Enforce utf-8 validity in string() conversion from bytes.

Relies on SIMD-optimized library https://github.com/cyb70289/utf8.

PiperOrigin-RevId: 353736473

Author: kyessenov

conformance/BUILD

@@ -90,8 +90,6 @@ cc_binary(
             # uncommented when the spec changes to truncation rather than rounding.
             "--skip_test=conversions/int/double_nearest,double_nearest_neg,double_half_away_neg,double_half_away_pos",
             "--skip_test=conversions/uint/double_nearest,double_nearest_int,double_half_away",
-            # TODO(issues/82): Unexpected behavior when converting invalid bytes to string.
-            "--skip_test=conversions/string/bytes_invalid",
             # TODO(issues/96): Well-known type conversion support.
             "--skip_test=proto2/literal_wellknown",
             "--skip_test=proto3/literal_wellknown",

eval/public/BUILD

@@ -180,6 +180,7 @@ cc_library(
         ":cel_function_adapter",
         ":cel_function_registry",
         ":cel_options",
+        "//base:unilib",
         "//eval/public/containers:container_backed_list_impl",
         "@com_google_absl//absl/numeric:int128",
         "@com_google_absl//absl/status",

eval/public/builtin_func_registrar.cc

@@ -16,6 +16,7 @@
 #include "eval/public/cel_options.h"
 #include "eval/public/containers/container_backed_list_impl.h"
 #include "re2/re2.h"
+#include "base/unilib.h"
 
 namespace google {
 namespace api {
@@ -1145,16 +1146,15 @@ absl::Status RegisterStringConversionFunctions(
     return absl::OkStatus();
   }
 
-  // TODO(issues/82): ensure the bytes conversion to string handles UTF-8
-  // properly, and avoids unncessary allocations.
-  // bytes -> string
-  auto status = FunctionAdapter<CelValue::StringHolder, CelValue::BytesHolder>::
-      CreateAndRegister(
+  auto status =
+      FunctionAdapter<CelValue, CelValue::BytesHolder>::CreateAndRegister(
           builtin::kString, false,
-          [](Arena* arena,
-             CelValue::BytesHolder value) -> CelValue::StringHolder {
-            return CelValue::StringHolder(
-                Arena::Create<std::string>(arena, std::string(value.value())));
+          [](Arena* arena, CelValue::BytesHolder value) -> CelValue {
+            if (UniLib::IsStructurallyValid(value.value())) {
+              return CelValue::CreateStringView(value.value());
+            }
+            return CreateErrorValue(arena, "invalid UTF-8 bytes value",
+                                    absl::StatusCode::kInvalidArgument);
           },
           registry);
   if (!status.ok()) return status;

eval/public/builtin_func_test.cc

@@ -1686,6 +1686,15 @@ TEST_F(BuiltinsTest, BytesToString) {
   EXPECT_EQ(result_value.StringOrDie().value(), "abcd");
 }
 
+TEST_F(BuiltinsTest, BytesToStringInvalid) {
+  std::string input = "\xFF";
+  std::vector<CelValue> args = {CelValue::CreateBytes(&input)};
+  CelValue result_value;
+  ASSERT_NO_FATAL_FAILURE(
+      PerformRun(builtin::kString, {}, args, &result_value));
+  ASSERT_TRUE(result_value.IsError());
+}
+
 TEST_F(BuiltinsTest, StringToString) {
   std::string input = "abcd";
   std::vector<CelValue> args = {CelValue::CreateString(&input)};

eval/public/cel_value.h

@@ -162,6 +162,9 @@ class CelValue {
 
   static CelValue CreateString(StringHolder holder) { return CelValue(holder); }
 
+  // Returns a string value from a string_view. Warning: the caller is
+  // responsible for the lifecycle of the backing string. Prefer CreateString
+  // instead.
   static CelValue CreateStringView(absl::string_view value) {
     return CelValue(StringHolder(value));
   }

testutil/BUILD

@@ -49,6 +49,7 @@ cc_library(
 
 cc_library(
     name = "test_data_io",
+    testonly = True,
     srcs = [
         "test_data_io.cc",
     ],
@@ -76,6 +77,7 @@ cc_library(
 #        third_party/cel/spec/testdata/unique_values.textpb
 cc_binary(
     name = "test_data_gen",
+    testonly = True,
     srcs = [
         "test_data_gen.cc",
     ],
@@ -110,6 +112,7 @@ cc_test(
 
 cc_library(
     name = "util",
+    testonly = True,
     hdrs = [
         "util.h",
     ],