Add syntax for escaped field selectors. (#1002)

Add syntax for escaped field selectors.

Introduces syntax for escaped fields that are not normally
syntactically legal field selections with backticks.

This PR is limited to fields, but would also consider adding for namespaced
names (e.g. `in.example.Foo`) in a later update.

* Changes applied from intial draft feedback:

- Reduce set of allowed escaped identifiers
- Additional testing

cel/cel_test.go

@@ -2171,6 +2171,53 @@ func TestParserRecursionLimit(t *testing.T) {
 	}
 }
 
+func TestQuotedFields(t *testing.T) {
+	testCases := []struct {
+		expr        string
+		errorSubstr string
+		out         ref.Val
+	}{
+		{
+			expr: "{'key-1': 64}.`key-1`",
+			out:  types.Int(64),
+		},
+		{
+			expr:        "{'key-1': 64}.`key-2`",
+			errorSubstr: "no such key: key-2",
+		},
+		{
+			expr: "has({'key-1': 64}.`key-1`)",
+			out:  types.True,
+		},
+		{
+			expr: "has({'key-1': 64}.`key-2`)",
+			out:  types.False,
+		},
+	}
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.expr, func(t *testing.T) {
+			env := testEnv(t, ParserRecursionLimit(10),
+				EnableIdentifierEscapeSyntax())
+			out, err := interpret(t, env,
+				tc.expr, map[string]any{})
+
+			if tc.errorSubstr != "" {
+				if err == nil || !strings.Contains(err.Error(), tc.errorSubstr) {
+					t.Fatalf("prg.Eval() wanted error containing '%s' got %v", tc.errorSubstr, err)
+				}
+			}
+
+			if tc.out != nil {
+				if tc.out != out {
+					t.Errorf("prg.Eval() wanted %v got %v", tc.out, out)
+				}
+			}
+		})
+
+	}
+}
+
 func TestDynamicDispatch(t *testing.T) {
 	env := testEnv(t,
 		HomogeneousAggregateLiterals(),

cel/env.go

@@ -615,6 +615,9 @@ func (e *Env) configure(opts []EnvOption) (*Env, error) {
 	if e.HasFeature(featureVariadicLogicalASTs) {
 		prsrOpts = append(prsrOpts, parser.EnableVariadicOperatorASTs(true))
 	}
+	if e.HasFeature(featureIdentEscapeSyntax) {
+		prsrOpts = append(prsrOpts, parser.EnableIdentEscapeSyntax(true))
+	}
 	e.prsr, err = parser.NewParser(prsrOpts...)
 	if err != nil {
 		return nil, err

cel/options.go

@@ -65,6 +65,9 @@ const (
 	// Enable error generation when a presence test or optional field selection is
 	// performed on a primitive type.
 	featureEnableErrorOnBadPresenceTest
+
+	// Enable escape syntax for field identifiers (`).
+	featureIdentEscapeSyntax
 )
 
 // EnvOption is a functional interface for configuring the environment.
@@ -618,6 +621,12 @@ func EnableMacroCallTracking() EnvOption {
 	return features(featureEnableMacroCallTracking, true)
 }
 
+// EnableIdentifierEscapeSyntax enables identifier escaping (`) syntax for
+// fields.
+func EnableIdentifierEscapeSyntax() EnvOption {
+	return features(featureIdentEscapeSyntax, true)
+}
+
 // CrossTypeNumericComparisons makes it possible to compare across numeric types, e.g. double < int
 func CrossTypeNumericComparisons(enabled bool) EnvOption {
 	return features(featureCrossTypeNumericComparisons, enabled)

ext/protos_test.go

@@ -42,12 +42,21 @@ func TestProtos(t *testing.T) {
 		{expr: `proto.getExt(ExampleType{}, google.expr.proto2.test.nested_example) == ExampleType{}`},
 		{expr: `proto.getExt(ExampleType{}, google.expr.proto2.test.ExtendedExampleType.extended_examples) == []`},
 		{expr: `proto.getExt(ExampleType{}, google.expr.proto2.test.ExtendedExampleType.enum_ext) == GlobalEnum.GOO`},
+		{expr: "ExampleType{`in`: 64}.`in` == 64"},
+		// TODO(issue/1095): legal parse, but can't assign extension fields without updates to type checker and runtime
+		// ExampleType{`google.expr.proto2.test.int32_ext`: 42}.`google.expr.proto2.test.int32_ext` == 42
 		{expr: `proto.getExt(msg, google.expr.proto2.test.int32_ext) == 42`},
+		{expr: "msg.`google.expr.proto2.test.int32_ext` == 42"},
 		{expr: `proto.getExt(msg, google.expr.proto2.test.int32_wrapper_ext) == 21`},
+		{expr: "msg.`google.expr.proto2.test.int32_wrapper_ext` == 21"},
 		{expr: `proto.hasExt(msg, google.expr.proto2.test.nested_example)`},
+		{expr: "has(msg.`google.expr.proto2.test.nested_example`)"},
 		{expr: `proto.getExt(msg, google.expr.proto2.test.nested_example) == ExampleType{name: 'nested'}`},
+		{expr: "msg.`google.expr.proto2.test.nested_example` == ExampleType{name: 'nested'}"},
 		{expr: `proto.getExt(msg, google.expr.proto2.test.ExtendedExampleType.extended_examples) == ['example1', 'example2']`},
+		{expr: "msg.`google.expr.proto2.test.ExtendedExampleType.extended_examples` == ['example1', 'example2']"},
 		{expr: `proto.getExt(msg, google.expr.proto2.test.ExtendedExampleType.enum_ext) == GlobalEnum.GAZ`},
+		{expr: "msg.`google.expr.proto2.test.ExtendedExampleType.enum_ext` == GlobalEnum.GAZ"},
 	}
 
 	env := testProtosEnv(t)
@@ -174,6 +183,15 @@ func TestProtosParseErrors(t *testing.T) {
 		| proto.getExt(ExtendedExampleType{}, has(google.expr.proto2.test.int32_ext))
 		| .......................................^`,
 		},
+		{
+			expr: `ExampleType{}.in`,
+			err: `ERROR: <input>:1:15: Syntax error: no viable alternative at input '.in'
+			| ExampleType{}.in
+			| ..............^
+		   ERROR: <input>:1:17: Syntax error: mismatched input '<EOF>' expecting {'[', '{', '(', '.', '-', '!', 'true', 'false', 'null', NUM_FLOAT, NUM_INT, NUM_UINT, STRING, BYTES, IDENTIFIER}
+			| ExampleType{}.in
+			| ................^`,
+		},
 	}
 	env := testProtosEnv(t)
 	for i, tst := range protosTests {
@@ -198,6 +216,7 @@ func testProtosEnv(t *testing.T, opts ...cel.EnvOption) *cel.Env {
 		cel.Container("google.expr.proto2.test"),
 		cel.Types(&proto2pb.ExampleType{}, &proto2pb.ExternalMessageType{}),
 		cel.Variable("msg", cel.ObjectType("google.expr.proto2.test.ExampleType")),
+		cel.EnableIdentifierEscapeSyntax(),
 		Protos(),
 	}
 	env, err := cel.NewEnv(append(baseOpts, opts...)...)

parser/gen/CEL.g4

@@ -52,13 +52,14 @@ unary
 
 member
     : primary                                                       # PrimaryExpr
-    | member op='.' (opt='?')? id=IDENTIFIER                        # Select
+    | member op='.' (opt='?')? id=escapeIdent                       # Select
     | member op='.' id=IDENTIFIER open='(' args=exprList? ')'       # MemberCall
     | member op='[' (opt='?')? index=expr ']'                       # Index
     ;
 
 primary
-    : leadingDot='.'? id=IDENTIFIER (op='(' args=exprList? ')')?    # IdentOrGlobalCall
+    : leadingDot='.'? id=IDENTIFIER                                # Ident
+    | leadingDot='.'? id=IDENTIFIER (op='(' args=exprList? ')')     # GlobalCall
     | '(' e=expr ')'                                                # Nested
     | op='[' elems=listInit? ','? ']'                               # CreateList
     | op='{' entries=mapInitializerList? ','? '}'                   # CreateStruct
@@ -80,13 +81,18 @@ fieldInitializerList
     ;
 
 optField
-    : (opt='?')? IDENTIFIER
+    : (opt='?')? escapeIdent
     ;
 
 mapInitializerList
     : keys+=optExpr cols+=':' values+=expr (',' keys+=optExpr cols+=':' values+=expr)*
     ;
 
+escapeIdent
+    : id=IDENTIFIER      # SimpleIdentifier
+    | id=ESC_IDENTIFIER  # EscapedIdentifier
+;
+
 optExpr
     : (opt='?')? e=expr
     ;
@@ -198,3 +204,4 @@ STRING
 BYTES : ('b' | 'B') STRING;
 
 IDENTIFIER : (LETTER | '_') ( LETTER | DIGIT | '_')*;
+ESC_IDENTIFIER : '`' (LETTER | DIGIT | '_' | '.' | '-' | '/' | ' ')+ '`';

parser/gen/CEL.tokens

@@ -34,6 +34,7 @@ NUM_UINT=33
 STRING=34
 BYTES=35
 IDENTIFIER=36
+ESC_IDENTIFIER=37
 '=='=1
 '!='=2
 'in'=3