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Always bind any free tyvars of the given type, regardless of whether we have a forall at the top.

For pattern type sigs, we do want to bring those type variables into scope, even if there's a forall at the top which usually stops that happening, e.g:

\ (x :: forall a. a -> b) -> e

Here we do bring b into scope.

RULES can also use AlwaysBind, such as in the following example:

{-# RULES \"f\" forall (x :: forall a. a -> b). f x = ... b ... #-}

This only applies to RULES that do not explicitly bind their type variables. If a RULE explicitly quantifies its type variables, then NeverBind is used instead. See also Note [Pattern signature binders and scoping] in GHC.Hs.Type.

Unless there's forall at the top, do the same thing as AlwaysBind. This is only ever used in places where the "forall-or-nothing" rule is in effect. See Note [forall-or-nothing rule].

Never bind any free tyvars. This is used for RULES that have both explicit type and term variable binders, e.g.:

{-# RULES \"const\" forall a. forall (x :: a) y. const x y = x #-}

The presence of the type variable binder forall a. implies that the free variables in the types of the term variable binders x and y are not bound. In the example above, there are no such free variables, but if the user had written (y :: b) instead of y in the term variable binders, then b would be rejected for being out of scope. See also Note [Pattern signature binders and scoping] in GHC.Hs.Type.