haskellPackages.mkDerivation: add doHaddockInterfaces option

This adds a new builder option `doHaddockInterfaces` to enable the -haddock flag in GHC,
which results in Haddock comments parsed at compile-time and embedded in
interface files. These are used by the :doc command in GHCi, as well as IDE
tools like ghcide and hls to display docs on hover.

The `-haddock` flag has been around since at least 8.2, even though it does not
get a mention in the GHC Users guide.

There are two downsides to turning on this flag:

1. Increased compile times, since Haddocks must be parsed and then encoded
2. Haddock parse errors now become compile errors for GHC < 9.0.1
   (https://gitlab.haskell.org/ghc/ghc/-/issues/8944)

Thus we only enable the feature if we have GHC 9.0.1 and haddock is
enabled; when 9.0.1 becomes the default GHC, we may need to reevaluate
the performance concern.

Co-authored-by: sternenseemann <sternenseemann@systemli.org>

pkgs/development/haskell-modules/generic-builder.nix

@@ -56,6 +56,7 @@ in
 , mainProgram ? null
 , doCoverage ? false
 , doHaddock ? !(ghc.isHaLVM or false)
+, doHaddockInterfaces ? doHaddock && lib.versionAtLeast ghc.version "9.0.1"
 , passthru ? {}
 , pkg-configDepends ? [], libraryPkgconfigDepends ? [], executablePkgconfigDepends ? [], testPkgconfigDepends ? [], benchmarkPkgconfigDepends ? []
 , testDepends ? [], testHaskellDepends ? [], testSystemDepends ? [], testFrameworkDepends ? []
@@ -226,7 +227,11 @@ let
   ] ++ optionals isCross ([
     "--configure-option=--host=${stdenv.hostPlatform.config}"
   ] ++ crossCabalFlags
-  ) ++ optionals enableSeparateBinOutput ["--bindir=${binDir}"];
+  ) ++ optionals enableSeparateBinOutput [
+    "--bindir=${binDir}"
+  ] ++ optionals (doHaddockInterfaces && isLibrary) [
+    "--ghc-options=-haddock"
+  ];
 
   setupCompileFlags = [
     (optionalString (!coreSetup) "-${nativePackageDbFlag}=$setupPackageConfDir")