Haskell
If all dependencies to build GHC (with Hadrian) are installed, the Haskell plugin works out of the box. It installs the required Haskell Language Server automatically.
C
I (Sven) got the best results with the clangd plugin. It requires clangd to be installed and a compile_commands.json file to be in place.
compile_commands.json contains all build parameters for all C files. It can easily be generated with bear. You just have to prefix your build command with bear, e.g.
bear -- make [...]bear -- hadrian/build [...]Nix
If you're using ghc.nix, you'll very likely want use it the provide the dependencies for the previously described plugins.
This can easily be done with the Nix Environment Selector plugin.