(fix) skip parser error when there's likely another preprocessor invo…

…lved

sveltejs#1259
sveltejs#1479
sveltejs#1558

This may give false negatives for people who do not use Vite/forgot to create a svelte.config.js when they use rollup or webpack, but that's likely OK since we can think about these diagnostics as being opt-in: you add a preprocessor config when you want the Svelte warnings, too. Most people who have this problem also likely use TypeScript or a CSS preprocessor. When using TS, errors are caught through the TS intellisense. When using a CSS preprocessor, the only warning they'll miss is the unused CSS warning, which is apparent anyway since those styles are not visible in the browser.

packages/language-server/src/plugins/svelte/features/getDiagnostics.ts

@@ -91,6 +91,13 @@ async function createParserErrorDiagnostic(error: any, document: Document) {
             document.scriptInfo || document.moduleScriptInfo
         );
 
+        if (!document.config?.preprocess && document.getText().includes('lang="')) {
+            Logger.error(
+                `Parsing ${document.getFilePath()} failed. No preprocess config found but lang tag exists. Skip showing error because they likely use other preprocessors.`
+            );
+            return [];
+        }
+
         if (isInStyle || isInScript) {
             diagnostic.message +=
                 '\n\nIf you expect this syntax to work, here are some suggestions: ';