Add support for rendering HTML comments

This is done via Fragments because they are the least common components.
That way the additional branching has minimal impact

.changeset/shy-hotels-impress.md

@@ -0,0 +1,7 @@
+---
+'preact-render-to-string': minor
+---
+
+Add ability to render comments via `<Fragment comment="my-comment" />`. When the `comment` prop is present all children of that `Fragment` will be ignored. This PR only supports that in server environments as it's useful to mark islands and other things. It's not supported in the browser.
+
+We picked a `Fragment` for this as it's the least common component and therefore the branch in our code with the least impact on perf by adding another if-check.

src/index.js

@@ -165,6 +165,12 @@ function _renderToString(vnode, context, isSvgMode, selectValue, parent) {
 	// Invoke rendering on Components
 	if (typeof type === 'function') {
 		if (type === Fragment) {
+			// Fragments are the least use components of core that's why
+			// branching here for comments has the least effect on perf.
+			if (props.comment) {
+				return '<!--' + encodeEntities(props.comment || '') + '-->';
+			}
+
 			rendered = props.children;
 		} else {
 			contextType = type.contextType;

test/render.test.js

@@ -1309,6 +1309,22 @@ describe('render', () => {
 		expect(render(<div>{() => {}}</div>)).to.equal('<div></div>');
 	});
 
+	describe('HTML Comments', () => {
+		it('should render HTML comments via Fragments', () => {
+			expect(render(<Fragment comment="foo" />)).to.equal('<!--foo-->');
+		});
+
+		it('should ignore children with comment prop', () => {
+			expect(
+				render(
+					<Fragment comment="foo">
+						<p>foo</p>
+					</Fragment>
+				)
+			).to.equal('<!--foo-->');
+		});
+	});
+
 	describe('vnode masks (useId)', () => {
 		it('should skip component top level Fragment child', () => {
 			const Wrapper = ({ children }) => <Fragment>{children}</Fragment>;