Stack rebuilds whole application in file-watch mode, when it should be able to incrementally recompile

[danbroooks]

I have opened this issue from https://github.com/yesodweb/yesod/issues/1661, where the context of my issue is I wish to switch between yesod-devel for my yesod application, and run stack test, without needing to re-compile the whole of my app. Doing so currently seems to trigger a full re-build, and I have narrowed down my problem to the stack commands I believe are being executed in this context.

If I build my application with tests like so:

stack build --fast --test --flag myapp:library-only --flag myapp:dev

-- outputs:
myapp> build (lib + exe + test)

And then run this command (which is the raw command from yesod devel, minus the --exec "/home/dan/.local/bin/yesod devel-signal" flag):

stack build --fast --file-watch myapp:lib --flag myapp:dev --flag myapp:library-only

I get this output:

Success! Waiting for next file change.
Type help for available commands. Press enter to force a rebuild.

Then when I make a file change, this results in the whole lib project needing to be rebuilt, and the following output:

myapp> build (lib)

My thought process here though is that the whole library has been built as part of the first test command, and therefore should not need to trigger an entire re-build from scratch, and instead should be able to incrementally re-compile from the file I have changed and onwards.

---

As part of my troubleshooting with stack I have been adding logging here and there to some of the files in src/Stack/Build, and have shifted a few things around in order to make the code a bit easier to follow, and also get at certain values in different places.

Would a refactor PR be welcomed with these changes to the project?

[MaciekFlis]

Could this be related to #4977 ?