Kill some unnecessary varSetElems

When you do `varSetElems (tyCoVarsOfType x)` it's equivalent to
`tyCoVarsOfTypeList x`.

Why? If you look at the implementation:
```
tyCoVarsOfTypeList ty = runFVList $ tyCoVarsOfTypeAcc ty
tyCoVarsOfType ty = runFVSet $ tyCoVarsOfTypeAcc ty
```
they use the same helper function. The helper function returns a
deterministically ordered list and a set. The only difference
between the two is which part of the result they take. It is redundant
to take the set and then immediately convert it to a list.

This helps with determinism and we eventually want to replace the uses
of `varSetElems` with functions that don't leak the values of uniques.
This change gets rid of some instances that are easy to kill.

I chose not to annotate every place where I got rid of `varSetElems`
with a comment about non-determinism, because once we get rid of
`varSetElems` it will not be possible to do the wrong thing.

Test Plan: ./validate

Reviewers: goldfire, austin, simonmar, bgamari, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2115

GHC Trac Issues: #4012

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/specialise/Rules.hs b/compiler/specialise/Rules.hs
index 24b52d3db7..3adad1c83e 100644
--- a/compiler/specialise/Rules.hs
+++ b/compiler/specialise/Rules.hs
@@ -33,7 +33,7 @@ import Module           ( Module, ModuleSet, elemModuleSet )
 import CoreSubst
 import OccurAnal        ( occurAnalyseExpr )
 import CoreFVs          ( exprFreeVars, exprsFreeVars, bindFreeVars
-                        , rulesFreeVarsDSet, exprsOrphNames )
+                        , rulesFreeVarsDSet, exprsOrphNames, exprFreeVarsList )
 import CoreUtils        ( exprType, eqExpr, mkTick, mkTicks,
                           stripTicksTopT, stripTicksTopE )
 import PprCore          ( pprRules )
@@ -898,7 +898,7 @@ match_tmpl_var :: RuleMatchEnv
 match_tmpl_var renv@(RV { rv_lcl = rn_env, rv_fltR = flt_env })
                subst@(RS { rs_id_subst = id_subst, rs_bndrs = let_bndrs })
                v1' e2
-  | any (inRnEnvR rn_env) (varSetElems (exprFreeVars e2))
+  | any (inRnEnvR rn_env) (exprFreeVarsList e2)
   = Nothing     -- Occurs check failure
                 -- e.g. match forall a. (\x-> a x) against (\y. y y)