add preprocessor nesting

This allows for developers to ensure that certain `markup` functions
are run *after* other `script` or `style` functions have run.

Author: happycollision

site/content/docs/04-compile-time.md

@@ -312,6 +312,51 @@ const { code } = await svelte.preprocess(source, [
 });
 ```
 
+An additional level of nesting allows more control over the timing of subsequent `markup` functions.
+
+```js
+const svelte = require('svelte/compiler');
+
+const { code } = await svelte.preprocess(source, [
+	[
+		{
+			markup: () => {
+				console.log('this runs first');
+			},
+			script: () => {
+				console.log('this runs third');
+			},
+			style: () => {
+				console.log('this runs fifth');
+			}
+		},
+		{
+			markup: () => {
+				console.log('this runs second');
+			},
+			script: () => {
+				console.log('this runs fourth');
+			},
+			style: () => {
+				console.log('this runs sixth');
+			}
+		}
+	],
+	{
+		markup: () => {
+			console.log('this runs seventh');
+		},
+		script: () => {
+			console.log('this runs eighth');
+		},
+		style: () => {
+			console.log('this runs ninth');
+		}
+	}
+], {
+	filename: 'App.svelte'
+});
+```
 
 ### `svelte.walk`
 

src/compiler/preprocess/index.ts

@@ -195,27 +195,26 @@ async function process_markup(filename: string, process: MarkupPreprocessor, sou
 	}
 }
 
-export default async function preprocess(
-	source: string,
-	preprocessor: PreprocessorGroup | PreprocessorGroup[],
-	options?: { filename?: string }
-): Promise<Processed> {
-	// @ts-ignore todo: doublecheck
-	const filename = (options && options.filename) || preprocessor.filename; // legacy
-
-	const preprocessors = preprocessor ? (Array.isArray(preprocessor) ? preprocessor : [preprocessor]) : [];
+function ensureArray<T>(x: T): T extends readonly any[] ? T : T[] {
+	// @ts-ignore
+	return Array.isArray(x)? x : [x];
+}
 
-	const markup = preprocessors.map(p => p.markup).filter(Boolean);
-	const script = preprocessors.map(p => p.script).filter(Boolean);
-	const style = preprocessors.map(p => p.style).filter(Boolean);
-
-	const result = new PreprocessResult(source, filename);
+async function preprocessGrouped<T extends PreprocessResult>(
+	result: T,
+	preprocessorGroup: PreprocessorGroup[]|PreprocessorGroup
+): Promise<void> {
+	const preprocessors = ensureArray(preprocessorGroup);
+	
+	const markup = preprocessors.map((p) => p.markup).filter(Boolean);
+	const script = preprocessors.map((p) => p.script).filter(Boolean);
+	const style = preprocessors.map((p) => p.style).filter(Boolean);
 
 	// TODO keep track: what preprocessor generated what sourcemap?
 	// to make debugging easier = detect low-resolution sourcemaps in fn combine_mappings
 
 	for (const process of markup) {
-		result.update_source(await process_markup(filename, process, result));
+		result.update_source(await process_markup(result.filename, process, result));
 	}
 
 	for (const process of script) {
@@ -226,5 +225,23 @@ export default async function preprocess(
 		result.update_source(await process_tag('style', preprocess, result));
 	}
 
+	return;
+}
+
+export default async function preprocess(
+	source: string,
+	preprocessor: PreprocessorGroup | Array<PreprocessorGroup | PreprocessorGroup[]> = [],
+	options?: { filename?: string }
+): Promise<Processed> {
+	// @ts-ignore todo: doublecheck
+	const filename = (options && options.filename) || preprocessor.filename; // legacy
+	const result = new PreprocessResult(source, filename);
+
+	const preprocessors = ensureArray(preprocessor);
+  
+	for (const preprocessorGrouping of preprocessors) {
+		await preprocessGrouped(result, preprocessorGrouping);
+	}
+
 	return result.to_processed();
 }