Warn about unused dependencies
GHC loads packages lazily, so at the end of compilation (at least in --make mode) it knows all the dependencies, required to compile the code. See eps_PIT in ExternalPackageState. If we compare this list with the list of packages, passed on command like via --package-id, then we'll get a list of unused dependencies.
I propose to add a flag -Wunused-packages, off by default, which will report the unused packages if any.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 8.6.1 |
| Type | FeatureRequest |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
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