fix: only escape characters in SSR template (#10555)

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 (https://github.com/sveltejs/svelte/issues/10359#issuecomment-1949678228)

.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}