The GHC API doesn't provide a good hscTarget option for tooling

Tools like ghc-mod typically just want TypecheckedModules. Sadly, the GHC API currently doesn't provide a good way to get at these in all cases (see this ghc-mod ticket). Each of the options we offer are cursed with their own limitations (largely quoting from the ghc-mod ticket),

HscNothing

At first glance this looks like what you would want. But...

• Pros

  * Doesn't generate code of any sort and is therefore rather lightweight

• Cons

  * It lacks support for Template Haskell

  * Has trouble with foreign exports

  * Fails to issue pattern match checker warnings (#10600 (closed))

HscInterpreted

Okay, so HscNothing doesn't work. Maybe HscInterpreted is better?

• Pros

  * Supports Template Haskell

• Cons

  * Can't deal with unboxed tuples (#1257 (closed))

  * Slower as we need to produce unnecessary bytecode

  * Large memory footprint

  * Also can't deal with foreign exports

HscAsm

• Pros

  * Supports all compilable code

• Cons

  * Slow

  * Produces .o files

This is quite unfortunate. It seems like we need something in between HscNothing and HscInterpreted which is willing to use the interpreter to evaluate Template Haskell splices when necessary, but doesn't produce bytecode. Unfortunately it's unclear what to do about foreign export (but arguably most tools would be fine with some approximate representation).