Fix CEL types for repeated field item expressions (#92)

For repeated items, CEL types were being miscalculated for variables in
the expression resulting in type-check/compilation errors

Fixes #91

internal/evaluator/builder.go

@@ -270,7 +270,7 @@ func (bldr *Builder) processIgnoreEmpty(
 func (bldr *Builder) processFieldExpressions(
 	fieldDesc protoreflect.FieldDescriptor,
 	fieldConstraints *validate.FieldConstraints,
-	_ bool,
+	forItems bool,
 	eval *value,
 	_ MessageCache,
 ) error {
@@ -279,7 +279,7 @@ func (bldr *Builder) processFieldExpressions(
 		return nil
 	}
 
-	celTyp := celext.ProtoFieldToCELType(fieldDesc, false, false)
+	celTyp := celext.ProtoFieldToCELType(fieldDesc, false, forItems)
 	opts := append(
 		celext.RequiredCELEnvOptions(fieldDesc),
 		cel.Variable("this", celTyp),

proto/tests/example/v1/validations.proto

@@ -165,3 +165,10 @@ message CelMapOnARepeated {
     string name = 1;
   }
 }
+
+message RepeatedItemCel {
+  repeated string paths = 1 [(buf.validate.field).repeated.items.cel = {
+    id: "paths.no_space",
+    expression: "!this.startsWith(' ')"
+  }];
+}

validator_test.go

@@ -184,3 +184,17 @@ func TestValidator_Validate_CelMapOnARepeated(t *testing.T) {
 	valErr := &ValidationError{}
 	assert.ErrorAs(t, err, &valErr)
 }
+
+func TestValidator_Validate_RepeatedItemCel(t *testing.T) {
+	t.Parallel()
+	val, err := New()
+	require.NoError(t, err)
+	msg := &pb.RepeatedItemCel{Paths: []string{"foo"}}
+	err = val.Validate(msg)
+	require.NoError(t, err)
+	msg.Paths = append(msg.Paths, " bar")
+	err = val.Validate(msg)
+	valErr := &ValidationError{}
+	assert.ErrorAs(t, err, &valErr)
+	assert.Equal(t, "paths.no_space", valErr.Violations[0].ConstraintId)
+}