Rodrigo Mesquita pushed to branch wip/romes/fix-pm-faststring at Glasgow Haskell Compiler / GHC

Commits:

• 1d04353f
  by Rodrigo Mesquita at 2026-06-17T15:58:38+01:00

  fix: Save FastStrings in the PMC
  
  There is no point in adding the unique to the occurrence FastString we
  create, since it is part of the Id anyway.
  
  Adding it to the FastString, meant each FastString was unique
  unnecessarily!
  
  In a separate branch, running the compiler on test `InstanceMatching`
  observed 30000 `FastString`s created by this code path.
  
  Plus, `fsLit "pm"` follows the existing pattern in `mkPmId`.
  

1 changed file:

• compiler/GHC/HsToCore/Pmc/Solver.hs

Changes:

          -- return the new inert set and increment the sequence number n

        ; return (TySt (n+1) <$> mb_new_inert) }

--- | Allocates a fresh 'EvVar' name for 'PredTy's.

+-- | Allocates a fresh 'EvVar' name for 'PredTy's

+-- (just like 'mkPmId').

 nameTyCt :: PredType -> DsM EvVar

 nameTyCt pred_ty = do

   unique <- getUniqueM

-  let occname = mkVarOccFS (fsLit ("pm_"++show unique))

+  let occname = mkVarOccFS $ fsLit "pm"

   return (mkUserLocalOrCoVar occname unique ManyTy pred_ty noSrcSpan)

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