config;
+
+import com.google.auto.value.AutoValue;
+import com.google.devtools.build.lib.cmdline.Label;
+
+/**
+ * Describes a requirement on a specific toolchain type.
+ */
+@AutoValue
+public abstract class ToolchainTypeRequirement {
+  /** Returns a builder for a new {@Link ToolchainTypeRequirement}. */
+  public static Builder builder() {
+    return new AutoValue_ToolchainTypeRequirement.Builder()
+        .mandatory(true);
+  }
+
+  /** Returns the label of the toolchain type that is requested. */
+  public abstract Label toolchainType();
+
+  /**
+   * Returns whether the toolchain type is mandatory or optional. An optional toolchain type which
+   * cannot be found will be skipped, but a mandatory toolchain type which cannot be found will stop
+   * the build with an error.
+   */
+  public abstract boolean mandatory();
+
+  /** A builder for a new {@link ToolchainTypeRequirement}. */
+  @AutoValue.Builder
+  public interface Builder {
+    /** Sets the toolchain type. */
+    Builder toolchainType(Label toolchainType);
+    /** Sets whether the toolchain type is mandatory. */
+    Builder mandatory(boolean mandatory);
+
+    /** Returns the newly built {@link ToolchainTypeRequirement}. */
+    ToolchainTypeRequirement build();
+  }
+}

src/test/java/com/google/devtools/build/lib/analysis/testing/ToolchainTypeRequirementSubject.java

@@ -0,0 +1,61 @@
+// Copyright 2022 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.analysis.testing;
+
+import static com.google.common.truth.Truth.assertAbout;
+
+import com.google.common.truth.ComparableSubject;
+import com.google.common.truth.FailureMetadata;
+import com.google.common.truth.Subject;
+import com.google.devtools.build.lib.analysis.config.ToolchainTypeRequirement;
+import com.google.devtools.build.lib.cmdline.Label;
+
+/** A Truth {@link Subject} for {@link ToolchainTypeRequirement}. */
+public class ToolchainTypeRequirementSubject extends Subject {
+  // Static data.
+
+  /** Entry point for test assertions related to {@link ToolchainTypeRequirement}. */
+  public static ToolchainTypeRequirementSubject assertThat(
+      ToolchainTypeRequirement toolchainTypeRequirement) {
+    return assertAbout(ToolchainTypeRequirementSubject::new).that(toolchainTypeRequirement);
+  }
+
+  /** Static method for getting the subject factory (for use with assertAbout()). */
+  public static Subject.Factory<ToolchainTypeRequirementSubject, ToolchainTypeRequirement>
+      toolchainTypeRequirements() {
+    return ToolchainTypeRequirementSubject::new;
+  }
+
+  // Instance fields.
+
+  private final ToolchainTypeRequirement actual;
+
+  protected ToolchainTypeRequirementSubject(
+      FailureMetadata failureMetadata, ToolchainTypeRequirement subject) {
+    super(failureMetadata, subject);
+    this.actual = subject;
+  }
+
+  public ComparableSubject<Label> toolchainType() {
+    return check("toolchainType").that(actual.toolchainType());
+  }
+
+  public void isMandatory() {
+    check("mandatory").that(actual.mandatory()).isTrue();
+  }
+
+  public void isOptional() {
+    check("mandatory").that(actual.mandatory()).isFalse();
+  }
+}