Support `ProgramOption`s to disable string conversion and list/string concatenation

[sergiitk]

Feature request checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request
• If this is a bug, please file a Bug Report.

Change
To make CEL environment setup consistent across CEL implementations, I propose to add equivalent runtime options that control string conversion and list/string concatenation equivalent to those found in CEL-cpp:

// Enable string() overloads.
bool enable_string_conversion = true;

// Enable string concatenation overload.
bool enable_string_concat = true;

// Enable list concatenation overload.
bool enable_list_concat = true;

Example

// Actual option names up to the implementer.
opts := cel.EvalOptions(cel.OptDisableDisableStringConcat |
	cel.OptDisableDisableListConcat | cel.OptDisableDisableListConcat)
prg, err := env.Program(ast, opts)

Related

• Corresponding cel-java request: Support CelOptions to disable string conversion and list/string concatenation cel-java#502
• Internal ref: go/grpc-cel-integration

[sergiitk]

I was able to achieve the desired behavior using the new Config feature:

celLibSubset := env.NewLibrarySubset().AddExcludedFunctions(
	[]*env.Function{
		{Name: operators.Add, Overloads: []*env.Overload{
			{ID: overloads.AddBytes},
			{ID: overloads.AddList},
			{ID: overloads.AddString},
		}},
		{Name: overloads.TypeConvertString},
	}...,
)

celConfig := env.NewConfig("grpc-unified-matcher").
	SetStdLib(celLibSubset).
	AddVariables(
		// variables
	)

matcherEnv, err := cel.NewCustomEnv(cel.FromConfig(celConfig))

In my specific setup where I get AST from an external source, I needed to additionally run the check phase before building the program to surface the validation errors:

func NewCheckedCelProgram(ast *cel.Ast) (cel.Program, error) {
	ast, checkIssues := celRuntimeEnv.Check(ast)
	if checkIssues != nil {
		return nil, fmt.Errorf("error checking AST: %v", checkIssues)
	}
	prg, err := celRuntimeEnv.Program(ast)
	if err != nil {
		return nil, fmt.Errorf("error creating CEL program from AST: %v", err)
	}
	return prg, nil
}

Additionally, having dedicated options to disable string/list concat and string conversion (perhaps via config) would be more convenient.