tsc: watch --build slower than watch without --build

[mastodon0]

Bug Report

🔎 Search Terms

tsc, watch, --build, performance, tryReuseStructureFromOldProgram

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about watch, --build

⏯ Playground Link

Repo link

💻 Code

n/a

🙁 Actual behavior

Watching a project using --build takes longer to compile than omitting the --build option. The difference could be up to multiple seconds, depending on the project size.

Here is the trace for my sample repo running tsc --build tsconfig.json --watch after saving notImported.ts

trace-slow.txt

Here is the trace when running tsc --project tsconfig.json --watch and saving the same file.

trace_fast.txt

The big difference seems to be that --build doesn't run tryReuseStructureFromOldProgram.
I tried to poke around in the code and it seems that the program state gets released during emitBuildInfo

🙂 Expected behavior

Watching using tsc --build --watch should be able to reuse the last program state to speed up compile times

[andrewbranch]

Duplicate of #45082, likely also the same thing as #46420

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.