Sourcemap generation consuming significant resources while sourcemaps are disabled

[aem]

Description

When running tsgo --build on a project with source maps and declaration maps disabled, a significant amount of CPU and memory is spent on what seemingly should be unused source map-related functions.

tsgo setup

// <root>/tsconfig.json
//  ...
    "declaration": true,
    "emitDeclarationOnly": true,
    "sourceMap": false,
    "declarationMap": false,
    "emit": false,
//  ...

tsgo version: 7.0.0-dev.20251204.1

Details

After migrating our monorepo to tsgo with a significant local performance improvement, I went to test the new setup in CI and discovered worse performance than typescript@5.8.3 on a CI machine with 4 CPU cores and 16gb memory. I ran tsgo with profiling enabled (profiles attached below) and noticed (with the help of Claude) that source map emission accounted for ~60% of cumulative CPU time during compilation despite source maps and declaration maps both being disabled during the build:

         0     0% 90.69%    126.39s 29.24%  github.com/microsoft/typescript-go/internal/printer.(*Printer).emitSourceMapsAfterNode
         0     0% 90.69%    125.28s 28.99%  github.com/microsoft/typescript-go/internal/printer.(*Printer).emitSourceMapsBeforeNode

The memory profile looks much less interesting, I suspect the ~18GiB of memory consumed by our project is just related to having ~1mm LOC and heavy use of intersection types, but I'm surprised to see the source map printer consuming the majority of CPU time while all maps are disabled.

Profiles

82514-cpuprofile.pb.gz
82514-memprofile.pb.gz

[jakebailey]

[jakebailey]

What is your full config? Do you have inlineSourceMaps enabled? Everything I'm reading implies this code should not be running if source maps are enabled.

Though, the perf here is certainly quite bad in general.

[aem]

No inline source maps enabled, and I assume since we're only emitting declaration files anyway that setting wouldn't apply? Full tsconfig is here!

{
  "compilerOptions": {
    "diagnostics": true,
    "allowJs": true,
    "tsBuildInfoFile": "build/root.tsbuildinfo",
    "outDir": "./build/",
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "declaration": true,
    "emitDeclarationOnly": true,
    "sourceMap": false,
    "declarationMap": false,
    "isolatedModules": true,
    "jsx": "preserve",
    "lib": ["dom", "dom.iterable", "esnext"],
    "module": "ESNext",
    "moduleResolution": "bundler",
    "noEmit": true,
    "noImplicitAny": true,
    "noImplicitThis": true,
    "resolveJsonModule": true,
    "skipLibCheck": true,
    "strict": false,
    "strictNullChecks": true,
    "target": "esnext",
    "paths": {
      "*": ["./src/*"]
    },
    "experimentalDecorators": true,
    "incremental": true,
    "plugins": [
      {
        "name": "next"
      }
    ]
  },
  "exclude": [
    "**/node_modules",
    "**/build",
    "**/.*",
    // ...
  ],
  "include": [
    // ...
  ],
  "references": [
    { "path": "src" },
    { "path": "functionsV2" }
  ]
}