refactor: improve Validation error message for placeholder

Author: kenjis

system/Validation/Validation.php

@@ -790,7 +790,9 @@ protected function fillPlaceholders(array $rules, array $data): array
                         // Check if the validation rule for the placeholder exists
                         if ($placeholderRules === null) {
                             throw new LogicException(
-                                'No validation rules for the placeholder: ' . $field
+                                'No validation rules for the placeholder: "' . $field
+                                . '". You must set the validation rules for the field.'
+                                . ' See <https://codeigniter4.github.io/userguide/libraries/validation.html#validation-placeholders>.'
                             );
                         }
 

tests/system/Validation/StrictRules/DatabaseRelatedRulesTest.php

@@ -17,6 +17,7 @@
 use Config\Database;
 use Config\Services;
 use InvalidArgumentException;
+use LogicException;
 use Tests\Support\Validation\TestRules;
 
 /**
@@ -139,6 +140,34 @@ public function testIsUniqueWithIgnoreValuePlaceholder(): void
         $this->assertTrue($this->validation->run($data));
     }
 
+    public function testIsUniqueWithPlaceholderAndNoValidationRulesForIt(): void
+    {
+        $this->expectException(LogicException::class);
+        $this->expectExceptionMessage('No validation rules for the placeholder: "id". You must set the validation rules for the field.');
+
+        $this->hasInDatabase('user', [
+            'name'    => 'Derek',
+            'email'   => 'derek@world.co.uk',
+            'country' => 'GB',
+        ]);
+
+        $row = Database::connect()
+            ->table('user')
+            ->limit(1)
+            ->get()
+            ->getRow();
+
+        $data = [
+            'id'    => $row->id,
+            'email' => 'derek@world.co.uk',
+        ];
+
+        $this->validation->setRules([
+            'email' => 'is_unique[user.email,id,{id}]',
+        ]);
+        $this->validation->run($data);
+    }
+
     public function testIsUniqueByManualRun(): void
     {
         Database::connect()