Introduce string(bool) support at runtime

PiperOrigin-RevId: 817382186

Author: TristonianJones

eval/public/builtin_func_registrar_test.cc

@@ -264,6 +264,11 @@ INSTANTIATE_TEST_SUITE_P(
          {},
          CelValue::CreateBool(true),
          OverflowChecksEnabled()},
+        {"StringToBool",
+         "string(true) + string(false)",
+         {},
+         CelValue::CreateStringView("truefalse"),
+         OverflowChecksEnabled()},
     }),
     [](const testing::TestParamInfo<BuiltinFuncParamsTest::ParamType>& info) {
       return info.param.test_name;

runtime/standard/type_conversion_functions.cc

@@ -152,6 +152,15 @@ absl::Status RegisterStringConversionFunctions(FunctionRegistry& registry,
           registry);
   CEL_RETURN_IF_ERROR(status);
 
+  // bool -> string
+  status = UnaryFunctionAdapter<StringValue, bool>::RegisterGlobalOverload(
+      cel::builtin::kString,
+      [](bool value) -> StringValue {
+        return StringValue(value ? "true" : "false");
+      },
+      registry);
+  CEL_RETURN_IF_ERROR(status);
+
   // double -> string
   status = UnaryFunctionAdapter<StringValue, double>::RegisterGlobalOverload(
       cel::builtin::kString,

runtime/standard/type_conversion_functions_test.cc

@@ -104,6 +104,7 @@ TEST(RegisterTypeConversionFunctions, RegisterStringConversionFunctions) {
   EXPECT_THAT(
       registry.FindStaticOverloads(builtin::kString, false, {Kind::kAny}),
       UnorderedElementsAre(
+          MatchesUnaryDescriptor(builtin::kString, false, Kind::kBool),
           MatchesUnaryDescriptor(builtin::kString, false, Kind::kInt),
           MatchesUnaryDescriptor(builtin::kString, false, Kind::kDouble),
           MatchesUnaryDescriptor(builtin::kString, false, Kind::kUint),