Fix default behavior of swift_prefix to remain unset in managed mode (#4245)

Author: doriable

CHANGELOG.md

@@ -2,7 +2,8 @@
 
 ## [Unreleased]
 
-- No changes yet.
+- Fix default behavior for `swift_prefix` to remain unset when no override is provided in
+  managed mode.
 
 ## [v1.62.0] - 2025-12-29
 

private/bufpkg/bufimage/bufimagemodify/bufimagemodify_test.go

@@ -99,7 +99,6 @@ func TestModifyImage(t *testing.T) {
 					PhpMetadataNamespace: proto.String(`Foo\Empty_\GPBMetadata`),
 					PhpNamespace:         proto.String(`Foo\Empty_`),
 					RubyPackage:          proto.String("Foo::Empty"),
-					SwiftPrefix:          proto.String("Foo_Empty_"),
 				},
 				"foo_empty/without_package.proto": {
 					// CcEnableArena's default value is true
@@ -126,7 +125,7 @@ func TestModifyImage(t *testing.T) {
 					PhpNamespace:         proto.String(`Bar\All`),
 					PyGenericServices:    proto.Bool(false),
 					RubyPackage:          proto.String("Bar::All"),
-					SwiftPrefix:          proto.String("Bar_All_"),
+					SwiftPrefix:          proto.String("bar"),
 				},
 				"bar_all/without_package.proto": {
 					CcEnableArenas:       proto.Bool(true),

private/bufpkg/bufimage/bufimagemodify/file_option.go

@@ -380,10 +380,10 @@ func modifySwiftPrefix(
 		bufconfig.FileOptionUnspecified,
 		bufconfig.FileOptionUnspecified,
 		func(bufimage.ImageFile) stringOverrideOptions {
-			return stringOverrideOptions{value: swiftPrefixValue(imageFile)}
+			return stringOverrideOptions{}
 		},
-		func(imageFile bufimage.ImageFile, _ stringOverrideOptions) string {
-			return swiftPrefixValue(imageFile)
+		func(imageFile bufimage.ImageFile, stringOverrideOptions stringOverrideOptions) string {
+			return stringOverrideOptions.value
 		},
 		func(options *descriptorpb.FileOptions) string {
 			return options.GetSwiftPrefix()

private/bufpkg/bufimage/bufimagemodify/override.go

@@ -422,20 +422,3 @@ func rubyPackageValue(imageFile bufimage.ImageFile) string {
 	}
 	return strings.Join(packageParts, "::")
 }
-
-// swiftPrefixValue returns the swift_prefix for the given ImageFile based on its
-// package declaration. If the image file doesn't have a package declaration, an
-// empty string is returned.
-func swiftPrefixValue(imageFile bufimage.ImageFile) string {
-	pkg := imageFile.FileDescriptorProto().GetPackage()
-	if pkg == "" {
-		return ""
-	}
-	packageParts := strings.Split(pkg, ".")
-	for i, part := range packageParts {
-		packageParts[i] = xstrings.ToPascalCase(part)
-	}
-	// We add a "_" as the suffix since this emulates the default behavior if swift_prefix
-	// is not set.
-	return strings.Join(packageParts, "_") + "_"
-}