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Created by Sebastian Graf@sgraf812Developer

Simplifier panics ("Tick in rule") after SpecConstr and CSE due to `postInlineUnconditionally` inside RULE LHS

In !3187 (closed) I rather unexpectedly hit a compiler panic. To reproduce, build a validate-x86_64-linux-deb9-dwarf job (source notes -g and -O2 for SpecConstr are essential) of !3187 (closed). Then you'll eventually get

 ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 8.11.0.20200508:
	Tick in rule
  src<libraries/Cabal/Cabal/Distribution/Simple/GHC/Internal.hs:456:30-58>
  src<libraries/Cabal/Cabal/Distribution/Simple/Flag.hs:95:1-13>
  ds60_sskS
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1218:37 in ghc:GHC.Utils.Outputable
        pprPanic, called at compiler/GHC/Core/Rules.hs:723:5 in ghc:GHC.Core.Rules
Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

while compiling libraries/Cabal/Cabal/Distribution/Simple/GHC.hs. I couldn't come up with a smaller reproducer yet. But I managed to pin-point the faulty behavior to a flaw in a rather painful debugging process to postInlineUnconditionally/occurrence analysis.

TLDR; As far as I can tell we need something like the occ_rules_only taint from weakLoopBreaker for OccMany/basically all other OccInfo fields to keep postInlineUnconditionally from triggering when a varaible occurs in an LHS of a RULE.

Edited by Sebastian Graf
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