From ccb92bd07e4adf88dd49b60d3bd71688f37cca51 Mon Sep 17 00:00:00 2001
From: Fabian Meumertzheim <fabian@meumertzhe.im>
Date: Wed, 15 Jan 2025 10:08:51 +0100
Subject: [PATCH] Show a conflict warning

---
 .../java/net/starlark/java/eval/Debug.java    | 24 +++++++++++++++++++
 .../starlark/java/eval/StarlarkThread.java    | 21 +++++++++++++---
 .../eval/StarlarkThreadDebuggingTest.java     |  2 +-
 3 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/src/main/java/net/starlark/java/eval/Debug.java b/src/main/java/net/starlark/java/eval/Debug.java
index e820b1469a6361..4b05a4349b04fb 100644
--- a/src/main/java/net/starlark/java/eval/Debug.java
+++ b/src/main/java/net/starlark/java/eval/Debug.java
@@ -19,6 +19,7 @@
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Predicate;
 import javax.annotation.Nullable;
+import net.starlark.java.annot.StarlarkBuiltin;
 import net.starlark.java.syntax.Location;
 
 /** Debugger API. */
@@ -37,6 +38,29 @@ public interface Debugger {
     void close();
   }
 
+  /** A distinguished value that carries a message. */
+  @StarlarkBuiltin(
+      name = "debugger_message",
+      doc = "A distinguished value that carries a message.",
+      documented = false)
+  public static final class DebuggerMessage implements StarlarkValue {
+    private final String message;
+
+    public DebuggerMessage(String message) {
+      this.message = message;
+    }
+
+    @Override
+    public String toString() {
+      return "<" + message + ">";
+    }
+
+    @Override
+    public void repr(Printer printer) {
+      printer.append("<").append(message).append(">");
+    }
+  }
+
   /** A Starlark value that can expose additional information to a debugger. */
   public interface ValueWithDebugAttributes extends StarlarkValue {
     /**
diff --git a/src/main/java/net/starlark/java/eval/StarlarkThread.java b/src/main/java/net/starlark/java/eval/StarlarkThread.java
index 66bfe737b8d0f3..f2083bf56d596d 100644
--- a/src/main/java/net/starlark/java/eval/StarlarkThread.java
+++ b/src/main/java/net/starlark/java/eval/StarlarkThread.java
@@ -19,7 +19,9 @@
 import com.google.common.collect.ImmutableMap;
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.Objects;
 import java.util.concurrent.atomic.AtomicInteger;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.Immutable;
@@ -182,8 +184,15 @@ public Location getLocation() {
 
     @Override
     public ImmutableMap<String, Object> getLocals() {
+      // TODO: List comprehensions introduce new locals that can shadow outer locals. Find a way to
+      //  accurately represent them and their values in a situation such as:
+      //
+      //  def foo(x):
+      //    x += [[j(x) for x in i(x)] + h(x) for x in f(x) if g(x)]
+      //    return k(x)
+      //
       // TODO(adonovan): provide a more efficient API.
-      ImmutableMap.Builder<String, Object> env = ImmutableMap.builder();
+      LinkedHashMap<String, Object> env = new LinkedHashMap<>();
       if (fn instanceof StarlarkFunction) {
         for (int i = 0; i < locals.length; i++) {
           Object local = locals[i];
@@ -191,11 +200,17 @@ public ImmutableMap<String, Object> getLocals() {
             local = ((StarlarkFunction.Cell) local).x;
           }
           if (local != null) {
-            env.put(((StarlarkFunction) fn).rfn.getLocals().get(i).getName(), local);
+            env.merge(
+                ((StarlarkFunction) fn).rfn.getLocals().get(i).getName(),
+                local,
+                (oldValue, newValue) ->
+                    Objects.equals(oldValue, newValue)
+                        ? oldValue
+                        : new Debug.DebuggerMessage("different values in scope"));
           }
         }
       }
-      return env.buildKeepingLast();
+      return ImmutableMap.copyOf(env);
     }
 
     @Override
diff --git a/src/test/java/net/starlark/java/eval/StarlarkThreadDebuggingTest.java b/src/test/java/net/starlark/java/eval/StarlarkThreadDebuggingTest.java
index a2cf9f0598cbdf..025ec044b130a3 100644
--- a/src/test/java/net/starlark/java/eval/StarlarkThreadDebuggingTest.java
+++ b/src/test/java/net/starlark/java/eval/StarlarkThreadDebuggingTest.java
@@ -315,7 +315,7 @@ def g(a, y, z):
         .isEqualTo(
             """
                 <toplevel> @ main.star:5:2 local={}
-                g @ main.star:4:4 local={a=[0, 1, 2, 3, 4, 5], y=5, z=6, x=5}
+                g @ main.star:4:4 local={a=[0, 1, 2, 3, 4, 5], y=5, z=6, x=<different values in scope>}
                 f @ builtin:12 local={}
                 """);
   }