Update the no-op ToolchainContext to throw an EvalException.

This is only possible from starlark rules with no toolchain context, which are typically build settings. Such rules should not attempt to use toolchains as they are part of configuration.

PiperOrigin-RevId: 368200348

src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkToolchainContext.java

@@ -43,9 +43,9 @@ public abstract class StarlarkToolchainContext implements ToolchainContextApi {
       new ToolchainContextApi() {
         @Override
         public Object getIndex(
-            StarlarkThread starlarkThread, StarlarkSemantics semantics, Object key) {
-          // TODO(jcater): throw Starlark.errorf instead of returning NONE.
-          return Starlark.NONE;
+            StarlarkThread starlarkThread, StarlarkSemantics semantics, Object key)
+            throws EvalException {
+          throw Starlark.errorf("Toolchains are not valid in this context");
         }
 
         @Override

src/test/java/com/google/devtools/build/lib/starlark/StarlarkRuleContextTest.java

@@ -3233,14 +3233,15 @@ public void testNoToolchainContext() throws Exception {
     scratch.file(
         "test/rule.bzl",
         "def _sample_impl(ctx):",
-        "    info = ctx.toolchains['//:toolchain_type']",
-        "    if info != None:",
-        "        fail('Toolchain should be empty')",
+        "    # This should raise an error.",
+        "    ctx.toolchains['//:toolchain_type']",
+        "    fail('Toolchain was not empty')",
         "sample_setting = rule(",
         "    implementation = _sample_impl,",
         "    build_setting = config.bool(flag = True),",
         ")");
-    getConfiguredTarget("//test:test");
-    assertNoEvents();
+    assertThrows(AssertionError.class, () -> getConfiguredTarget("//test:test"));
+    assertContainsEvent("Toolchains are not valid in this context");
+    assertDoesNotContainEvent("Toolchain was not empty");
   }
 }