Float-out is not idempotent

Consider

f x = BIG

where BIG is bottoming. In HEAD, the float-out pass decides to float BIG to the top level, producing

lvl x = BIG
f x = lvl x

which is obviously stupid.

There's a whole Note to explain this problem and how we avoid it:

Note [Floating from a RHS]
~~~~~~~~~~~~~~~~~~~~~~~~~~
When float the RHS of a let-binding, we don't always want to apply
lvlMFE to the body of a lambda, as we usually do, because the entire
binding body is already going to the right place (dest_lvl).

A particular example is the top level.  Consider
   concat = /\ a -> foldr ..a.. (++) []
We don't want to float the body of the lambda to get
   lvl    = /\ a -> foldr ..a.. (++) []
   concat = /\ a -> lvl a
That would be stupid.

But I forgot to take account of the possibility that abs_vars might abstract over Ids. Easily fixed.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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