[api-extractor] Fix crash when a source map segment has no source

apps/api-extractor/src/collector/SourceMapper.ts

@@ -113,7 +113,9 @@ export class SourceMapper {
       column: sourceFileColumn
     });
 
-    if (!nearestMappingItem) return;
+    // A segment with only a generated column marks code that has no original source. The "source-map"
+    // package reports it with a null originalLine, despite the MappingItem typings.
+    if (!nearestMappingItem || nearestMappingItem.originalLine === null) return;
 
     const mappedFilePath: string = path.resolve(path.dirname(sourceFilePath), nearestMappingItem.source);
 

build-tests/api-extractor-scenarios/config/heft.json

@@ -27,7 +27,7 @@
                 {
                   "sourcePath": "src",
                   "destinationFolders": ["lib-dts"],
-                  "fileExtensions": [".d.ts"]
+                  "fileExtensions": [".d.ts", ".d.ts.map"]
                 }
               ]
             }

build-tests/api-extractor-scenarios/etc/sourceMapWithoutSource/api-extractor-scenarios.api.json

@@ -0,0 +1,217 @@
+{
+  "metadata": {
+    "toolPackage": "@microsoft/api-extractor",
+    "toolVersion": "[test mode]",
+    "schemaVersion": 1011,
+    "oldestForwardsCompatibleVersion": 1001,
+    "tsdocConfig": {
+      "$schema": "https://developer.microsoft.com/json-schemas/tsdoc/v0/tsdoc.schema.json",
+      "noStandardTags": true,
+      "tagDefinitions": [
+        {
+          "tagName": "@alpha",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@beta",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@defaultValue",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@decorator",
+          "syntaxKind": "block",
+          "allowMultiple": true
+        },
+        {
+          "tagName": "@deprecated",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@eventProperty",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@example",
+          "syntaxKind": "block",
+          "allowMultiple": true
+        },
+        {
+          "tagName": "@experimental",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@inheritDoc",
+          "syntaxKind": "inline"
+        },
+        {
+          "tagName": "@internal",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@label",
+          "syntaxKind": "inline"
+        },
+        {
+          "tagName": "@link",
+          "syntaxKind": "inline",
+          "allowMultiple": true
+        },
+        {
+          "tagName": "@override",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@packageDocumentation",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@param",
+          "syntaxKind": "block",
+          "allowMultiple": true
+        },
+        {
+          "tagName": "@privateRemarks",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@public",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@readonly",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@remarks",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@returns",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@sealed",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@see",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@throws",
+          "syntaxKind": "block",
+          "allowMultiple": true
+        },
+        {
+          "tagName": "@typeParam",
+          "syntaxKind": "block",
+          "allowMultiple": true
+        },
+        {
+          "tagName": "@virtual",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@jsx",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@jsxRuntime",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@jsxFrag",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@jsxImportSource",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@betaDocumentation",
+          "syntaxKind": "modifier"
+        },
+        {
+          "tagName": "@internalRemarks",
+          "syntaxKind": "block"
+        },
+        {
+          "tagName": "@preapproved",
+          "syntaxKind": "modifier"
+        }
+      ],
+      "supportForTags": {
+        "@alpha": true,
+        "@beta": true,
+        "@defaultValue": true,
+        "@decorator": true,
+        "@deprecated": true,
+        "@eventProperty": true,
+        "@example": true,
+        "@experimental": true,
+        "@inheritDoc": true,
+        "@internal": true,
+        "@label": true,
+        "@link": true,
+        "@override": true,
+        "@packageDocumentation": true,
+        "@param": true,
+        "@privateRemarks": true,
+        "@public": true,
+        "@readonly": true,
+        "@remarks": true,
+        "@returns": true,
+        "@sealed": true,
+        "@see": true,
+        "@throws": true,
+        "@typeParam": true,
+        "@virtual": true,
+        "@betaDocumentation": true,
+        "@internalRemarks": true,
+        "@preapproved": true
+      },
+      "reportUnsupportedHtmlElements": false
+    }
+  },
+  "kind": "Package",
+  "canonicalReference": "api-extractor-scenarios!",
+  "docComment": "",
+  "name": "api-extractor-scenarios",
+  "preserveMemberOrder": false,
+  "members": [
+    {
+      "kind": "EntryPoint",
+      "canonicalReference": "api-extractor-scenarios!",
+      "name": "",
+      "preserveMemberOrder": false,
+      "members": [
+        {
+          "kind": "Variable",
+          "canonicalReference": "api-extractor-scenarios!foo:var",
+          "docComment": "/**\n * @public\n */\n",
+          "excerptTokens": [
+            {
+              "kind": "Content",
+              "text": "foo: "
+            },
+            {
+              "kind": "Content",
+              "text": "string"
+            }
+          ],
+          "fileUrlPath": "lib-dts/sourceMapWithoutSource/index.d.ts",
+          "isReadonly": true,
+          "releaseTag": "Public",
+          "name": "foo",
+          "variableTypeTokenRange": {
+            "startIndex": 1,
+            "endIndex": 2
+          }
+        }
+      ]
+    }
+  ]
+}

build-tests/api-extractor-scenarios/etc/sourceMapWithoutSource/api-extractor-scenarios.api.md

@@ -0,0 +1,12 @@
+## API Report File for "api-extractor-scenarios"
+
+> Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
+
+```ts
+
+// @public (undocumented)
+export const foo: string;
+
+// (No @packageDocumentation comment for this package)
+
+```

build-tests/api-extractor-scenarios/etc/sourceMapWithoutSource/rollup.d.ts

@@ -0,0 +1,4 @@
+/** @public */
+export declare const foo: string;
+
+export { }

build-tests/api-extractor-scenarios/src/sourceMapWithoutSource/index.d.ts

@@ -0,0 +1,2 @@
+/** @public */
+export declare const foo: string;

common/changes/@microsoft/api-extractor/fix-api-extractor-sourceless-mapping_2026-10-20-12-00.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "comment": "Fix a crash (\"Target is a folder, not a file\") when a .d.ts.map contains a mapping segment with no source, such as those emitted by rolldown.",
+      "type": "patch",
+      "packageName": "@microsoft/api-extractor"
+    }
+  ],
+  "packageName": "@microsoft/api-extractor"
+}