Give all build settings a "help" attribute

It looks like Bazel adds this automatically, even though it doesn't seem
to be documented anywhere. For now we don't actually attempt to capture
the value and store it anywhere. This is something that we may do at
some point in the future.

Author: EdSchouten

pkg/model/starlark/builtins.go

@@ -91,6 +91,7 @@ func labelSetting[TReference object.BasicReference, TMetadata model_core.Referen
 
 	var name string
 	var buildSettingDefault string
+	var help string
 	singletonList := false
 	var visibility []pg_label.ResolvedLabel
 	labelOrStringUnpackerInto := NewLabelOrStringUnpackerInto[TReference, TMetadata](currentPackage)
@@ -101,6 +102,7 @@ func labelSetting[TReference object.BasicReference, TMetadata model_core.Referen
 		// None to implement command line flags that don't point
 		// to anything by default.
 		"build_setting_default", unpack.Bind(thread, &buildSettingDefault, unpack.IfNotNone(unpack.Stringer(labelOrStringUnpackerInto))),
+		"help?", unpack.Bind(thread, &help, unpack.String),
 		// Extension: --platforms is a list at the command line
 		// level, but it can only be set to a single value at a
 		// time during analysis. As part of transitions it still

pkg/model/starlark/rule.go

@@ -115,9 +115,9 @@ func (r *rule[TReference, TMetadata]) CallInternal(thread *starlark.Thread, args
 				"package_metadata", "tags", "target_compatible_with",
 				"testonly", "visibility":
 				return nil, fmt.Errorf("rule uses attribute with reserved name %#v", nameStr)
-			case "build_setting_default":
+			case "build_setting_default", "help":
 				if buildSetting != nil {
-					return nil, fmt.Errorf("rule uses attribute with name \"build_setting_default\", which is reserved for build settings", nameStr)
+					return nil, fmt.Errorf("rule uses attribute with name %#v, which is reserved for build settings", nameStr)
 				}
 			case "args", "flaky", "local", "shard_count", "size", "timeout":
 				if test {
@@ -186,6 +186,7 @@ func (r *rule[TReference, TMetadata]) CallInternal(thread *starlark.Thread, args
 
 	var buildSettingDefault starlark.Value
 	if buildSetting != nil {
+		var help string
 		mandatoryUnpackers = append(
 			mandatoryUnpackers,
 			"build_setting_default",
@@ -195,6 +196,7 @@ func (r *rule[TReference, TMetadata]) CallInternal(thread *starlark.Thread, args
 				unpack.Canonicalize(buildSetting.buildSettingType.GetCanonicalizer(currentPackage)),
 			),
 		)
+		optionalUnpackers = append(optionalUnpackers, "help?", unpack.Bind(thread, &help, unpack.String))
 	}
 
 	if test {