Prevent nesting forms

packages/block-library/src/form/index.js

@@ -7,6 +7,11 @@ import metadata from './block.json';
 import save from './save';
 import variations from './variations';
 
+/**
+ * WordPress dependencies
+ */
+import { addFilter } from '@wordpress/hooks';
+
 const { name } = metadata;
 
 export { metadata, name };
@@ -18,3 +23,31 @@ export const settings = {
 };
 
 export const init = () => initBlock( { name, metadata, settings } );
+
+// Prevent adding forms inside forms.
+const DISALLOWED_PARENTS = [ 'core/form' ];
+addFilter(
+	'blockEditor.__unstableCanInsertBlockType',
+	'removeTemplatePartsFromPostTemplates',
+	(
+		canInsert,
+		blockType,
+		rootClientId,
+		{ getBlock, getBlockParentsByBlockName }
+	) => {
+		if ( blockType.name !== 'core/form' ) {
+			return canInsert;
+		}
+
+		for ( const disallowedParentType of DISALLOWED_PARENTS ) {
+			const hasDisallowedParent =
+				getBlock( rootClientId )?.name === disallowedParentType ||
+				getBlockParentsByBlockName( rootClientId, disallowedParentType )
+					.length;
+			if ( hasDisallowedParent ) {
+				return false;
+			}
+		}
+		return true;
+	}
+);