[GHC] #14123: Figure out invariants surrounding ticks in Core

[GHC]

#14123: Figure out invariants surrounding ticks in Core
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  highest        |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In the past we have had a number of issues stemming from ticks breaking
 various Core analyses in GHC,

  * #13233: a tick separates a levity-polymorphic tycon (e.g. an unboxed
 tuple tycon) from its levity arguments, resulting in a `typePrimRep`
 panic.
  * ticket:8472#comment:22: a tick wraps a primitive string literal,
 causing `CoreToStg` to fail to notice it is a string resulting in a panic
  * #14122: Another literal string issue which I have yet to debug but
 seems very likely to be tick-related.

 We have merged a stop-gap solution/hack
 (f5b275a239d2554c4da0b7621211642bf3b10650) to the `ghc-8.2` branch however
 need a more principled solution in the long-term (e.g. before 8.4). We
 will need to start by defining precisely where ticks are allowed and add
 appropriate logic to CoreLint to verify that this invariant is upheld.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14123>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler