[api-extractor] Source map segment without a source crashes SourceMapper with "Target is a folder, not a file" #6156

Summary

API Extractor crashes when a .d.ts.map contains a mapping segment that has only a generated column and no source. The source map spec allows these segments, and rolldown 1.2.14 now emits them in declaration maps (rolldown/rolldown#11170). We hit this upgrading rolldown in sanity-io/sdk#1445.

Repro steps

1. Create these files:

package.json

{"name":"ae-repro","version":"1.0.0","types":"dist/index.d.ts"}

src/index.ts

export const foo: string = "foo"

dist/index.d.ts

export declare const foo: string

dist/index.d.ts.map (the second segment, qB, has one field: a generated column with no source)

{"version":3,"file":"index.d.ts","names":[],"sources":["../src/index.ts"],"mappings":"AAAA,qB"}

api-extractor.json

{
  "projectFolder": ".",
  "mainEntryPointFilePath": "<projectFolder>/dist/index.d.ts",
  "apiReport": {"enabled": true, "reportFolder": "<projectFolder>/etc/"},
  "docModel": {"enabled": false},
  "dtsRollup": {"enabled": false},
  "compiler": {"overrideTsconfig": {"compilerOptions": {"strict": true}}}
}

2. mkdir etc && npx @microsoft/api-extractor@7.59.4 run --local

Expected result: The run completes and reports ae-missing-release-tag for foo. It does when the mappings are AAAA,qBAAqB.

Actual result:

ERROR: Target is a folder, not a file: undefined
Target is a folder, not a file: undefined
EISDIR: illegal operation on a directory, read
    at Object.readFileSync (node:fs)
    at FileSystem.readFile (@rushstack/node-core-library/src/FileSystem.ts:1066:12)
    at SourceMapper.#getMappedSourceLocation (@microsoft/api-extractor/src/collector/SourceMapper.ts:130:56)

Details

SourceMapper.#getMappedSourceLocation takes the nearest mapping item and runs path.resolve(path.dirname(sourceFilePath), nearestMappingItem.source). For a source-less segment source is empty, so the path resolves to the .d.ts file's folder. FileSystem.exists returns true for it, and the read fails.

Treating a mapping item with no source as "no mapping" would fix it, so the original .d.ts location is used.

Standard questions