Haddock fails to run plugins with GHC 8.6.1 alpha

[mpickering]

GHC has changed so that plugins are only loaded once rather than just before each time they are used.
I suspect that Haddock needs to follow this change and explicitly loads the plugins as well.

@christiaanb suggested some code like this in a suitable place would fix things.

clash-lang/clash-compiler@8d32505#diff-c50e1570ba401340dde73490fed2c8cdR189

[alexbiehl]

I suspect

haddock/haddock-api/src/Haddock.hs

Lines 428 to 445 in 1c40763

	withGhc' :: String -> [String] -> (DynFlags -> Ghc a) -> IO a
	withGhc' libDir flags ghcActs = runGhc (Just libDir) $ do
	dynflags <- getSessionDynFlags
	dynflags' <- parseGhcFlags (gopt_set dynflags Opt_Haddock) {
	hscTarget = HscNothing,
	ghcMode = CompManager,
	ghcLink = NoLink
	}
	-- We disable pattern match warnings because than can be very
	-- expensive to check
	let dynflags'' = unsetPatternMatchWarnings $
	updOptLevel 0 $
	gopt_unset dynflags' Opt_SplitObjs
	-- ignore the following return-value, which is a list of packages
	-- that may need to be re-linked: Haddock doesn't do any
	-- dynamic or static linking at all!
	_ <- setSessionDynFlags dynflags''
	ghcActs dynflags''

would be a good place for that.

[sheaf]

This doesn't seem to work on Windows. See #905 (comment).