Skylark: Show list of fields in error message for struct objects.

--
MOS_MIGRATED_REVID=102341687

1 files changed, 8 insertions, 3 deletions

diff --git a/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java b/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java
index 2d477667d5..2d5cee7cb5 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/ClassObject.java
@@ -61,6 +61,8 @@ public interface ClassObject {
           + "See the global <a href=\"globals.html#struct\">struct</a> function "
           + "for more details.")
   public class SkylarkClassObject implements ClassObject {
+    /** Error message to use when errorMessage argument is null. */
+    private static final String DEFAULT_ERROR_MESSAGE = "'struct' object has no attribute '%s'";
 
     private final ImmutableMap<String, Object> values;
     private final Location creationLoc;
@@ -73,13 +75,13 @@ public interface ClassObject {
     public SkylarkClassObject(Map<String, Object> values, String errorMessage) {
       this.values = ImmutableMap.copyOf(values);
       this.creationLoc = null;
-      this.errorMessage = errorMessage;
+      this.errorMessage = Preconditions.checkNotNull(errorMessage);
     }
 
     public SkylarkClassObject(Map<String, Object> values, Location creationLoc) {
       this.values = ImmutableMap.copyOf(values);
       this.creationLoc = Preconditions.checkNotNull(creationLoc);
-      this.errorMessage = null;
+      this.errorMessage = DEFAULT_ERROR_MESSAGE;
     }
 
     @Override
@@ -110,7 +112,10 @@ public interface ClassObject {
 
     @Override
     public String errorMessage(String name) {
-      return errorMessage != null ? String.format(errorMessage, name) : null;
+      String suffix =
+          "Available attributes: "
+              + Joiner.on(", ").join(Ordering.natural().sortedCopy(values.keySet()));
+      return String.format(errorMessage, name) + "\n" + suffix;
     }
 
     /**