fix: use Babel to strip comments and unmap line numbers

packages/bundle-source/demo/circular/a.js

@@ -1,5 +1,8 @@
 // eslint-disable-next-line import/no-cycle
 import { bar } from './b';
 
+/**
+ * @type {import('./b').Bar}
+ */
 export const foo = 'Foo';
 export default bar;

packages/bundle-source/demo/circular/b.js

@@ -1,4 +1,9 @@
 // eslint-disable-next-line import/no-cycle
 import { foo } from './a';
 
+/*
+ * --------->
+ * <!-- foo as bar -->
+ */
+console.error('here am I');
 export { foo as bar };

packages/bundle-source/src/index.js

@@ -4,6 +4,7 @@ import resolve0 from '@rollup/plugin-node-resolve';
 import commonjs0 from '@rollup/plugin-commonjs';
 import * as babelParser from '@agoric/babel-parser';
 import babelGenerate from '@babel/generator';
+import babelTraverse from '@babel/traverse';
 import { makeTransform } from '@agoric/transform-eventual-send';
 
 import { SourceMapConsumer } from 'source-map';
@@ -12,9 +13,14 @@ const DEFAULT_MODULE_FORMAT = 'nestedEvaluate';
 const DEFAULT_FILE_PREFIX = '/bundled-source';
 const SUPPORTED_FORMATS = ['getExport', 'nestedEvaluate'];
 
-// eslint-disable-next-line no-useless-concat
-const IMPORT_RE = new RegExp('\\b(import)' + '(\\s*(?:\\(|/[/*]))', 'g');
-const HTML_COMMENT_RE = new RegExp(`(?:${'<'}!--|--${'>'})`, 'g');
+const removeComments = comments => {
+  if (!comments) {
+    return;
+  }
+  // We remove all the JS comments to prevent misidentifying HTML
+  // comments, or import expressions.
+  comments.splice(0, comments.length);
+};
 
 export function tildotPlugin() {
   const transformer = makeTransform(babelParser, babelGenerate);
@@ -73,51 +79,58 @@ export default async function bundleSource(
     if (isEntry) {
       entrypoint = fileName;
     }
-    let unmappedCode = code;
+
+    // Parse the rolled-up chunk with Babel.
+    const ast = (babelParser.parse || babelParser)(code);
+
+    let unmapLoc;
     if (
       moduleFormat === 'nestedEvaluate' &&
       !fileName.startsWith('_virtual/')
     ) {
-      // TODO: Should parse the generated chunk with Babel.
-      // We rearrange the generated chunk according to its sourcemap to move
-      // its source lines back to the right place, like Babel generator's
-      // `retainLines: true`.
-      unmappedCode = '';
+      // We rearrange the rolled-up chunk according to its sourcemap to move
+      // its source lines back to the right place.
       // eslint-disable-next-line no-await-in-loop
       const consumer = await new SourceMapConsumer(chunk.map);
-      const genLines = code.split('\n');
-      let lastLine = 1;
-      for (let genLine = 0; genLine < genLines.length; genLine += 1) {
-        const pos = consumer.originalPositionFor({
-          line: genLine + 1,
-          column: 0,
-        });
-        const { line: origLine } = pos;
-
-        const srcLine = origLine === null ? lastLine : origLine;
-        const priorChar = unmappedCode[unmappedCode.length - 1];
-        if (
-          srcLine === lastLine &&
-          !['\n', ';', '{', undefined].includes(priorChar) &&
-          !['}'].includes(genLines[genLine][0])
-        ) {
-          unmappedCode += `;`;
+      let lastPos = ast.loc.start;
+      unmapLoc = loc => {
+        if (!loc) {
+          return;
         }
-        while (lastLine < srcLine) {
-          unmappedCode += `\n`;
-          lastLine += 1;
+        for (const pos of ['start', 'end']) {
+          if (loc[pos]) {
+            const newPos = consumer.originalPositionFor(loc[pos]);
+            if (newPos.source !== null) {
+              lastPos = loc[pos];
+              lastPos.line = newPos.line;
+              lastPos.column = newPos.column;
+            }
+            loc[pos] = lastPos;
+          }
         }
-        unmappedCode += genLines[genLine];
-      }
+      };
     }
 
-    // Rewrite apparent import expressions so that they don't fail under SES.
-    // We also do apparent HTML comments.
-    const defangedCode = unmappedCode
-      .replace(IMPORT_RE, '$1notreally')
-      .replace(HTML_COMMENT_RE, '<->');
-    // console.log(`<<<<<< ${fileName}\n${defangedCode}\n>>>>>>>>`);
-    sourceBundle[fileName] = defangedCode;
+    babelTraverse(ast, {
+      enter(p) {
+        const { loc, leadingComments, trailingComments } = p.node;
+        // Remove all comments.
+        removeComments(leadingComments);
+        removeComments(trailingComments);
+        if (p.node.type.startsWith('Comment')) {
+          p.replaceWithMultiple([]);
+          return;
+        }
+        // If not a comment, and we are unmapping the source maps,
+        // then do it for this location.
+        if (unmapLoc) {
+          unmapLoc(loc);
+        }
+      },
+    });
+
+    // Now generate the sources with the new positions.
+    sourceBundle[fileName] = babelGenerate(ast, { retainLines: true }).code;
   }
 
   if (!entrypoint) {