Use custom build and config hook.

  This is necessary and useful for two reasons:

  1. It removes the need of pre-building the Rust library beforehand. It
     is now done as part of the Haskell build itself.

  2. It works around Cabal's limitation of not begin able to refer to an
     extra C library using a relative path. See also: (haskell/cabal#2641)

cddl/Setup.hs

@@ -0,0 +1,70 @@
+import Prelude
+
+import Data.Maybe
+  ( fromJust
+  )
+import Distribution.Simple
+  ( Args
+  , UserHooks (..)
+  , defaultMainWithHooks
+  , simpleUserHooks
+  )
+import Distribution.Simple.LocalBuildInfo
+  ( LocalBuildInfo (..),
+  )
+import Distribution.Simple.Setup
+  ( BuildFlags (..)
+  , ConfigFlags (..)
+  , fromFlag
+  )
+import Distribution.Simple.UserHooks
+  ( UserHooks (..)
+  )
+import Distribution.Simple.Utils
+  ( rawSystemExit
+  )
+import System.Directory
+  ( getCurrentDirectory
+  )
+
+import qualified Distribution.PackageDescription as Pkg
+
+main :: IO ()
+main =
+  defaultMainWithHooks
+    simpleUserHooks
+      { confHook = rustConfHook
+      , buildHook = rustBuildHook
+      }
+
+rustConfHook
+  :: (Pkg.GenericPackageDescription, Pkg.HookedBuildInfo)
+  -> ConfigFlags
+  -> IO LocalBuildInfo
+rustConfHook (description, buildInfo) flags = do
+  localBuildInfo <- confHook simpleUserHooks (description, buildInfo) flags
+  let packageDescription = localPkgDescr localBuildInfo
+  let library = fromJust $ Pkg.library packageDescription
+  let libraryBuildInfo = Pkg.libBuildInfo library
+  dir <- getCurrentDirectory
+  return localBuildInfo
+    { localPkgDescr = packageDescription
+      { Pkg.library = Just library
+        { Pkg.libBuildInfo = libraryBuildInfo
+          { Pkg.extraLibDirs = (dir ++ "/target/release") : Pkg.extraLibDirs libraryBuildInfo
+          }
+        }
+      }
+    }
+
+rustBuildHook
+  :: Pkg.PackageDescription
+  -> LocalBuildInfo
+  -> UserHooks
+  -> BuildFlags
+  -> IO ()
+rustBuildHook description localBuildInfo hooks flags = do
+  putStrLn "[🦀] Compiling Rust dependencies..."
+  putStrLn "[🦀] cargo build --release"
+  rawSystemExit (fromFlag $ buildVerbosity flags) "cargo" ["build", "--release"]
+  buildHook simpleUserHooks description localBuildInfo hooks flags

cddl/cddl.cabal

@@ -10,8 +10,14 @@ author:                 KtorZ <matthias.benkort@cardanofoundation.org>
 maintainer:             matthias.benkort@cardanofoundation.org
 license:                Apache-2.0
 license-files:          LICENSE
-build-type:             Simple
-extra-source-files:     cbits/libcddl.h
+build-type:             Custom
+extra-source-files:     Setup.hs
+                        cbits/libcddl.h
+
+custom-setup
+  setup-depends:        , base
+                        , directory
+                        , Cabal
 
 common project-config
   default-language:     Haskell2010
@@ -46,7 +52,6 @@ test-suite unit
                         Paths_cddl
   build-depends:        , base
                         , base16-bytestring
-                        , bytestring
                         , cddl
                         , hspec
                         , text