feat(svelte5): enable hmr

.changeset/shiny-drinks-dress.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': minor
+---
+
+feat(svelte5): enable hmr option in dev

packages/vite-plugin-svelte/src/index.js

@@ -178,7 +178,8 @@ export function svelte(inlineOptions) {
 			},
 
 			handleHotUpdate(ctx) {
-				if (!options.hot || !options.emitCss) {
+				// @ts-expect-error svelte4 does not have hmr option
+				if ((!options.hot && !options.compilerOptions.hmr) || !options.emitCss) {
 					return;
 				}
 				const svelteRequest = requestParser(ctx.file, false, ctx.timestamp);

packages/vite-plugin-svelte/src/utils/compile.js

@@ -203,7 +203,8 @@ export const _createCompileSvelte = (makeHot) => {
  * @returns {Function | undefined}
  */
 function buildMakeHot(options) {
-	const needsMakeHot = options.hot !== false && options.isServe && !options.isProduction;
+	const needsMakeHot =
+		!isSvelte5 && options.hot !== false && options.isServe && !options.isProduction;
 	if (needsMakeHot) {
 		// @ts-ignore
 		const hotApi = options?.hot?.hotApi;

packages/vite-plugin-svelte/src/utils/log.js

@@ -263,9 +263,6 @@ export function isDebugNamespaceEnabled(namespace) {
 
 export function logSvelte5Warning() {
 	const notice = `You are using Svelte ${VERSION}. Svelte 5 support is experimental, breaking changes can occur in any release until this notice is removed.`;
-	const wip = [
-		'svelte-inspector is disabled until dev mode implements node to code mapping',
-		'hmr for .svelte files is disabled until hmr api is implemented'
-	];
+	const wip = ['svelte-inspector is disabled until dev mode implements node to code mapping'];
 	log.warn(`${notice}\nwork in progress:\n - ${wip.join('\n - ')}\n`);
 }

packages/vite-plugin-svelte/src/utils/options.js

@@ -26,7 +26,7 @@ import {
 import { isCommonDepWithoutSvelteField } from './dependencies.js';
 import { VitePluginSvelteStats } from './vite-plugin-svelte-stats.js';
 import { VitePluginSvelteCache } from './vite-plugin-svelte-cache.js';
-import { isSvelte5 } from './svelte-version.js';
+import { isSvelte5, isSvelte5WithHMRSupport } from './svelte-version.js';
 
 const allowedPluginOptions = new Set([
 	'include',
@@ -194,17 +194,24 @@ export function resolveOptions(preResolveOptions, viteConfig, cache) {
 	const css = preResolveOptions.emitCss ? 'external' : 'injected';
 	/** @type {Partial<import('../public.d.ts').Options>} */
 	const defaultOptions = {
-		hot: viteConfig.isProduction
-			? false
-			: {
-					injectCss: css === 'injected',
-					partialAccept: !!viteConfig.experimental?.hmrPartialAccept
-				},
 		compilerOptions: {
 			css,
 			dev: !viteConfig.isProduction
 		}
 	};
+	if (isSvelte5) {
+		if (isSvelte5WithHMRSupport) {
+			// @ts-expect-error svelte4 does not have hmr option
+			defaultOptions.compilerOptions.hmr = !viteConfig.isProduction;
+		}
+	} else {
+		defaultOptions.hot = viteConfig.isProduction
+			? false
+			: {
+					injectCss: css === 'injected',
+					partialAccept: !!viteConfig.experimental?.hmrPartialAccept
+				};
+	}
 	/** @type {Partial<import('../types/options.d.ts').ResolvedOptions>} */
 	const extraOptions = {
 		root: viteConfig.root,
@@ -231,45 +238,52 @@ export function resolveOptions(preResolveOptions, viteConfig, cache) {
  */
 function enforceOptionsForHmr(options) {
 	if (isSvelte5) {
-		// TODO add hmr options for svelte5 once it is supported and update utils/log.js#logSvelte5Warning
-		options.hot = false;
-	}
-	if (options.hot) {
-		if (!options.compilerOptions.dev) {
-			log.warn('hmr is enabled but compilerOptions.dev is false, forcing it to true');
-			options.compilerOptions.dev = true;
+		if (options.hot && isSvelte5WithHMRSupport) {
+			log.warn(
+				'svelte 5 has hmr integrated in core. Please remove the hot option and use compilerOptions.hmr instead'
+			);
+			delete options.hot;
+			// @ts-expect-error hmr option doesn't exist in svelte4
+			options.compilerOptions.hmr = true;
 		}
-		if (options.emitCss) {
-			if (options.hot !== true && options.hot.injectCss) {
-				log.warn('hmr and emitCss are enabled but hot.injectCss is true, forcing it to false');
-				options.hot.injectCss = false;
-			}
-			const css = options.compilerOptions.css;
-			if (css === true || css === 'injected') {
-				const forcedCss = 'external';
-				log.warn(
-					`hmr and emitCss are enabled but compilerOptions.css is ${css}, forcing it to ${forcedCss}`
-				);
-				options.compilerOptions.css = forcedCss;
+	} else {
+		if (options.hot) {
+			if (!options.compilerOptions.dev) {
+				log.warn('hmr is enabled but compilerOptions.dev is false, forcing it to true');
+				options.compilerOptions.dev = true;
 			}
-		} else {
-			if (options.hot === true || !options.hot.injectCss) {
-				log.warn(
-					'hmr with emitCss disabled requires option hot.injectCss to be enabled, forcing it to true'
-				);
-				if (options.hot === true) {
-					options.hot = { injectCss: true };
-				} else {
-					options.hot.injectCss = true;
+			if (options.emitCss) {
+				if (options.hot !== true && options.hot.injectCss) {
+					log.warn('hmr and emitCss are enabled but hot.injectCss is true, forcing it to false');
+					options.hot.injectCss = false;
+				}
+				const css = options.compilerOptions.css;
+				if (css === true || css === 'injected') {
+					const forcedCss = 'external';
+					log.warn(
+						`hmr and emitCss are enabled but compilerOptions.css is ${css}, forcing it to ${forcedCss}`
+					);
+					options.compilerOptions.css = forcedCss;
+				}
+			} else {
+				if (options.hot === true || !options.hot.injectCss) {
+					log.warn(
+						'hmr with emitCss disabled requires option hot.injectCss to be enabled, forcing it to true'
+					);
+					if (options.hot === true) {
+						options.hot = { injectCss: true };
+					} else {
+						options.hot.injectCss = true;
+					}
+				}
+				const css = options.compilerOptions.css;
+				if (!(css === true || css === 'injected')) {
+					const forcedCss = 'injected';
+					log.warn(
+						`hmr with emitCss disabled requires compilerOptions.css to be enabled, forcing it to ${forcedCss}`
+					);
+					options.compilerOptions.css = forcedCss;
 				}
-			}
-			const css = options.compilerOptions.css;
-			if (!(css === true || css === 'injected')) {
-				const forcedCss = 'injected';
-				log.warn(
-					`hmr with emitCss disabled requires compilerOptions.css to be enabled, forcing it to ${forcedCss}`
-				);
-				options.compilerOptions.css = forcedCss;
 			}
 		}
 	}

packages/vite-plugin-svelte/src/utils/svelte-version.js

@@ -9,3 +9,9 @@ export const isSvelte4 = VERSION.startsWith('4.');
  * @type {boolean}
  */
 export const isSvelte5 = VERSION.startsWith('5.');
+
+/**
+ * @type {boolean}
+ */
+export const isSvelte5WithHMRSupport =
+	VERSION.startsWith('5.0.0-next.') && Number(VERSION.slice(11)) > 96;