make struct validation fail (#28)

Validating directly against a struct has problems (see #23).
We could marshal it to JSON and then unmarshal to a map[string]any,
but that is a performance footgun.
Have it fail for now.
We can add it back later if we choose to.

For #23.

Author: jba

jsonschema/validate.go

@@ -6,6 +6,7 @@ package jsonschema
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"hash/maphash"
 	"iter"
@@ -399,11 +400,13 @@ func (st *state) validate(instance reflect.Value, schema *Schema, callerAnns *an
 
 	// objects
 	// https://json-schema.org/draft/2020-12/json-schema-core#section-10.3.2
-	if instance.Kind() == reflect.Map || instance.Kind() == reflect.Struct {
-		if instance.Kind() == reflect.Map {
-			if kt := instance.Type().Key(); kt.Kind() != reflect.String {
-				return fmt.Errorf("map key type %s is not a string", kt)
-			}
+	// Validating structs is problematic. See https://github.com/google/jsonschema-go/issues/23.
+	if instance.Kind() == reflect.Struct {
+		return errors.New("cannot validate against a struct; see https://github.com/google/jsonschema-go/issues/23 for details")
+	}
+	if instance.Kind() == reflect.Map {
+		if kt := instance.Type().Key(); kt.Kind() != reflect.String {
+			return fmt.Errorf("map key type %s is not a string", kt)
 		}
 		// Track the evaluated properties for just this schema, to support additionalProperties.
 		// If we used anns here, then we'd be including properties evaluated in subschemas

jsonschema/validate_test.go

@@ -260,10 +260,9 @@ func TestStructInstance(t *testing.T) {
 			t.Fatal(err)
 		}
 		err = res.Validate(instance)
-		if err == nil && !tt.want {
-			t.Errorf("succeeded unexpectedly\nschema = %s", tt.s.json())
-		} else if err != nil && tt.want {
-			t.Errorf("Validate: %v\nschema = %s", err, tt.s.json())
+		// Validating a struct always fails.
+		if err == nil {
+			t.Error("struct validation succeeded")
 		}
 	}
 }
@@ -442,8 +441,9 @@ func TestStructEmbedding(t *testing.T) {
 				t.Fatalf("schema.Resolve() failed: %v", err)
 			}
 			// Validate a correct instance against the generated schema.
-			if err := resolved.Validate(tc.validInstance); err != nil {
-				t.Errorf("resolved.Validate() failed on a valid instance: %v", err)
+			// Struct validation always fails.
+			if err := resolved.Validate(tc.validInstance); err == nil {
+				t.Error("struct validation succeeded")
 			}
 		})
 	}