No join-point from an INLINE function with wrong arity

The main payload of this patch is NOT to make a join-point
from a function with an INLINE pragma and the wrong arity;
see Note [Join points and INLINE pragmas] in CoreOpt.
This is what caused Trac #13413.

But we must do the exact same thing in simpleOptExpr,
which drove me to the following refactoring:

* Move simpleOptExpr and simpleOptPgm from CoreSubst to a new
  module CoreOpt along with a few others (exprIsConApp_maybe,
  pushCoArg, etc)

  This eliminates a module loop altogether (delete
  CoreArity.hs-boot), and stops CoreSubst getting too huge.

* Rename Simplify.matchOrConvertToJoinPoint
     to joinPointBinding_maybe
  Move it to the new CoreOpt
  Use it in simpleOptExpr as well as in Simplify

* Define CoreArity.joinRhsArity and use it

1 files changed, 5 insertions, 0 deletions

diff --git a/compiler/coreSyn/CoreSyn.hs b/compiler/coreSyn/CoreSyn.hs
index ad504ac1b9..6762ed6fb2 100644
--- a/compiler/coreSyn/CoreSyn.hs
+++ b/compiler/coreSyn/CoreSyn.hs
@@ -589,6 +589,11 @@ Join points must follow these invariants:
      "join arity" (to distinguish from regular arity, which only counts values).
 
   2. For join arity n, the right-hand side must begin with at least n lambdas.
+     No ticks, no casts, just lambdas!  C.f. CoreUtils.joinRhsArity.
+
+  2a. Moreover, this same constraint applies to any unfolding of the binder.
+     Reason: if we want to push a continuation into the RHS we must push it
+     into the unfolding as well.
 
   3. If the binding is recursive, then all other bindings in the recursive group
      must also be join points.