+Note [Do not specialise diverging functions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Specialising a function that just diverges is a waste of code.
+Furthermore, it broke GHC (simpl014) thus:
+ {-# STR Sb #-}
+ f = \x. case x of (a,b) -> f x
+If we specialise f we get
+ f = \x. case x of (a,b) -> fspec a b
+But fspec doesn't have decent strictnes info. As it happened,
+(f x) :: IO t, so the state hack applied and we eta expanded fspec,
+and hence f. But now f's strictness is less than its arity, which
+breaks an invariant.
+
+Note [Forcing specialisation]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+With stream fusion and in other similar cases, we want to fully specialise
+some (but not necessarily all!) loops regardless of their size and the
+number of specialisations. We allow a library to specify this by annotating
+a type with ForceSpecConstr and then adding a parameter of that type to the
+loop. Here is a (simplified) example from the vector library:
+
+ data SPEC = SPEC | SPEC2
+ {-# ANN type SPEC ForceSpecConstr #-}
+
+ foldl :: (a -> b -> a) -> a -> Stream b -> a
+ {-# INLINE foldl #-}
+ foldl f z (Stream step s _) = foldl_loop SPEC z s
+ where
+ foldl_loop SPEC z s = case step s of
+ Yield x s' -> foldl_loop SPEC (f z x) s'
+ Skip -> foldl_loop SPEC z s'
+ Done -> z
+
+SpecConstr will spot the SPEC parameter and always fully specialise
+foldl_loop. Note that we can't just annotate foldl_loop since it isn't a
+top-level function but even if we could, inlining etc. could easily drop the
+annotation. We also have to prevent the SPEC argument from being removed by
+w/w which is why SPEC is a sum type. This is all quite ugly; we ought to come
+up with a better design.
+
+ForceSpecConstr arguments are spotted in scExpr' and scTopBinds which then set
+force_spec to True when calling specLoop. This flag makes specLoop and
+specialise ignore specConstrCount and specConstrThreshold when deciding
+whether to specialise a function.
+