Unparse Expr values to strings (#1164)

* Unparse Expr values to strings
* ExprToString test case

Author: TristonianJones

cel/io.go

@@ -99,7 +99,13 @@ func AstToParsedExpr(a *Ast) (*exprpb.ParsedExpr, error) {
 // Note, the conversion may not be an exact replica of the original expression, but will produce
 // a string that is semantically equivalent and whose textual representation is stable.
 func AstToString(a *Ast) (string, error) {
-	return parser.Unparse(a.NativeRep().Expr(), a.NativeRep().SourceInfo())
+	return ExprToString(a.NativeRep().Expr(), a.NativeRep().SourceInfo())
+}
+
+// ExprToString converts an AST Expr node back to a string using macro call tracking metadata from
+// source info if any macros are encountered within the expression.
+func ExprToString(e ast.Expr, info *ast.SourceInfo) (string, error) {
+	return parser.Unparse(e, info)
 }
 
 // RefValueToValue converts between ref.Val and google.api.expr.v1alpha1.Value.

cel/io_test.go

@@ -23,6 +23,8 @@ import (
 	"google.golang.org/protobuf/proto"
 
 	"github.com/google/cel-go/checker/decls"
+	celast "github.com/google/cel-go/common/ast"
+	"github.com/google/cel-go/common/operators"
 	"github.com/google/cel-go/common/types"
 
 	proto3pb "github.com/google/cel-go/test/proto3pb"
@@ -145,6 +147,52 @@ func TestAstToString(t *testing.T) {
 	}
 }
 
+func TestExprToString(t *testing.T) {
+	stdEnv, err := NewEnv(EnableMacroCallTracking())
+	if err != nil {
+		t.Fatalf("NewEnv() failed: %v", err)
+	}
+	in := "[a, b].filter(i, (i > 0) ? (-i + 4) : i)"
+	ast, iss := stdEnv.Parse(in)
+	if iss.Err() != nil {
+		t.Fatalf("stdEnv.Parse(%q) failed: %v", in, iss.Err())
+	}
+	expr, err := ExprToString(ast.NativeRep().Expr(), ast.NativeRep().SourceInfo())
+	if err != nil {
+		t.Fatalf("ExprToString(ast) failed: %v", err)
+	}
+	if expr != in {
+		t.Errorf("got %v, wanted %v", expr, in)
+	}
+
+	// Test sub-expression unparsing.
+	navExpr := celast.NavigateAST(ast.NativeRep())
+	condExpr := celast.MatchDescendants(navExpr, celast.FunctionMatcher(operators.Conditional))[0]
+	want := `(i > 0) ? (-i + 4) : i`
+	expr, err = ExprToString(condExpr, ast.NativeRep().SourceInfo())
+	if err != nil {
+		t.Fatalf("ExprToString(ast) failed: %v", err)
+	}
+	if expr != want {
+		t.Errorf("got %v, wanted %v", expr, want)
+	}
+
+	// Also passes with a nil source info, but only because the sub-expr doesn't contain macro calls.
+	expr, err = ExprToString(condExpr, nil)
+	if err != nil {
+		t.Fatalf("ExprToString(ast) failed: %v", err)
+	}
+	if expr != want {
+		t.Errorf("got %v, wanted %v", expr, want)
+	}
+
+	// Fails do to missing macro information.
+	_, err = ExprToString(ast.NativeRep().Expr(), nil)
+	if err == nil {
+		t.Error("ExprToString() succeeded, wanted error")
+	}
+}
+
 func TestAstToStringNil(t *testing.T) {
 	expr, err := AstToString(nil)
 	if err == nil || !strings.Contains(err.Error(), "unsupported expr") {