start working on hydration

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/element.js

@@ -205,7 +205,7 @@ export function build_attribute_value(value, context, memoize = (value) => value
 
 		return {
 			value: memoize(expression, chunk.metadata.expression),
-			has_state: chunk.metadata.expression.has_state
+			has_state: chunk.metadata.expression.has_state || chunk.metadata.expression.is_async
 		};
 	}
 

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -217,7 +217,24 @@ export function boundary(node, props, boundary_fn) {
 			hydrate_next();
 		}
 
+		const pending = props.pending;
+
+		if (hydrating && pending) {
+			boundary_effect = branch(() => pending(anchor));
+
+			// ...now what? we need to start rendering `boundary_fn` offscreen,
+			// and either insert the resulting fragment (if nothing suspends)
+			// or keep the pending effect alive until it unsuspends.
+			// not exactly sure how to do that.
+
+			// future work: when we have some form of async SSR, we will
+			// need to use hydration boundary comments to report whether
+			// the pending or main block was rendered for a given
+			// boundary, and hydrate accordingly
+		} else {
 			boundary_effect = branch(() => boundary_fn(anchor));
+		}
+
 		reset_is_throwing_error();
 	}, EFFECT_TRANSPARENT | BOUNDARY_EFFECT);