Looking up names in an "opportunistic" type checker plugin

[Erdi, Gergo]

PUBLIC

Hi,

I'm writing a type checker plugin that should do things whenever used in the context of a module that already has some definitions loaded. I've hacked together the following function using internal details that I feel I should not be using:

lookupOrigMaybe :: Module -> OccName -> TcPluginM (Maybe Name)
lookupOrigMaybe mod occ = do
    hsc_env <- getTopEnv
    unsafeTcPluginTcM $ liftIO $ updateNameCache (hsc_NC hsc_env) mod occ $ \cache0 -> do
        let name = lookupOrigNameCache cache0 mod occ
        return (cache0, name)

I would like to do this in a nicer way instead. It should be enough to check that the given module is already loaded, since for my use case, if the module is there but the name isn't, then all bets are off and my plugin might as well just error out. So I was hoping I could use `findImportedModule` followed by `lookupOrig`, but alas, `findImportedModule` fails me in one of two ways:


  1.  If I don't specify the package (i.e. use `NoPkgQual`), `findImportedModule` seems to return a `Found{}` result even when the module isn't actually loaded yet, leading to `lookupOrig` failing:

    attempting to use module 'main:Cortex.Stem.Relation.Types' (Cortex.Stem.Relation.Types) which is not loaded


  1.  If I do specify the package (using `ThisPkg`), then `findImportedModule` chokes in situations when the given unit would come "later" in the dependency graph than the currenty processed one:

ghc-mu-core-to-exp: panic! (the 'impossible' happened)
  GHC version 9.5.20220830:
        findImportModule

Cortex.Stem.Relation.Types
"main"
Just ghc-bignum
main
[ghc-bignum, cortex-prim]
Call stack:
    CallStack (from HasCallStack):
      callStackDoc, called at compiler/GHC/Utils/Panic.hs:188:37 in ghc-lib-0.20220830-FvKByKwp6ri5ICk6wiQO8W:GHC.Utils.Panic
      pprPanic, called at compiler/GHC/Unit/Finder.hs:160:32 in ghc-lib-0.20220830-FvKByKwp6ri5ICk6wiQO8W:GHC.Unit.Finder

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

So my question then, is what is the recommended way of looking up a loaded definition opportunistically from a type checker plugin?

Thanks,
            Gergo

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