Note [Core letrec invariant]

The Core letrec invariant:

  The right hand sides of all /top-level/ or /recursive/
  bindings must be of lifted type

See "Type#type_classification" in GHC.Core.Type
for the meaning of "lifted" vs. "unlifted".

For the non-top-level, non-recursive case see
Note [Core let-can-float invariant].

At top level, however, there are two exceptions to this rule:

(TL1) A top-level binding is allowed to bind primitive string literal,
      (which is unlifted).  See Note [Core top-level string literals].

(TL2) In Core, we generate a top-level binding for every non-newtype data
constructor worker or wrapper
      e.g.   data T = MkT Int
      we generate
             MkT :: Int -> T
             MkT = \x. MkT x
      (This binding looks recursive, but isn't; it defines a top-level, curried
      function whose body just allocates and returns the data constructor.)

      But if (a) the data constructor is nullary and (b) the data type is unlifted,
      this binding is unlifted.
      e.g.   data S :: UnliftedType where { S1 :: S, S2 :: S -> S }
      we generate
             S1 :: S   -- A top-level unlifted binding
             S1 = S1
      We allow this top-level unlifted binding to exist.