Compiler API (language service) becomes incredibly slow when `noEmitOnError = true`

[AlCalzone]

TypeScript Version: 3.0.0-dev.20180526

Search Terms: compiler api noEmitOnError

I'm building an in-memory version of the TypeScript compiler using the language service API. This allows to compile and generate declarations for files that never reach the hard disk. While debugging another issue, I found that setting noEmitOnError to true drastically increases compile times with minor changes to the compiled sources.
I'm not 100% sure that the problem doesn't come from me misusing the API, so forgive me if this is actually not an issue in TypeScript.

Code
This is too much for a short repro, so I created a repro repo here:
https://github.com/AlCalzone/virtual-tsc/tree/ts-repro
To reproduce, npm install and then npm test. The project will run a few simple compilations using the language service API. It contains two identical test cases, where one has noEmitOnError = true and the other one false.

Both test cases correctly compile correct code and both catch syntax errors.

Expected behavior:
Similar results for both.

Actual behavior:

  compiler API repro =>
    √ noEmitOnError = false (2265ms)
    √ noEmitOnError = true (13794ms)
  2 passing (16s)

Related Issues: none found

[RyanCavanaugh]

Didn't see anything obviously wrong with the way things were being called.

It'd be great if you could identify when this regressed

[AlCalzone]

TS 2.9.0-RC:

  compiler API repro =>
    √ noEmitOnError = false (2319ms)
    √ noEmitOnError = true (13683ms)

TS 2.8.3:

  compiler API repro =>
    √ noEmitOnError = false (2335ms)
    √ noEmitOnError = true (13755ms)

TS 2.7.2:

  compiler API repro =>
    √ noEmitOnError = false (2021ms)
    √ noEmitOnError = true (13184ms)

TS 2.6.2:

  compiler API repro =>
    √ noEmitOnError = false (1580ms)
    √ noEmitOnError = true (10238ms)

TS 2.5.3:

  compiler API repro =>
    √ noEmitOnError = false (1526ms)
    √ noEmitOnError = true (7848ms)

TS 2.4.2:
(with different signature of ServiceHost.readDirectory)

  compiler API repro =>
    √ noEmitOnError = false (1546ms)
    √ noEmitOnError = true (7726ms)

TS 2.3.4:

  compiler API repro =>
    √ noEmitOnError = false (1538ms)
    √ noEmitOnError = true (7241ms)

TS 2.2.2:

  compiler API repro =>
    √ noEmitOnError = false (1049ms)
    √ noEmitOnError = true (3772ms)

TS 2.1.6:

  compiler API repro =>
    √ noEmitOnError = false (1063ms)
    √ noEmitOnError = true (3530ms)

TS 2.0 doesn't work.

[sheetalkamat]

The time spent in noEmitOnError is in calculation of declaration diagnostics which isn't done if noEmitOnError is false.
In the test, declaration = true for compiler options (link) is set and that's the reason you are seeing this.

[AlCalzone]

What exactly do you mean by "declaration diagnostics"?

Since I do need the declarations, is there a better way to detect that the compilation had errors than setting noEmitOnError to true and spending an extra 10 seconds? Or in other words: is it enough to test if there are any diagnostics with type === "error" to know that the compiled program will most likely not run?

I still find it weird that the time needed for that almost doubled from 2.5 to 2.7. Also when I try to reproduce the test manually using the tsc CLI, both settings for noEmitOnError take roughly 2 seconds.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.