Support merging sourcemaps when transformed file is fully replaced

packages/babel-core/package.json

@@ -48,7 +48,7 @@
     "./src/transformation/util/clone-deep.ts": "./src/transformation/util/clone-deep-browser.ts"
   },
   "dependencies": {
-    "@ampproject/remapping": "^2.0.0",
+    "@ampproject/remapping": "^2.1.0",
     "@babel/code-frame": "workspace:^",
     "@babel/generator": "workspace:^",
     "@babel/helper-compilation-targets": "workspace:^",

packages/babel-core/src/transformation/file/merge-map.ts

@@ -6,77 +6,27 @@ export default function mergeSourceMap(
   map: SourceMap,
   source: string,
 ): SourceMap {
-  const outputSources = map.sources;
-
-  let result;
-  if (outputSources.length > 1) {
-    // When there are multiple output sources, we can't always be certain which
-    // source represents the file we just transformed.
-    const index = outputSources.indexOf(source);
-
-    // If we can't find the source, we fall back to the legacy behavior of
-    // outputting an empty sourcemap.
-    if (index === -1) {
-      result = emptyMap(inputMap);
-    } else {
-      result = mergeMultiSource(inputMap, map, index);
+  const result = remapping(rootless(map), (s, ctx) => {
+    if (s === source) {
+      // We empty the source location, which will prevent the sourcemap from
+      // becoming relative to the input's location. Eg, if we're transforming a
+      // file 'foo/bar.js', and it is a transformation of a `baz.js` file in the
+      // same directory, the expected output is just `baz.js`. Without this step,
+      // it would become `foo/baz.js`.
+      ctx.source = "";
+
+      return rootless(inputMap);
     }
-  } else {
-    result = mergeSingleSource(inputMap, map);
-  }
+
+    return null;
+  });
 
   if (typeof inputMap.sourceRoot === "string") {
     result.sourceRoot = inputMap.sourceRoot;
   }
   return result;
 }
 
-// A single source transformation is the default, and easiest to handle.
-function mergeSingleSource(inputMap: SourceMap, map: SourceMap): SourceMap {
-  return remapping([rootless(map), rootless(inputMap)], () => null);
-}
-
-// Transformation generated an output from multiple source files. When this
-// happens, it's ambiguous which source was the transformed file, and which
-// source is from the transformation process. We use remapping's multisource
-// behavior, returning the input map when we encounter the transformed file.
-function mergeMultiSource(inputMap: SourceMap, map: SourceMap, index: number) {
-  // We empty the source index, which will prevent the sourcemap from becoming
-  // relative the the input's location. Eg, if we're transforming a file
-  // 'foo/bar.js', and it is a transformation of a `baz.js` file in the same
-  // directory, the expected output is just `baz.js`. Without this step, it
-  // would become `foo/baz.js`.
-  map.sources[index] = "";
-
-  let count = 0;
-  return remapping(rootless(map), () => {
-    if (count++ === index) return rootless(inputMap);
-    return null;
-  });
-}
-
-// Legacy behavior of the old merger was to output a sourcemap without any
-// mappings but with copied sourcesContent. This only happens if there are
-// multiple output files and it's ambiguous which one is the transformed file.
-function emptyMap(inputMap: SourceMap) {
-  const inputSources = inputMap.sources;
-
-  const sources = [];
-  const sourcesContent = inputMap.sourcesContent?.filter((content, i) => {
-    if (typeof content !== "string") return false;
-
-    sources.push(inputSources[i]);
-    return true;
-  });
-
-  return {
-    ...inputMap,
-    sources,
-    sourcesContent,
-    mappings: "",
-  };
-}
-
 function rootless(map: SourceMap): SourceMap {
   return {
     ...map,

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-multiple-output-sources-complete-replace/options.json

@@ -0,0 +1,4 @@
+{
+  "inputSourceMap": true,
+  "plugins": ["./plugin.js"]
+}

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-multiple-output-sources-complete-replace/output.js

@@ -0,0 +1,2 @@
+"bar";
+"baz";

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-multiple-output-sources-complete-replace/plugin.js

@@ -0,0 +1,52 @@
+module.exports = function (babel) {
+  const { types: t } = babel;
+
+  return {
+    visitor: {
+      Program(path) {
+        const { file } = this;
+        const { sourceFileName } = file.opts.generatorOpts;
+
+        // This injects the sourcesContent, though I don't imagine anyone's
+        // doing it.
+        file.code = {
+            [sourceFileName]: file.code,
+            'bar.js': '<bar />',
+            'baz.js': 'baz();',
+        };
+      },
+
+      CallExpression(path) {
+        const callee = path.node;
+        const { loc } = callee;
+
+        // This filename will cause a second source file to be generated in the
+        // output sourcemap.
+        loc.filename = "bar.js";
+        loc.start.column = 1;
+        loc.end.column = 4;
+
+        const node = t.stringLiteral('bar');
+        node.loc = loc;
+        path.replaceWith(node);
+      },
+
+      Function(path) {
+        const callee = path.node;
+        const { loc } = callee;
+
+        // This filename will cause a second source file to be generated in the
+        // output sourcemap.
+        loc.filename = "baz.js";
+        loc.start.column = 0;
+        loc.start.line = 1;
+        loc.end.column = 3;
+        loc.end.line = 1;
+
+        const node = t.stringLiteral('baz');
+        node.loc = loc;
+        path.replaceWith(node);
+      },
+    },
+  };
+};

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-multiple-output-sources-complete-replace/source-map.json

@@ -0,0 +1,13 @@
+{
+  "mappings": "AAAC;ACAD,K",
+  "names": [],
+  "sources": [
+    "bar.js",
+    "baz.js"
+  ],
+  "sourcesContent": [
+    "<bar />",
+    "baz();"
+  ],
+  "version": 3
+}

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-multiple-output-sources/plugin.js

@@ -20,7 +20,7 @@ module.exports = function (babel) {
         path.replaceWith(node);
 
         // This injects the sourcesContent, though I don't imagine anyone's
-          // doing it.
+        // doing it.
         file.code = {
             [sourceFileName]: file.code,
             'test.js': '<bar />',

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-sources-complete-replace/options.json

@@ -0,0 +1,4 @@
+{
+  "inputSourceMap": true,
+  "plugins": ["./plugin.js"]
+}

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-sources-complete-replace/output.js

@@ -0,0 +1 @@
+"bar";

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-sources-complete-replace/plugin.js

@@ -0,0 +1,38 @@
+module.exports = function (babel) {
+  const { types: t } = babel;
+
+  return {
+    visitor: {
+      Program(path) {
+        const { file } = this;
+        const { sourceFileName } = file.opts.generatorOpts;
+
+        // This injects the sourcesContent, though I don't imagine anyone's
+        // doing it.
+        file.code = {
+            [sourceFileName]: file.code,
+            'test.js': '<bar />',
+        };
+      },
+
+      CallExpression(path) {
+        const callee = path.node;
+        const { loc } = callee;
+
+        // This filename will cause a second source file to be generated in the
+        // output sourcemap.
+        loc.filename = "test.js";
+        loc.start.column = 1;
+        loc.end.column = 4;
+
+        const node = t.stringLiteral('bar');
+        node.loc = loc;
+        path.replaceWith(node);
+      },
+
+      Function(path) {
+        path.remove();
+      },
+    },
+  };
+};

packages/babel-core/test/fixtures/transformation/source-maps/input-source-map-sources-complete-replace/source-map.json

@@ -0,0 +1,7 @@
+{
+  "mappings": "AAAC",
+  "names": [],
+  "sources": ["test.js"],
+  "sourcesContent": ["<bar />"],
+  "version": 3
+}

yarn.lock

@@ -5,13 +5,12 @@ __metadata:
   version: 5
   cacheKey: 8
 
-"@ampproject/remapping@npm:^2.0.0":
-  version: 2.0.2
-  resolution: "@ampproject/remapping@npm:2.0.2"
+"@ampproject/remapping@npm:^2.0.0, @ampproject/remapping@npm:^2.1.0":
+  version: 2.1.0
+  resolution: "@ampproject/remapping@npm:2.1.0"
   dependencies:
-    "@jridgewell/trace-mapping": ^0.2.2
-    sourcemap-codec: 1.4.8
-  checksum: 5759df3715f0291cbf97099a9bb7202201a1a267e232ee1505418c768b9ae7281cd550b1da563a12808a06529eb1298744a6cabde21ac354fc8450044c7f2213
+    "@jridgewell/trace-mapping": ^0.3.0
+  checksum: 10ff0d4a559f930082f1a4c1b68dc521d5b1a75e0b8ab4829e9eedf6621386893e4a008f0db6c716f64db5d8eed49c0abcfbf3bd6ff11d5a00312454a9351ed4
   languageName: node
   linkType: hard
 
@@ -323,7 +322,7 @@ __metadata:
   version: 0.0.0-use.local
   resolution: "@babel/core@workspace:packages/babel-core"
   dependencies:
-    "@ampproject/remapping": ^2.0.0
+    "@ampproject/remapping": ^2.1.0
     "@babel/code-frame": "workspace:^"
     "@babel/generator": "workspace:^"
     "@babel/helper-compilation-targets": "workspace:^"
@@ -4091,13 +4090,20 @@ __metadata:
   languageName: node
   linkType: hard
 
-"@jridgewell/trace-mapping@npm:^0.2.2":
-  version: 0.2.5
-  resolution: "@jridgewell/trace-mapping@npm:0.2.5"
+"@jridgewell/sourcemap-codec@npm:^1.4.10":
+  version: 1.4.10
+  resolution: "@jridgewell/sourcemap-codec@npm:1.4.10"
+  checksum: 247229218edbe165dcf0a5ae0c4b81bff1b5438818bb09221f756681fe158597fdf25c2a803f9260453b299c98c7e01ddebeb1555cda3157d987cd22c08605ef
+  languageName: node
+  linkType: hard
+
+"@jridgewell/trace-mapping@npm:^0.3.0":
+  version: 0.3.2
+  resolution: "@jridgewell/trace-mapping@npm:0.3.2"
   dependencies:
     "@jridgewell/resolve-uri": ^3.0.3
-    sourcemap-codec: 1.4.8
-  checksum: 7ac0a2992f4a8d16c1cbe03bccbcfbd1e96bf5071b0a794dd97904a7588cc248b73e8091fabcb13dac8f40bc51297ee4ef98a6a870ca5a4dfb8e2dcbf6f33956
+    "@jridgewell/sourcemap-codec": ^1.4.10
+  checksum: b58be6b4133cbcb20bfd28c9ca843b8db9efa0bf1d7e0e9e26b2228dace94ad53161c996ab1d762d7c3955dfc398a7734e7b84a2493ae36b451f232234fbb257
   languageName: node
   linkType: hard
 
@@ -14168,7 +14174,7 @@ fsevents@^1.2.7:
   languageName: node
   linkType: hard
 
-"sourcemap-codec@npm:1.4.8, sourcemap-codec@npm:^1.4.4":
+"sourcemap-codec@npm:^1.4.4":
   version: 1.4.8
   resolution: "sourcemap-codec@npm:1.4.8"
   checksum: b57981c05611afef31605732b598ccf65124a9fcb03b833532659ac4d29ac0f7bfacbc0d6c5a28a03e84c7510e7e556d758d0bb57786e214660016fb94279316