fix(sourcemap): fall back to low-resolution line mapping

src/utils/collapseSourcemaps.ts

@@ -47,6 +47,7 @@ class Link {
 
 	traceMappings() {
 		const sources: string[] = [];
+		const sourceIndexMap = new Map<string, number>();
 		const sourcesContent: string[] = [];
 		const names: string[] = [];
 		const nameIndexMap = new Map<string, number>();
@@ -57,7 +58,7 @@ class Link {
 			const tracedLine: SourceMapSegment[] = [];
 
 			for (const segment of line) {
-				if (segment.length == 1) continue;
+				if (segment.length === 1) continue;
 				const source = this.sources[segment[1]];
 				if (!source) continue;
 
@@ -68,36 +69,34 @@ class Link {
 				);
 
 				if (traced) {
-					// newer sources are more likely to be used, so search backwards.
-					let sourceIndex = sources.lastIndexOf(traced.source.filename);
-					if (sourceIndex === -1) {
+					const {
+						column,
+						line,
+						name,
+						source: { content, filename }
+					} = traced;
+					let sourceIndex = sourceIndexMap.get(filename);
+					if (sourceIndex === undefined) {
 						sourceIndex = sources.length;
-						sources.push(traced.source.filename);
-						sourcesContent[sourceIndex] = traced.source.content;
+						sources.push(filename);
+						sourceIndexMap.set(filename, sourceIndex);
+						sourcesContent[sourceIndex] = content;
 					} else if (sourcesContent[sourceIndex] == null) {
-						sourcesContent[sourceIndex] = traced.source.content;
-					} else if (
-						traced.source.content != null &&
-						sourcesContent[sourceIndex] !== traced.source.content
-					) {
+						sourcesContent[sourceIndex] = content;
+					} else if (content != null && sourcesContent[sourceIndex] !== content) {
 						return error({
-							message: `Multiple conflicting contents for sourcemap source ${traced.source.filename}`
+							message: `Multiple conflicting contents for sourcemap source ${filename}`
 						});
 					}
 
-					const tracedSegment: SourceMapSegment = [
-						segment[0],
-						sourceIndex,
-						traced.line,
-						traced.column
-					];
+					const tracedSegment: SourceMapSegment = [segment[0], sourceIndex, line, column];
 
-					if (traced.name) {
-						let nameIndex = nameIndexMap.get(traced.name);
+					if (name) {
+						let nameIndex = nameIndexMap.get(name);
 						if (nameIndex === undefined) {
 							nameIndex = names.length;
-							names.push(traced.name);
-							nameIndexMap.set(traced.name, nameIndex);
+							names.push(name);
+							nameIndexMap.set(name, nameIndex);
 						}
 
 						(tracedSegment as SourceMapSegment)[4] = nameIndex;
@@ -118,13 +117,17 @@ class Link {
 		if (!segments) return null;
 
 		// binary search through segments for the given column
-		let i = 0;
-		let j = segments.length - 1;
+		let searchStart = 0;
+		let searchEnd = segments.length - 1;
 
-		while (i <= j) {
-			const m = (i + j) >> 1;
+		while (searchStart <= searchEnd) {
+			const m = (searchStart + searchEnd) >> 1;
 			const segment = segments[m];
-			if (segment[0] === column) {
+
+			// If a sourcemap does not have sufficient resolution to contain a
+			// necessary mapping, e.g. because it only contains line information, we
+			// use the best approximation we could find
+			if (segment[0] === column || searchStart === searchEnd) {
 				if (segment.length == 1) return null;
 				const source = this.sources[segment[1]];
 				if (!source) return null;
@@ -136,9 +139,9 @@ class Link {
 				);
 			}
 			if (segment[0] > column) {
-				j = m - 1;
+				searchEnd = m - 1;
 			} else {
-				i = m + 1;
+				searchStart = m + 1;
 			}
 		}
 

src/utils/getOriginalLocation.ts

@@ -2,35 +2,30 @@ import type { DecodedSourceMapOrMissing, ExistingDecodedSourceMap } from '../rol
 
 export function getOriginalLocation(
 	sourcemapChain: readonly DecodedSourceMapOrMissing[],
-	location: { column: number; line: number; name?: string; source?: string }
+	location: { column: number; line: number }
 ): { column: number; line: number } {
 	const filteredSourcemapChain = sourcemapChain.filter(
 		(sourcemap): sourcemap is ExistingDecodedSourceMap => !!sourcemap.mappings
 	);
-
-	while (filteredSourcemapChain.length > 0) {
+	traceSourcemap: while (filteredSourcemapChain.length > 0) {
 		const sourcemap = filteredSourcemapChain.pop()!;
 		const line = sourcemap.mappings[location.line - 1];
-		let locationFound = false;
-
-		if (line !== undefined) {
-			for (const segment of line) {
-				if (segment[0] >= location.column) {
-					if (segment.length === 1) break;
+		if (line) {
+			const filteredLine = line.filter(
+				(segment): segment is [number, number, number, number] => segment.length > 1
+			);
+			const lastSegment = filteredLine[filteredLine.length - 1];
+			for (const segment of filteredLine) {
+				if (segment[0] >= location.column || segment === lastSegment) {
 					location = {
 						column: segment[3],
-						line: segment[2] + 1,
-						name: segment.length === 5 ? sourcemap.names[segment[4]] : undefined,
-						source: sourcemap.sources[segment[1]]
+						line: segment[2] + 1
 					};
-					locationFound = true;
-					break;
+					continue traceSourcemap;
 				}
 			}
 		}
-		if (!locationFound) {
-			throw new Error("Can't resolve original location of error.");
-		}
+		throw new Error("Can't resolve original location of error.");
 	}
 	return location;
 }

test/function/samples/cannot-resolve-sourcemap-warning/_config.js

@@ -7,7 +7,7 @@ module.exports = {
 		plugins: {
 			name: 'test-plugin',
 			transform() {
-				return { code: 'export default this', map: { mappings: 'X' } };
+				return { code: 'export default this', map: { mappings: '' } };
 			}
 		}
 	},

test/function/samples/warning-low-resolution-location/_config.js

@@ -0,0 +1,37 @@
+const path = require('path');
+const { encode } = require('sourcemap-codec');
+const ID_MAIN = path.join(__dirname, 'main.js');
+
+module.exports = {
+	description: 'handles when a low resolution sourcemap is used to report an error',
+	options: {
+		plugins: {
+			name: 'test-plugin',
+			transform() {
+				// each entry of each line consist of
+				// [generatedColumn, sourceIndex, sourceLine, sourceColumn];
+				// this mapping only maps the first line to itself
+				const decodedMap = [[[0], [0, 0, 0, 0], [1]]];
+				return { code: 'export default this', map: { mappings: encode(decodedMap), sources: [] } };
+			}
+		}
+	},
+	warnings: [
+		{
+			code: 'THIS_IS_UNDEFINED',
+			frame: `
+1: console.log('original source');
+   ^`,
+			id: ID_MAIN,
+			loc: {
+				column: 0,
+				file: ID_MAIN,
+				line: 1
+			},
+			message:
+				"The 'this' keyword is equivalent to 'undefined' at the top level of an ES module, and has been rewritten",
+			pos: 15,
+			url: 'https://rollupjs.org/guide/en/#error-this-is-undefined'
+		}
+	]
+};

test/function/samples/warning-low-resolution-location/main.js

@@ -0,0 +1 @@
+console.log('original source');

test/sourcemaps/samples/transform-low-resolution/_config.js

@@ -0,0 +1,47 @@
+const assert = require('assert');
+const MagicString = require('magic-string');
+const { SourceMapConsumer } = require('source-map');
+const { encode } = require('sourcemap-codec');
+const getLocation = require('../../getLocation');
+
+module.exports = {
+	description: 'handles combining low-resolution and high-resolution source-maps when transforming',
+	options: {
+		output: { name: 'bundle' },
+		plugins: [
+			{
+				transform(code) {
+					// each entry of each line consist of
+					// [generatedColumn, sourceIndex, sourceLine, sourceColumn];
+					// this mapping only maps the second line to the first with no column
+					// details
+					const decodedMap = [[], [[0, 0, 0, 0]]];
+					return {
+						code: `console.log('added');\n${code}`,
+						map: { mappings: encode(decodedMap) }
+					};
+				}
+			},
+			{
+				transform(code) {
+					const s = new MagicString(code);
+					s.prepend("console.log('second');\n");
+
+					return {
+						code: s.toString(),
+						map: s.generateMap({ hires: true })
+					};
+				}
+			}
+		]
+	},
+	async test(code, map) {
+		const smc = await new SourceMapConsumer(map);
+
+		const generatedLoc = getLocation(code, code.indexOf("'baz'"));
+		const originalLoc = smc.originalPositionFor(generatedLoc);
+
+		assert.strictEqual(originalLoc.line, 1);
+		assert.strictEqual(originalLoc.column, 0);
+	}
+};

test/sourcemaps/samples/transform-low-resolution/main.js

@@ -0,0 +1 @@
+export let foo = 'bar'; foo += 'baz';