optimize merge_tables, verbose remapper error

src/compiler/preprocess/index.ts

@@ -217,10 +217,21 @@ export default async function preprocess(
 	// remapper can throw error
 	// `Transformation map ${i} must have exactly one source file.`
 	// for 0 <= i <= (sourcemap_list.length - 2)
-	const map: ReturnType<typeof remapper> =
-		sourcemap_list.length == 0
-			? null
-			: remapper(sourcemap_list as any, () => null, true); // true: skip optional field `sourcesContent`
+
+	let map: ReturnType<typeof remapper>;
+	try {
+		map =
+			sourcemap_list.length == 0
+				? null
+				: remapper(sourcemap_list as any, () => null, true); // true: skip optional field `sourcesContent`
+	} catch (error) {
+		throw { ...error, message: error.message +
+			'\n\ncould not combine sourcemaps:\n' +
+			JSON.stringify((sourcemap_list as any).map(m => {
+				return { ...m, mappings: JSON.stringify(m.mappings).slice(0, 100)+' ....'};
+			}), null, 2)
+		};
+	}
 
 	if (map && !map.file) delete map.file; // skip optional field `file`
 

src/compiler/utils/string_with_sourcemap.ts

@@ -53,6 +53,12 @@ function merge_tables<T>(this_table: T[], other_table): [T[], number[], boolean]
 			has_changed = true;
 		}
 	}
+	if (has_changed) {
+		if (idx_map.find((val, idx) => val != idx) === undefined) {
+			// idx_map is identity map [0, 1, 2, 3, 4, ....]
+			has_changed = false;
+		}
+	}
 	return [new_table, idx_map, has_changed];
 }
 

test/sourcemaps/samples/sourcemap-sources/test.js

@@ -1,12 +1,12 @@
 export function test({ assert, preprocessed, js, css }) {
 
-	const msg_expected = 'Transformation map 0 must have exactly one source file.';
+	assert.notEqual(preprocessed.error, undefined, 'expected preprocessed.error');
 
-  assert.notEqual(preprocessed.error, undefined, 'expected preprocessed.error');
+	const msg_expected_prefix = 'Transformation map 0 must have exactly one source file.';
 
 	assert.equal(
-		preprocessed.error.message.slice(0, msg_expected.length),
-		msg_expected
+		preprocessed.error.message.slice(0, msg_expected_prefix.length),
+		msg_expected_prefix
 	);
 
 }