Generalize low-resolution sourcemap handling and add tests

src/utils/collapseSourcemaps.ts

@@ -117,34 +117,23 @@ class Link {
 		const segments = this.mappings[line];
 		if (!segments) return null;
 
-		// Sometimes a high-resolution sourcemap will be preceded in the sourcemap chain
-		// by a low-resolution sourcemap. We can detect this by checking if the mappings
-		// array for this line only contains a segment for column zero. In that case, we
-		// want to fall back to a low-resolution mapping instead of returning null.
-		if (segments.length == 1 && segments[0][0] == 0) {
-			const segment = segments[0];
-			if (segment.length == 1) {
-				return null;
-			}
-			const source = this.sources[segment[1]] || null;
-			return source?.traceSegment(
-				segment[2],
-				segment[3],
-				segment.length === 5 ? this.names[segment[4]] : name
-			);
-		}
-
 		// 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;
+				if (!source) {
+					return null;
+				}
 
 				return source.traceSegment(
 					segment[2],
@@ -153,9 +142,9 @@ class Link {
 				);
 			}
 			if (segment[0] > column) {
-				j = m - 1;
+				searchEnd = m - 1;
 			} else {
-				i = m + 1;
+				searchStart = m + 1;
 			}
 		}
 

src/utils/getOriginalLocation.ts

@@ -7,35 +7,25 @@ export function getOriginalLocation(
 	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) {
-			// Sometimes a high-resolution sourcemap will be preceded in the sourcemap chain
-			// by a low-resolution sourcemap. We can detect this by checking if the mappings
-			// array for this line only contains a segment for column zero. In that case, we
-			// want to fall back to a low-resolution mapping instead of throwing an error.
-			const segment =
-				line.length == 1 && line[0][0] == 0
-					? line[0]
-					: line.find(segment => segment[0] >= location.column);
 
-			if (segment && segment.length !== 1) {
-				locationFound = true;
-				location = {
-					column: segment[3],
-					line: segment[2] + 1,
-					name: segment.length === 5 ? sourcemap.names[segment[4]] : undefined,
-					source: sourcemap.sources[segment[1]]
-				};
+		if (line?.length) {
+			for (const segment of line) {
+				if (segment[0] >= location.column || line.length === 1) {
+					if (segment.length === 1) break;
+					location = {
+						column: segment[3],
+						line: segment[2] + 1,
+						name: segment.length === 5 ? sourcemap.names[segment[4]] : undefined,
+						source: sourcemap.sources[segment[1]]
+					};
+					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/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]]];
+				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';