.tsbuildinfo file should be created when the noEmit flag is enabled

[vkrol]

TypeScript Version: 3.4.0-dev.20190330

Search Terms: incremental, noEmit, no emit, tsbuildinfo

Code:
https://github.com/vkrol/typescript-incremental-no-emit

{
  "compilerOptions": {
    "outDir": "./dist",
    "incremental": true,
    "noEmit": true
  }
}

Expected behavior:
.tsbuildinfo file should be created when the noEmit flag is enabled.

Actual behavior:
.tsbuildinfo file is not created when the noEmit flag is enabled.

[OliverJAsh]

This would be useful so we can still benefit from fast rebuilds when using tsc for type checking only.

[sheetalkamat]

Though emitting a file (in this case .tsbuildinfo) when --noEmit is specified seems inappropriate.
cc: @DanielRosenwasser and @RyanCavanaugh for their input.

[denis-sokolov]

The way I use these terms, “emit” refers to the valuable output of the application, something we’d put to stdout in unix terms. The tsbuildinfo file is a cache that is supposed to be invisible to the consumer. The fact that it happens to be a file that we happen to write to disk is rather an irrelevant implementation detail.

[RyanCavanaugh]

"tsc emits anything when --noEmit is specified" is a follow-up bug report to this one.

The problem here is that incremental is on by default if composite is on, so this is an actual risky change because it's going to mean new build artifacts in unexpected places for people using --noEmit today.

If the build info file path is specified manually, then writing it out does seem OK

[denis-sokolov]

It tsbuildinfo a build artifact in an unexpected place, though? What if we write this cache to /tmp, the Windows registry, or whatever other storage, as long as the user does not see it? (edit: to be clear, the previous sentence is a hypothetical to help identify the meaning of tsbuildinfo, not a real suggestion) It seems like the explicitly thorough solution would be to split the noEmit option into typecheckButNoBuildOutput and noTouchingMyFilesystem.

[vkrol]

If the build info file path is specified manually, then writing it out does seem OK

Good idea 👍

[fmal]

@RyanCavanaugh

If the build info file path is specified manually, then writing it out does seem OK

i second this!

[META-DREAMER]

This would be incredibly useful in react native since the typescript code is parsed with the packager using Babel 7 so for type checking we have to use tsc --noEmit

[oleg-codaio]

I was looking into just ditching the --noEmit flag in order to use --incremental, but even though the subsequent build times decreased by half, the initial build time went up by quite a bit due to emit time.

I'm not super familiar with the implementation, but it seems like type checking via --noEmit should still be able to benefit from the signatures/dependencies stored in the build info cache file, and this would be great for use cases like speeding up type checking in CI by caching and reusing the build file periodically.