Add causes parameter to fail and use it for extension tags

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/SingleExtensionEvalFunction.java

@@ -259,6 +259,7 @@ private ModuleExtensionContext createContext(
             StarlarkBazelModule.create(
                 abridgedModule,
                 extension,
+                extensionId,
                 usagesValue.getRepoMappings().get(moduleKey),
                 usagesValue.getExtensionUsages().get(moduleKey)));
       } catch (ExternalDepsException e) {

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/StarlarkBazelModule.java

@@ -101,6 +101,7 @@ private StarlarkBazelModule(String name, String version, Tags tags, boolean isRo
   public static StarlarkBazelModule create(
       AbridgedModule module,
       ModuleExtension extension,
+      ModuleExtensionId extensionId,
       RepositoryMapping repoMapping,
       @Nullable ModuleExtensionUsage usage)
       throws ExternalDepsException {
@@ -130,7 +131,7 @@ public static StarlarkBazelModule create(
       // (for example, String to Label).
       typeCheckedTags
           .get(tag.getTagName())
-          .add(TypeCheckedTag.create(tagClass, tag, labelConverter));
+          .add(TypeCheckedTag.create(tagClass, tag, labelConverter, tag.getTagName(), extensionId));
     }
     return new StarlarkBazelModule(
         module.getName(),

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/TypeCheckedTag.java

@@ -23,25 +23,42 @@
 import javax.annotation.Nullable;
 import net.starlark.java.annot.StarlarkBuiltin;
 import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.FailureCause;
+import net.starlark.java.eval.Printer;
+import net.starlark.java.eval.StarlarkSemantics;
 import net.starlark.java.eval.Structure;
 import net.starlark.java.spelling.SpellChecker;
+import net.starlark.java.syntax.Location;
 
 /**
  * A {@link Tag} whose attribute values have been type-checked against the attribute schema define
  * in the {@link TagClass}.
  */
 @StarlarkBuiltin(name = "bazel_module_tag", documented = false)
-public class TypeCheckedTag implements Structure {
+public class TypeCheckedTag implements FailureCause, Structure {
+
   private final TagClass tagClass;
   private final Object[] attrValues;
 
-  private TypeCheckedTag(TagClass tagClass, Object[] attrValues) {
+  // The properties below are only used for error reporting.
+  private final Location location;
+  private final String tagClassName;
+  private final String extensionName;
+
+  private TypeCheckedTag(TagClass tagClass, Object[] attrValues, Location location,
+      String tagClassName, String extensionName) {
     this.tagClass = tagClass;
     this.attrValues = attrValues;
+    this.location = location;
+    this.tagClassName = tagClassName;
+    this.extensionName = extensionName;
   }
 
-  /** Creates a {@link TypeCheckedTag}. */
-  public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter labelConverter)
+  /**
+   * Creates a {@link TypeCheckedTag}.
+   */
+  public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter labelConverter,
+      String tagClassName, ModuleExtensionId extensionId)
       throws ExternalDepsException {
     Object[] attrValues = new Object[tagClass.getAttributes().size()];
     for (Map.Entry<String, Object> attrValue : tag.getAttributeValues().entrySet()) {
@@ -95,7 +112,8 @@ public static TypeCheckedTag create(TagClass tagClass, Tag tag, LabelConverter l
         attrValues[i] = Attribute.valueToStarlark(attr.getDefaultValueUnchecked());
       }
     }
-    return new TypeCheckedTag(tagClass, attrValues);
+    return new TypeCheckedTag(tagClass, attrValues, tag.getLocation(), tagClassName,
+        extensionId.getExtensionName());
   }
 
   @Override
@@ -123,4 +141,28 @@ public ImmutableCollection<String> getFieldNames() {
   public String getErrorMessageForUnknownField(String field) {
     return "unknown attribute " + field;
   }
+
+  @Override
+  public void debugPrint(Printer printer, StarlarkSemantics semantics) {
+    printer.append(extensionName);
+    printer.append('.');
+    printer.append(tagClassName);
+    printer.append("(\n");
+
+    for (int i = 0; i < attrValues.length; i++) {
+      Attribute attr = tagClass.getAttributes().get(i);
+      printer.append("    ");
+      printer.append(attr.getPublicName());
+      printer.append(" = ");
+      printer.repr(attrValues[i]);
+      printer.append(",\n");
+    }
+
+    printer.append(")");
+  }
+
+  @Override
+  public Location getLocation() {
+    return location;
+  }
 }

src/main/java/net/starlark/java/eval/BUILD

@@ -22,6 +22,7 @@ java_library(
         "Eval.java",
         "EvalException.java",
         "EvalUtils.java",
+        "FailureCause.java",
         "FlagGuardedValue.java",
         "FormatParser.java",
         "GuardedValue.java",
@@ -40,6 +41,7 @@ java_library(
         "RangeList.java",
         "RegularTuple.java",
         "Sequence.java",
+        "SilentCloseable.java",
         "SingletonTuple.java",
         "Starlark.java",
         "StarlarkCallable.java",

src/main/java/net/starlark/java/eval/EvalException.java

@@ -31,13 +31,16 @@
 /** An EvalException indicates an Starlark evaluation error. */
 public class EvalException extends Exception {
 
+  private static final ImmutableList<StarlarkThread.CallStackEntry> NO_CALLSTACK_SENTINEL =
+      ImmutableList.of();
+
   // The call stack associated with this error.
   // It is initially null, but is set by the interpreter to a non-empty
   // stack when popping a frame. Thus an exception newly created by a
   // built-in function has no stack until it is thrown out of a function call.
   @Nullable private ImmutableList<StarlarkThread.CallStackEntry> callstack;
 
-  /** Constructs an EvalException. Use {@link Starlak#errorf} if you want string formatting. */
+  /** Constructs an EvalException. Use {@link Starlark#errorf} if you want string formatting. */
   public EvalException(String message) {
     this(message, /*cause=*/ null);
   }
@@ -57,6 +60,12 @@ public EvalException(Throwable cause) {
     super(getCauseMessage(cause), cause);
   }
 
+  static EvalException createWithoutCallStack(String message) {
+    EvalException e = new EvalException(message);
+    e.callstack = NO_CALLSTACK_SENTINEL;
+    return e;
+  }
+
   private static String getCauseMessage(Throwable cause) {
     String msg = cause.getMessage();
     return msg != null ? msg : cause.toString();
@@ -107,7 +116,7 @@ public final String getMessageWithStack() {
    * source line for each stack frame is obtained from the provided SourceReader.
    */
   public final String getMessageWithStack(SourceReader src) {
-    if (callstack != null) {
+    if (callstack != null && callstack != NO_CALLSTACK_SENTINEL) {
       return formatCallStack(callstack, getMessage(), src);
     }
     return getMessage();

src/main/java/net/starlark/java/eval/FailureCause.java

@@ -0,0 +1,39 @@
+package net.starlark.java.eval;
+
+import net.starlark.java.annot.StarlarkBuiltin;
+import net.starlark.java.syntax.Location;
+
+/**
+ * A {@link StarlarkValue} that can be used as a cause in calls to {@code link}.
+ *
+ * <p>Implementations should override {@link StarlarkValue#debugPrint(Printer, StarlarkSemantics)}
+ * and ensure that, combined with {@link FailureCause#getLocation()}, this provides sufficient
+ * information for users to understand the cause of a failure without a stack trace.
+ */
+@StarlarkBuiltin(name = "failure_cause",
+    doc = "Can be passed to the <code>cause</code> parameter of "
+        + "<a href=\"globals.html#fail\"><code>fail</code></a> to add context to an error message.")
+public interface FailureCause extends StarlarkValue {
+
+  Location getLocation();
+
+  default void appendCauseTo(Printer printer, StarlarkSemantics semantics) {
+    printer.append(Starlark.type(this));
+    Location location = getLocation();
+    printer.append(" in file \"");
+    printer.append(location.file());
+    printer.append('\"');
+    if (location.line() != 0) {
+      printer.append(", line ");
+      printer.append(location.line());
+      if (location.column() != 0) {
+        printer.append(", column ");
+        printer.append(location.column());
+      }
+    }
+    printer.append(", with value:\n");
+    try (SilentCloseable ignored = printer.indent()) {
+      debugPrint(printer, semantics);
+    }
+  }
+}

src/main/java/net/starlark/java/eval/MethodLibrary.java

@@ -673,6 +673,16 @@ public StarlarkList<?> dir(Object object, StarlarkThread thread) throws EvalExce
                 "Deprecated. Causes an optional prefix containing this string to be added to the"
                     + " error message.",
             positional = false,
+            named = true),
+        @Param(
+            name = "causes",
+            allowedTypes = {
+              @ParamType(type = Sequence.class, generic1 = FailureCause.class),
+            },
+            defaultValue = "[]",
+            doc = "A list of <code>failure_causes</code> to append to the error message. If this "
+                + "list is not empty, no stack trace is printed.",
+            positional = false,
             named = true)
       },
       extraPositionals =
@@ -682,8 +692,9 @@ public StarlarkList<?> dir(Object object, StarlarkThread thread) throws EvalExce
                   "A list of values, formatted with str and joined with spaces, that appear in the"
                       + " error message."),
       useStarlarkThread = true)
-  public void fail(Object msg, Object attr, Tuple args, StarlarkThread thread)
+  public void fail(Object msg, Object attr, Sequence<?> contexts0, Tuple args, StarlarkThread thread)
       throws EvalException {
+    Sequence<FailureCause> causes = Sequence.cast(contexts0, FailureCause.class, "causes");
     List<String> elems = new ArrayList<>();
     // msg acts like a leading element of args.
     if (msg != Starlark.NONE) {
@@ -696,7 +707,21 @@ public void fail(Object msg, Object attr, Tuple args, StarlarkThread thread)
     if (attr != Starlark.NONE) {
       str = String.format("attribute %s: %s", attr, str);
     }
-    throw Starlark.errorf("%s", str);
+
+    if (causes.isEmpty()) {
+      throw Starlark.errorf("%s", str);
+    } else {
+      Printer p = new Printer();
+      p.append(str);
+      p.append("\nCauses:");
+      try (SilentCloseable ignored = p.indent()) {
+        for (FailureCause cause : causes) {
+          p.append('\n');
+          cause.appendCauseTo(p, thread.getSemantics());
+        }
+      }
+      throw EvalException.createWithoutCallStack(p.toString());
+    }
   }
 
   @StarlarkMethod(

src/main/java/net/starlark/java/eval/Printer.java

@@ -14,6 +14,7 @@
 package net.starlark.java.eval;
 
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import com.google.errorprone.annotations.CheckReturnValue;
 import java.util.Arrays;
 import java.util.IllegalFormatException;
 import java.util.List;
@@ -36,6 +37,7 @@ public class Printer {
   // indicating a cycle.
   private Object[] stack;
   private int depth;
+  private String indent = "";
 
   /** Creates a printer that writes to the given buffer. */
   public Printer(StringBuilder buffer) {
@@ -47,24 +49,47 @@ public Printer() {
     this(new StringBuilder());
   }
 
+  /** Increases the indent level by one tab.
+   *  Should be called at the beginning of a new line.
+   */
+  @CheckReturnValue
+  public final SilentCloseable indent() {
+    indent += '\t';
+    buffer.append('\t');
+    return () -> indent = indent.substring(0, indent.length() - 1);
+  }
+
   /** Appends a char to the printer's buffer */
   @CanIgnoreReturnValue
   public final Printer append(char c) {
     buffer.append(c);
+    if (c == '\n') {
+      buffer.append(indent);
+    }
     return this;
   }
 
   /** Appends a char sequence to the printer's buffer */
   @CanIgnoreReturnValue
   public final Printer append(CharSequence s) {
-    buffer.append(s);
-    return this;
+    if (indent.isEmpty()) {
+      buffer.append(s);
+      return this;
+    } else {
+      return append(s, 0, s.length());
+    }
   }
 
   /** Appends a char subsequence to the printer's buffer */
   @CanIgnoreReturnValue
   public final Printer append(CharSequence s, int start, int end) {
-    buffer.append(s, start, end);
+    if (indent.isEmpty()) {
+      buffer.append(s, start, end);
+    } else {
+      for (int i = start; i < end; i++) {
+        append(s.charAt(i));
+      }
+    }
     return this;
   }
 

src/main/java/net/starlark/java/eval/SilentCloseable.java

@@ -0,0 +1,6 @@
+package net.starlark.java.eval;
+
+public interface SilentCloseable extends AutoCloseable {
+    @Override
+    void close();
+}

src/test/java/com/google/devtools/build/lib/bazel/bzlmod/StarlarkBazelModuleTest.java

@@ -89,7 +89,9 @@ public void basic() throws Exception {
 
     StarlarkBazelModule moduleProxy =
         StarlarkBazelModule.create(
-            abridgedModule, extension, module.getRepoMappingWithBazelDepsOnly(), usage);
+            abridgedModule, extension,
+            ModuleExtensionId.create(Label.parseCanonicalUnchecked("@foo//:extensions.bzl"), "ext"),
+            module.getRepoMappingWithBazelDepsOnly(), usage);
 
     assertThat(moduleProxy.getName()).isEqualTo("foo");
     assertThat(moduleProxy.getVersion()).isEqualTo("1.0");
@@ -136,7 +138,11 @@ public void unknownTagClass() throws Exception {
             ExternalDepsException.class,
             () ->
                 StarlarkBazelModule.create(
-                    abridgedModule, extension, module.getRepoMappingWithBazelDepsOnly(), usage));
+                    abridgedModule, extension,
+                    ModuleExtensionId.create(Label.parseCanonicalUnchecked("@foo//:extensions.bzl"),
+                        "ext"),
+                    module.getRepoMappingWithBazelDepsOnly(),
+                    usage));
     assertThat(e).hasMessageThat().contains("does not have a tag class named blep");
   }
 }