feat: more information when hydration fails (#11649)

In the case of an invalid child element, we already get information about the parent and the child, but in other cases where a mismatch could occur you're pretty much on your own.

This adds a bit more context to hydration_mismatch warnings — 'The error occurred near ...'

Author: Rich-Harris

.changeset/violet-mails-trade.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: more information when hydration fails

packages/svelte/messages/client-errors/errors.md

@@ -36,13 +36,9 @@
 
 > Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
 
-## hydration_missing_marker_close
+## hydration_failed
 
-> Missing hydration closing marker
-
-## hydration_missing_marker_open
-
-> Missing hydration opening marker
+> Failed to hydrate the application
 
 ## lifecycle_legacy_only
 

packages/svelte/messages/client-warnings/warnings.md

@@ -6,6 +6,8 @@
 
 > Hydration failed because the initial UI does not match what was rendered on the server
 
+> Hydration failed because the initial UI does not match what was rendered on the server. The error occurred near %location%
+
 ## lifecycle_double_unmount
 
 > Tried to unmount a component that was not mounted

packages/svelte/scripts/process-messages/index.js

@@ -163,6 +163,7 @@ function transform(name, dest) {
 					type: 'Literal',
 					value: text
 				};
+				prev_vars = vars;
 				continue;
 			}
 

packages/svelte/src/constants.js

@@ -22,6 +22,7 @@ export const TEMPLATE_USE_IMPORT_NODE = 1 << 1;
 export const HYDRATION_START = '[';
 export const HYDRATION_END = ']';
 export const HYDRATION_END_ELSE = `${HYDRATION_END}!`; // used to indicate that an `{:else}...` block was rendered
+export const HYDRATION_ERROR = {};
 
 export const UNINITIALIZED = Symbol();
 

packages/svelte/src/internal/client/dom/hydration.js

@@ -1,5 +1,6 @@
-import { HYDRATION_END, HYDRATION_START } from '../../../constants.js';
-import * as e from '../errors.js';
+import { DEV } from 'esm-env';
+import { HYDRATION_END, HYDRATION_START, HYDRATION_ERROR } from '../../../constants.js';
+import * as w from '../warnings.js';
 
 /**
  * Use this variable to guard everything related to hydration code so it can be treeshaken out
@@ -67,5 +68,16 @@ export function hydrate_anchor(node) {
 		nodes.push(current);
 	}
 
-	e.hydration_missing_marker_close();
+	let location;
+
+	if (DEV) {
+		// @ts-expect-error
+		const loc = node.parentNode?.__svelte_meta?.loc;
+		if (loc) {
+			location = `${loc.file}:${loc.line}:${loc.column}`;
+		}
+	}
+
+	w.hydration_mismatch(location);
+	throw HYDRATION_ERROR;
 }

packages/svelte/src/internal/client/errors.js

@@ -161,34 +161,18 @@ export function effect_update_depth_exceeded() {
 }
 
 /**
- * Missing hydration closing marker
+ * Failed to hydrate the application
  * @returns {never}
  */
-export function hydration_missing_marker_close() {
+export function hydration_failed() {
 	if (DEV) {
-		const error = new Error(`${"hydration_missing_marker_close"}\n${"Missing hydration closing marker"}`);
+		const error = new Error(`${"hydration_failed"}\n${"Failed to hydrate the application"}`);
 
 		error.name = 'Svelte error';
 		throw error;
 	} else {
 		// TODO print a link to the documentation
-		throw new Error("hydration_missing_marker_close");
-	}
-}
-
-/**
- * Missing hydration opening marker
- * @returns {never}
- */
-export function hydration_missing_marker_open() {
-	if (DEV) {
-		const error = new Error(`${"hydration_missing_marker_open"}\n${"Missing hydration opening marker"}`);
-
-		error.name = 'Svelte error';
-		throw error;
-	} else {
-		// TODO print a link to the documentation
-		throw new Error("hydration_missing_marker_open");
+		throw new Error("hydration_failed");
 	}
 }
 

packages/svelte/src/internal/client/render.js

@@ -6,7 +6,7 @@ import {
 	empty,
 	init_operations
 } from './dom/operations.js';
-import { HYDRATION_START, PassiveDelegatedEvents } from '../../constants.js';
+import { HYDRATION_ERROR, HYDRATION_START, PassiveDelegatedEvents } from '../../constants.js';
 import { flush_sync, push, pop, current_component_context } from './runtime.js';
 import { effect_root, branch } from './reactivity/effects.js';
 import {
@@ -153,7 +153,7 @@ export function hydrate(component, options) {
 			}
 
 			if (!node) {
-				e.hydration_missing_marker_open();
+				throw HYDRATION_ERROR;
 			}
 
 			const anchor = hydrate_anchor(node);
@@ -167,22 +167,20 @@ export function hydrate(component, options) {
 			return instance;
 		}, false);
 	} catch (error) {
-		if (
-			!hydrated &&
-			options.recover !== false &&
-			/** @type {Error} */ (error).message.includes('hydration_missing_marker_close')
-		) {
-			w.hydration_mismatch();
+		if (error === HYDRATION_ERROR) {
+			if (options.recover === false) {
+				e.hydration_failed();
+			}
 
 			// If an error occured above, the operations might not yet have been initialised.
 			init_operations();
 			clear_text_content(target);
 
 			set_hydrating(false);
 			return mount(component, options);
-		} else {
-			throw error;
 		}
+
+		throw error;
 	} finally {
 		set_hydrating(!!previous_hydrate_nodes);
 		set_hydrate_nodes(previous_hydrate_nodes);

packages/svelte/src/internal/client/warnings.js

@@ -21,11 +21,12 @@ export function hydration_attribute_changed(attribute, html, value) {
 }
 
 /**
- * Hydration failed because the initial UI does not match what was rendered on the server
+ * Hydration failed because the initial UI does not match what was rendered on the server. The error occurred near %location%
+ * @param {string | undefined | null} [location]
  */
-export function hydration_mismatch() {
+export function hydration_mismatch(location) {
 	if (DEV) {
-		console.warn(`%c[svelte] ${"hydration_mismatch"}\n%c${"Hydration failed because the initial UI does not match what was rendered on the server"}`, bold, normal);
+		console.warn(`%c[svelte] ${"hydration_mismatch"}\n%c${location ? `Hydration failed because the initial UI does not match what was rendered on the server. The error occurred near ${location}` : "Hydration failed because the initial UI does not match what was rendered on the server"}`, bold, normal);
 	} else {
 		// TODO print a link to the documentation
 		console.warn("hydration_mismatch");
@@ -66,7 +67,7 @@ export function ownership_invalid_binding(parent, child, owner) {
  */
 export function ownership_invalid_mutation(component, owner) {
 	if (DEV) {
-		console.warn(`%c[svelte] ${"ownership_invalid_mutation"}\n%c${`${component} mutated a value owned by ${owner}. This is strongly discouraged. Consider passing values to child components with \`bind:\`, or use a callback instead`}`, bold, normal);
+		console.warn(`%c[svelte] ${"ownership_invalid_mutation"}\n%c${component ? `${component} mutated a value owned by ${owner}. This is strongly discouraged. Consider passing values to child components with \`bind:\`, or use a callback instead` : "Mutating a value outside the component that created it is strongly discouraged. Consider passing values to child components with `bind:`, or use a callback instead"}`, bold, normal);
 	} else {
 		// TODO print a link to the documentation
 		console.warn("ownership_invalid_mutation");