Fix and cover case when preprocessor returns no source map

src/compiler/utils/string_with_sourcemap.ts

@@ -167,11 +167,13 @@ export class StringWithSourcemap {
 	}
 
 	static from_processed(string: string, map?: DecodedSourceMap): StringWithSourcemap {
+		const line_count = string.split('\n').length;
+
 		if (map) {
 			// ensure that count of source map mappings lines 
 			// is equal to count of generated code lines
 			// (some tools may produce less)
-			const missing_lines = string.split('\n').length - map.mappings.length;
+			const missing_lines = line_count - map.mappings.length;
 			for (let i = 0; i < missing_lines; i++) {
 				map.mappings.push([]);
 			}
@@ -182,7 +184,6 @@ export class StringWithSourcemap {
 		map = { version: 3, names: [], sources: [], mappings: [] };
 
 		// add empty SourceMapSegment[] for every line
-		const line_count = (string.match(/\n/g) || '').length;
 		for (let i = 0; i < line_count; i++) map.mappings.push([]);
 		return new StringWithSourcemap(string, map);
 	}

test/sourcemaps/helpers.ts

@@ -43,6 +43,36 @@ export function assert_mapped(
 	);
 }
 
+type AssertNotMappedParameters = {
+	code: string;
+	filename?: string;
+	preprocessed: any;
+};
+
+export function assert_not_mapped(
+	{ code, filename, preprocessed }: AssertNotMappedParameters
+) {
+	if (filename === undefined) filename = 'input.svelte';
+
+	const transformed_loc = preprocessed.locate_1(code);
+	assert.notEqual(
+		transformed_loc,
+		undefined,
+		`failed to locate "${code}" in transformed "${filename}"`
+	);
+
+	assert.deepEqual(
+		preprocessed.mapConsumer.originalPositionFor(transformed_loc),
+		{
+			source: null,
+			name: null,
+			line: null,
+			column: null
+		},
+		`incorrect mappings for "${code}" in "${filename}"`
+	);
+}
+
 export function assert_not_located(
 	code: string,
 	locate: ReturnType<typeof getLocator>,

test/sourcemaps/samples/preprocess-no-map/_config.js

@@ -0,0 +1,13 @@
+export default {
+	css_map_sources: [],
+	preprocess: [
+		{
+			style: ({ content }) => {
+				return { code: content };
+			},
+			script: ({ content }) => {
+				return { code: content };
+			}
+		}
+	]
+};

test/sourcemaps/samples/preprocess-no-map/input.svelte

@@ -0,0 +1,13 @@
+<script>
+	export let name;
+
+	console.log(name);
+</script>
+
+<main>
+	<div>{name}</div>
+</main>
+
+<style>
+	main { font-weight: bold; }
+</style>

test/sourcemaps/samples/preprocess-no-map/test.js

@@ -0,0 +1,36 @@
+import { assert_mapped, assert_not_mapped } from '../../helpers';
+
+export function test({ input, preprocessed }) {
+	// markup (start)
+	assert_mapped({
+		code: '<script>',
+		input: input.locate,
+		preprocessed
+	});
+
+	// script
+	assert_not_mapped({
+		code: 'console.log(name);',
+		preprocessed
+	});
+	
+	// markup (middle)
+	assert_mapped({
+		code: '<div>{name}</div>',
+		input: input.locate,
+		preprocessed
+	});
+	
+	// style content
+	assert_not_mapped({
+		code: 'font-weight: bold;',
+		preprocessed
+	});
+
+	// markup (end)
+	assert_mapped({
+		code: '</style>',
+		input: input.locate,
+		preprocessed
+	});
+}