From 247ce0c8b7250c7f40c3c66131ad00e9b98b5b34 Mon Sep 17 00:00:00 2001
From: Fabian Meumertzheim <fabian@meumertzhe.im>
Date: Thu, 17 Aug 2023 13:16:32 -0700
Subject: [PATCH] Use `debugPrint` instead of `str` for `fail` arguments

This allows non-hermetic information (such as Starlark `Location`s) to be included in `fail` messages without allowing access to it from Starlark. It also aligns the content of `fail` output with that of `print`.

This change has very mild implications for backwards compatibility: it only affects the contents of `fail` messages, which aren't accessible from Starlark; `debugPrint`'s default implementation is `str` and no built-in Starlark type overrides it; if a type overrides `debugPrint`, it would usually contain at least as detailed information as `str`; `debugPrint` is a "Bazelism" that isn't governed by the Starlark spec.

Work towards #17375

Closes #18818.

Co-authored-by: Alexandre Rostovtsev <arostovtsev@google.com>
PiperOrigin-RevId: 557916312
Change-Id: I8f202cd8530bcebb2d99f57745289b3992d03cac
---
 .../net/starlark/java/eval/MethodLibrary.java | 32 +++++++++++--------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/src/main/java/net/starlark/java/eval/MethodLibrary.java b/src/main/java/net/starlark/java/eval/MethodLibrary.java
index 5610061162982a..be5cf11651363b 100644
--- a/src/main/java/net/starlark/java/eval/MethodLibrary.java
+++ b/src/main/java/net/starlark/java/eval/MethodLibrary.java
@@ -15,13 +15,10 @@
 package net.starlark.java.eval;
 
 import com.google.common.base.Ascii;
-import com.google.common.base.Joiner;
 import com.google.common.collect.Ordering;
-import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Comparator;
 import java.util.Iterator;
-import java.util.List;
 import java.util.NoSuchElementException;
 import net.starlark.java.annot.Param;
 import net.starlark.java.annot.ParamType;
@@ -679,24 +676,33 @@ public StarlarkList<?> dir(Object object, StarlarkThread thread) throws EvalExce
           @Param(
               name = "args",
               doc =
-                  "A list of values, formatted with str and joined with spaces, that appear in the"
-                      + " error message."),
+                  "A list of values, formatted with debugPrint (which is equivalent to str by"
+                      + " default) and joined with spaces, that appear in the error message."),
       useStarlarkThread = true)
   public void fail(Object msg, Object attr, Tuple args, StarlarkThread thread)
       throws EvalException {
-    List<String> elems = new ArrayList<>();
+    Printer printer = new Printer();
+    boolean needSeparator = false;
+    if (attr != Starlark.NONE) {
+      printer.append("attribute ").append((String) attr).append(":");
+      needSeparator = true;
+    }
     // msg acts like a leading element of args.
     if (msg != Starlark.NONE) {
-      elems.add(Starlark.str(msg, thread.getSemantics()));
+      if (needSeparator) {
+        printer.append(" ");
+      }
+      printer.debugPrint(msg, thread.getSemantics());
+      needSeparator = true;
     }
     for (Object arg : args) {
-      elems.add(Starlark.str(arg, thread.getSemantics()));
-    }
-    String str = Joiner.on(" ").join(elems);
-    if (attr != Starlark.NONE) {
-      str = String.format("attribute %s: %s", attr, str);
+      if (needSeparator) {
+        printer.append(" ");
+      }
+      printer.debugPrint(arg, thread.getSemantics());
+      needSeparator = true;
     }
-    throw Starlark.errorf("%s", str);
+    throw Starlark.errorf("%s", printer.toString());
   }
 
   @StarlarkMethod(