Block API: Add hook for deprecated raw HTML support

blocks/api/serializer.js

@@ -47,6 +47,8 @@ export function getSaveElement( blockType, attributes ) {
 		saveElement = save( { attributes } );
 	}
 
+	saveElement = applyFilters( 'blocks.getSaveContent.saveElement', saveElement, blockType, attributes );
+
 	const addExtraContainerProps = ( element ) => {
 		if ( ! element || ! isObject( element ) ) {
 			return element;

blocks/hooks/deprecated.js

@@ -0,0 +1,46 @@
+/**
+ * WordPress dependencies
+ */
+import { Component, DangerousHTML } from '@wordpress/element';
+import { addFilter } from '@wordpress/hooks';
+
+class DangerousHTMLWithWarning extends Component {
+	constructor() {
+		super( ...arguments );
+
+		// Disable reason: We're intentionally logging a console warning
+		// advising the developer to upgrade usage.
+
+		// eslint-disable-next-line no-console
+		console.warn(
+			'Deprecated: String returns from block `save` is unsupported. ' +
+			'Use `wp.element.DangerousHTML` component instead.\n\n' +
+			'See: '
+		);
+	}
+
+	render() {
+		const { children } = this.props;
+
+		return <DangerousHTML>{ children }</DangerousHTML>;
+	}
+}
+
+/**
+ * Override save element for a block, providing support for deprecated string
+ * return value, logging a warning advising the developer to use the preferred
+ * DangerousHTML component instead.
+ *
+ * @param {(string|Object)} element Original block save return.
+ *
+ * @returns {Object} Filtered props applied to save element.
+ */
+export function shimDangerousHTML( element ) {
+	if ( typeof element !== 'string' ) {
+		return element;
+	}
+
+	return <DangerousHTMLWithWarning children={ element } />;
+}
+
+addFilter( 'blocks.getSaveContent.saveElement', 'core/deprecated/save-props', shimDangerousHTML );

blocks/hooks/index.js

@@ -3,5 +3,6 @@
  */
 import './anchor';
 import './custom-class-name';
+import './deprecated';
 import './generated-class-name';
 import './matchers';