stack rebuilds sublibraries that are not dependencies of the specified target

[kostmo]

Minimal testcase is here: https://github.com/kostmo/stack-sublibrary-rebuild

I am using stack version 2.15.5.

To reproduce the bug

1. Build the executable:

   stack build bug-repro:exe:bug-repro-exe
   

2. Then repeat to ensure all targets are up-to-date.
3. Insert a newline into src2/Lib2.hs and save
4. Re-run the above command to build the executable. Observe that files are rebuilt despite that the lib2 sublibrary is not a dependency of bug-repro-exe.

Equivalent cabal test

1. Build the executable:

   cabal build bug-repro:exe:bug-repro-exe
   

2. Then repeat to ensure all targets are up-to-date.
3. Insert a newline into src2/Lib2.hs and save
4. Re-run the above command to build the executable. Observe nothing is rebuilt; only "Up to date" is printed.

Potentially related issues

• Allow stack build to target sub-libraries #6042
• stack build copies unnecessary executables when asked to build just a single executable #3486

[mpilgrem]

@kostmo, thanks for reporting and for taking the time to create a minimal example.

What you are experiencing is an example of an long-standing issue/feature request known as 'component-based builds'. The most recent open issue pulling that together is:

• Component-based builds #6356

but there is a long history before that.

In short, Stack currently approaches building in terms of 'packages (with components)' rather than 'components (of packages)'.

I am going to close this issue - not because it is unimportant but because I am seeking to consolidate matters related to component-based builds under a single 'master' issue.