`ghc --make` recompiles unchanged files when using `-fplugin` OPTIONS

Consider this bare-bones module

{-# OPTIONS -fplugin GHC.TypeLits.Normalise #-}

module A where

compiling it will always say

$ ghc --make A.hs
[1 of 1] Compiling A                ( A.hs, A.o ) [GHC.TypeLits.Normalise changed]

even when the module was compiled before.

$ ghc A.hs -c -ddump-if-trace -ddump-hi-diffs

will give the reason:

imported module ‘GHC.TypeLits.Normalise’ is from package ‘ghc-typelits-natnormalise-0.5’, which is not among previous dependencies

Which is the probable cause: when writing the .hi file, the used plugins' package dependencies should be also written out. AFAICS they are extracted fron the DynFlags but for this comparison in

checkDependencies :: HscEnv -> ModSummary -> ModIface -> IfG RecompileRequired

but not found in the ModSummary that was originally written to disk.

This is pretty nasty for clash compilations and the suspected bug is documented here: https://github.com/clash-lang/ghc-typelits-natnormalise/issues/2

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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