fix: use user's Svelte parser when formatting if necessary (#2578)

This makes formatting "just work" if users use Svelte 5 but don't have prettier (and the plugin) installed in their workspace

Depends on sveltejs/prettier-plugin-svelte#471

#2542

Author: dummdidumm

packages/language-server/package.json

@@ -59,7 +59,7 @@
         "globrex": "^0.1.2",
         "lodash": "^4.17.21",
         "prettier": "~3.3.3",
-        "prettier-plugin-svelte": "^3.2.6",
+        "prettier-plugin-svelte": "^3.2.8",
         "svelte": "^4.2.19",
         "svelte2tsx": "workspace:~",
         "typescript": "^5.6.3",

packages/language-server/src/plugins/svelte/SveltePlugin.ts

@@ -127,11 +127,9 @@ export class SveltePlugin
         /**
          * Prettier v2 can't use v3 plugins and vice versa. Therefore, we need to check
          * which version of prettier is used in the workspace and import the correct
-         * version of the Svelte plugin. If user uses Prettier >= 3 and has no Svelte plugin
-         * then fall back to our built-in versions which are both v2 and compatible with
+         * version of the Svelte plugin. If user uses Prettier < 3 and has no Svelte plugin
+         * then fall back to our built-in versions which are both v3 and compatible with
          * each other.
-         * TODO switch this around at some point to load Prettier v3 by default because it's
-         * more likely that users have that installed.
          */
         const importFittingPrettier = async () => {
             const getConfig = async (p: any) => {
@@ -206,6 +204,15 @@ export class SveltePlugin
             return [];
         }
 
+        if (isFallback || !(await hasSveltePluginLoaded(prettier, resolvedPlugins))) {
+            // If the user uses Svelte 5 but doesn't have prettier installed, we need to provide
+            // the compiler path to the plugin so it can use its parser method; else it will crash.
+            const svelteCompilerInfo = getPackageInfo('svelte', filePath);
+            if (svelteCompilerInfo.version.major >= 5) {
+                config.svelte5CompilerPath = svelteCompilerInfo.path + '/compiler';
+            }
+        }
+
         // Prettier v3 format is async, v2 is not
         const formattedCode = await prettier.format(document.getText(), {
             ...config,