Use a dummy toolchain context for rules that don't have one.

Fixes #12610.

Closes #13162.

PiperOrigin-RevId: 361545255

Author: katre

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

@@ -76,6 +76,7 @@
 import com.google.devtools.build.lib.shell.ShellUtils;
 import com.google.devtools.build.lib.shell.ShellUtils.TokenizationException;
 import com.google.devtools.build.lib.starlarkbuildapi.StarlarkRuleContextApi;
+import com.google.devtools.build.lib.starlarkbuildapi.platform.ToolchainContextApi;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -701,9 +702,24 @@ public Dict<String, String> var() throws EvalException {
   }
 
   @Override
-  public ResolvedToolchainContext toolchains() throws EvalException {
+  public ToolchainContextApi toolchains() throws EvalException {
     checkMutable("toolchains");
-    return ruleContext.getToolchainContext();
+    ResolvedToolchainContext toolchainContext = ruleContext.getToolchainContext();
+    if (toolchainContext == null) {
+      // Starlark rules are easier if this cannot be null, so return a no-op value instead.
+      return new ToolchainContextApi() {
+        @Override
+        public Object getIndex(StarlarkSemantics semantics, Object key) {
+          return Starlark.NONE;
+        }
+
+        @Override
+        public boolean containsKey(StarlarkSemantics semantics, Object key) {
+          return false;
+        }
+      };
+    }
+    return toolchainContext;
   }
 
   @Override

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

@@ -3123,4 +3123,29 @@ public void testBuildFilePath() throws Exception {
     Object result = ev.eval("ruleContext.build_file_path");
     assertThat(result).isEqualTo("bar/BUILD");
   }
+
+  @Test
+  public void testNoToolchainContext() throws Exception {
+    // Build setting rules do not have a toolchain context, as they are part of the configuration.
+    scratch.file(
+        "test/BUILD",
+        "load(':rule.bzl', 'sample_setting')",
+        "toolchain_type(name = 'toolchain_type')",
+        "sample_setting(",
+        "    name = 'test',",
+        "    build_setting_default = True,",
+        ")");
+    scratch.file(
+        "test/rule.bzl",
+        "def _sample_impl(ctx):",
+        "    info = ctx.toolchains['//:toolchain_type']",
+        "    if info != None:",
+        "        fail('Toolchain should be empty')",
+        "sample_setting = rule(",
+        "    implementation = _sample_impl,",
+        "    build_setting = config.bool(flag = True),",
+        ")");
+    getConfiguredTarget("//test:test");
+    assertNoEvents();
+  }
 }