[6.3.0] Improve error on invalid -//foo and -@repo//foo options

src/main/java/com/google/devtools/common/options/OptionsParserImpl.java

@@ -476,6 +476,18 @@ private OptionsParserImplResult parse(
         continue; // not an option arg
       }
 
+      if (arg.startsWith("-//") || arg.startsWith("-@")) {
+        // Fail with a helpful error when an invalid option looks like an absolute negative target
+        // pattern or a typoed Starlark option.
+        throw new OptionsParsingException(
+            String.format(
+                "Invalid options syntax: %s\n"
+                    + "Note: Negative target patterns can only appear after the end of options"
+                    + " marker ('--'). Flags corresponding to Starlark-defined build settings"
+                    + " always start with '--', not '-'.",
+                arg));
+      }
+
       arg = swapShorthandAlias(arg);
 
       if (arg.equals("--")) { // "--" means all remaining args aren't options

src/test/java/com/google/devtools/common/options/OptionsParserTest.java

@@ -2369,6 +2369,35 @@ public void setOptionValueAtSpecificPriorityWithoutExpansion_expandedFlag_setsVa
         .containsExactly("--second=hello");
   }
 
+  @Test
+  public void negativeTargetPatternsInOptions_failsDistinctively() {
+    OptionsParser parser = OptionsParser.builder().optionsClasses(ExampleFoo.class).build();
+    OptionsParsingException e =
+        assertThrows(OptionsParsingException.class, () -> parser.parse("//foo", "-//bar", "//baz"));
+    assertThat(e).hasMessageThat().contains("-//bar");
+    assertThat(e)
+        .hasMessageThat()
+        .contains("Negative target patterns can only appear after the end of options marker");
+    assertThat(e)
+        .hasMessageThat()
+        .contains("Flags corresponding to Starlark-defined build settings always start with '--'");
+  }
+
+  @Test
+  public void negativeExternalTargetPatternsInOptions_failsDistinctively() {
+    OptionsParser parser = OptionsParser.builder().optionsClasses(ExampleFoo.class).build();
+    OptionsParsingException e =
+        assertThrows(
+            OptionsParsingException.class, () -> parser.parse("//foo", "-@repo//bar", "//baz"));
+    assertThat(e).hasMessageThat().contains("-@repo//bar");
+    assertThat(e)
+        .hasMessageThat()
+        .contains("Negative target patterns can only appear after the end of options marker");
+    assertThat(e)
+        .hasMessageThat()
+        .contains("Flags corresponding to Starlark-defined build settings always start with '--'");
+  }
+
   private static OptionInstanceOrigin createInvocationPolicyOrigin() {
     return createInvocationPolicyOrigin(/*implicitDependent=*/ null, /*expandedFrom=*/ null);
   }