fix: only escape characters in SSR template

Adjusts the escaping mechanism done for server compilation. For template literals it's now only applied when explicitly told, which is the case for generated literals from the html template. Fixes a bug where a template literal string inside the `@html` tag was wrongfully escaped (#10359 (comment))

.changeset/neat-boats-shake.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: only escape characters in SSR template

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -32,10 +32,11 @@ function t_string(value) {
 
 /**
  * @param {import('estree').Expression} value
+ * @param {boolean} [needs_escaping]
  * @returns {import('./types').TemplateExpression}
  */
-function t_expression(value) {
-	return { type: 'expression', value };
+function t_expression(value, needs_escaping = false) {
+	return { type: 'expression', value, needs_escaping };
 }
 
 /**
@@ -94,7 +95,8 @@ function serialize_template(template, out = b.id('out')) {
 			} else if (template_item.type === 'expression') {
 				const value = template_item.value;
 				if (value.type === 'TemplateLiteral') {
-					last.value.raw += sanitize_template_string(value.quasis[0].value.raw);
+					const raw = value.quasis[0].value.raw;
+					last.value.raw += template_item.needs_escaping ? sanitize_template_string(raw) : raw;
 					quasis.push(...value.quasis.slice(1));
 					expressions.push(...value.expressions);
 					continue;
@@ -198,7 +200,7 @@ function process_children(nodes, parent, { visit, state }) {
 			}
 		}
 
-		state.template.push(t_expression(b.template(quasis, expressions)));
+		state.template.push(t_expression(b.template(quasis, expressions), true));
 	}
 
 	for (let i = 0; i < nodes.length; i += 1) {

packages/svelte/src/compiler/phases/3-transform/server/types.d.ts

@@ -12,6 +12,7 @@ import type { ComponentAnalysis } from '../../types.js';
 export type TemplateExpression = {
 	type: 'expression';
 	value: Expression;
+	needs_escaping: boolean;
 };
 
 export type TemplateString = {

packages/svelte/tests/runtime-runes/samples/html-tag-escaping/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `s s s \\u73`
+});

packages/svelte/tests/runtime-runes/samples/html-tag-escaping/main.svelte

@@ -0,0 +1,5 @@
+{@html `\u{73}`}
+{@html '\u{73}'}
+{@html "\u{73}"}
+
+\u{73}