Improved error message when accessing undeclared configuration fragments.

Users will get an error when rules access configuration fragments without having declared them as required fragments.
The error message now contains more details on the cause of the error and may also include a possible Skylark fix, if applicable.

--
MOS_MIGRATED_REVID=102497559

1 files changed, 22 insertions, 7 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
index b6affcacf9..16982f0519 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
@@ -259,12 +259,20 @@ public final class RuleContext extends TargetContext
    */
   @Nullable
   public <T extends Fragment> T getFragment(Class<T> fragment, ConfigurationTransition config) {
+    return getFragment(fragment, fragment.getSimpleName(), "", config);
+  }
+
+  @Nullable
+  protected <T extends Fragment> T getFragment(Class<T> fragment, String name,
+      String additionalErrorMessage, ConfigurationTransition config) {
     // TODO(bazel-team): The fragments can also be accessed directly through BuildConfiguration.
     // Can we lock that down somehow?
     Preconditions.checkArgument(isLegalFragment(fragment, config),
-        "%s does not have access to '%s' in %s configuration", rule.getRuleClass(),
-        fragment.getSimpleName(), FragmentCollection.getConfigurationName(config));
-    return getConfiguration(config).getFragment(fragment);
+        "%s has to declare '%s' as a required fragment "
+        + "in %s configuration in order to access it.%s",
+        rule.getRuleClass(), name, FragmentCollection.getConfigurationName(config),
+        additionalErrorMessage);
+    return getConfiguration().getFragment(fragment);
   }
 
   @Nullable
@@ -275,9 +283,16 @@ public final class RuleContext extends TargetContext
 
   @Nullable
   public Fragment getSkylarkFragment(String name, ConfigurationTransition config) {
-    Class<? extends Fragment> fragmentClass =
-        getConfiguration(config).getSkylarkFragmentByName(name);
-    return (fragmentClass == null) ? null : getFragment(fragmentClass, config);
+    Class<? extends Fragment> fragmentClass = getConfiguration().getSkylarkFragmentByName(name);
+    if (fragmentClass == null) {
+      return null;
+    }
+    return getFragment(fragmentClass, name,
+        String.format(
+            " Please update the '%1$sfragments' argument of the rule definition "
+            + "(for example: %1$sfragments = [\"%2$s\"])",
+            (config == ConfigurationTransition.HOST) ? "host_" : "", name),
+        config);
   }
 
   public ImmutableCollection<String> getSkylarkFragmentNames(ConfigurationTransition config) {
@@ -293,7 +308,7 @@ public final class RuleContext extends TargetContext
     // NONE means target configuration.
     return isLegalFragment(fragment, ConfigurationTransition.NONE);
   }
-  
+
   protected BuildConfiguration getConfiguration(ConfigurationTransition config) {
     return config.equals(ConfigurationTransition.HOST) ? hostConfiguration : getConfiguration();
   }