Add inlay hints plugin

[jetjinser]

inlay hints usages: #2938.

task list

misc:

• tests
• detail config or version option (still updating)

feat:

• fixity
• local (let) binding
• implicit imports list
• lambda arguments
• function arguments
• do block binding
• record names in positional record construction
• holes ^[1]

[1]: It is currently impossible to find holes deterministically in HieASTs, so the current implementation is actually "type with an Unbound Var". There are two solutions I can think of: 1) modify GHC; 2) regex diagnostic.

config

The plugin configuration looks like this (neovim/lua):

settings = {
  haskell = {
    formattingProvider = "stylish-haskell",
    plugin = {
      ["inlay-hints"] = {
        config = {
          -- All inlay hints are disabled default

          -- enable all inlay hints
          all = true,

          -- or enable individually
          fixity = true,
          localBinding = true,
          -- hole = true,
        },
      },
    },
  },
},

The usages of inlay hints is still under public discussion

preview

fixity

fixity + local binding (`let`)

fixity + local binding (`let`) + hole type

[soulomoon]

Looking good.
We have an inlay hints GSoC candidate @michaelpj

[michaelpj]

This looks like a good proof-of-concept!

We'll definitely want to make this configurable - I suspect most people won't want fixities everywhere in their editor