fix: add deprecation warning to defaults prop (#393)

docs/preprocessing.md

@@ -72,7 +72,6 @@ The following options can be passed to the preprocessor. None are required:
 | `markupTagName` | `"template"`                                             | `string` that sets the name of the tag `svelte-preprocess` looks for markup in custom languages.<br><br>i.e `markup` makes it possible to write your markup between `<markup lang="..."></markup>` tag.                                                                                                                                                                                        |
 | `aliases`       | `null`                                                   | A list of tuples `[alias: string, language: string]` that correlates an `alias` to a `language`<br><br>i.e `['cst', 'customLanguage']` means<br>`<... src="./file.cst">`<br>`<... lang="cst">`<br>`<... type="text/customLanguage">`<br>`<... type="application/customLanguage">`<br>are treated as `customLanguage`.                                                                          |
 | preserve        | `[]`                                                     | A `string` list of languages/aliases that shouldn't pass through the preprocessor. (i.e `ld+json`)                                                                                                                                                                                                                                                                                             |
-| `defaults`      | `{ markup: 'html', script: 'javascript', style: 'css' }` | An `object` that defines the default languages of your components.<br><br>i.e: `{ script: 'typescript' }` makes TypeScript the default language, removing the need of adding `lang="ts"` to `script` tags.<br><br>**Note: It is generally not recommended to use this setting because not all tooling is able to deal with it, resulting in for example broken syntax highlighting for SCSS.** |
 | `sourceMap`     | `false`                                                  | If `true`, `svelte-preprocess` generates sourcemap for every language that supports it.                                                                                                                                                                                                                                                                                                        |
 
 ##### Configuring preprocessors

src/autoProcess.ts

@@ -63,6 +63,13 @@ export function sveltePreprocess(
     ...defaults,
   });
 
+  // todo: remove this on v5
+  if (defaults != null) {
+    console.warn(
+      '[svelte-preprocess] Deprecation notice: using the "defaults" option is no longer recommended and will be removed in the next major version. Instead, define the language being used explicitly via the lang attribute.\n\nSee https://github.com/sveltejs/svelte-preprocess/issues/362',
+    );
+  }
+
   const transformers = rest as Transformers;
 
   if (aliases?.length) {

src/types/index.ts

@@ -64,6 +64,7 @@ export type AutoPreprocessOptions = {
   markupTagName?: string;
   aliases?: Array<[string, string]>;
   preserve?: string[];
+  /** @deprecated Don't use "defaults" anymore, define the language being used explicitly instead */
   defaults?: {
     markup?: string;
     style?: string;