Less Tc inside simplCore (Phase 1 for #14391)

Simplifier depends on typechecker in two points: `thNameToGhcName`
(`lookupThName_maybe`, in particular)  and `lookupGlobal`. We want to
cut the ties in two steps.

1. (Presented in this commit), reimplement both functions in a way that
doesn't use typechecker.

2. (Should follow), do code moving: a) `lookupGlobal` should go in some
typechecker-free place; b) `thNameToGhcName` should leave simplifier,
because it is not used there at all (probably, it should be placed
somewhere where `GhcPlugins` can see it -- this is suggested by Joachim
on Trac).

Details
=======

We redesigned lookup interface a bit so that it exposes some
`IO`-equivalents of `Tc`-features in use.

First, `CoreMonad.hs` still calls `lookupGlobal` which is no longer
bound to the typechecker monad, but still resides in `TcEnv.hs` — it
should be moved out of Tc-land at some point (“Phase 2”) in the
future in order to achieve its part of the #14391's goal.

Second, `lookupThName_maybe` is eliminated from `CoreMonad.hs`
completely; this already achieves its part of the goal of #14391. Its
client, though, `thNameToGhcName`, is better to be moved in the future
also, for it is not used in the `CoreMonad.hs` (or anywhere else)
anyway. Joachim suggested “any module reexported by GhcPlugins (or
maybe even that module itself)”.

As a side goal, we removed `initTcForLookup` which was instrumental for
the past version of `lookupGlobal`. This, in turn, called for pushing
some more parts of the lookup interface from the `Tc`-monad to `IO`,
most notably, adding `IO`-version of `lookupOrig` and pushing
`dataConInfoPtrToName` to `IO`. The `lookupOrig` part, in turn,
triggered a slight redesign of name cache updating interface: we now
have both, `updNameCacheIO` and `updNameCacheTc`, both accepting `mod`
and `occ` to force them inside, instead of more error-prone outside
before. But all these hardly have to do anything with #14391, mere
refactoring.

Reviewers: simonpj, nomeata, bgamari, hvr

Reviewed By: simonpj, bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14391

Differential Revision: https://phabricator.haskell.org/D4503

1 files changed, 6 insertions, 9 deletions

diff --git a/testsuite/tests/ghc-api/T4891/T4891.hs b/testsuite/tests/ghc-api/T4891/T4891.hs
index b2f8cc464d..4aa4842640 100644
--- a/testsuite/tests/ghc-api/T4891/T4891.hs
+++ b/testsuite/tests/ghc-api/T4891/T4891.hs
@@ -54,13 +54,10 @@ chaseConstructor !hv = do
   case tipe closure  of
     Indirection _ -> chaseConstructor (ptrs closure ! 0)
     Constr -> do
-      withSession $ \hscEnv -> liftIO $ initTcForLookup hscEnv $ do
-        eDcname <- dataConInfoPtrToName (infoPtr closure)
-        case eDcname of
-          Left _       -> return ()
-          Right dcName -> do
-            liftIO $ putStrLn $ "Name: "      ++ showPpr dflags dcName
-            liftIO $ putStrLn $ "OccString: " ++ "'" ++ getOccString dcName ++ "'"
-            dc <- tcLookupDataCon dcName
-            liftIO $ putStrLn $ "DataCon: "   ++ showPpr dflags dc
+      withSession $ \hscEnv -> liftIO $ do
+        dcName <- dataConInfoPtrToName hscEnv (infoPtr closure)
+        putStrLn $ "Name: "      ++ showPpr dflags dcName
+        putStrLn $ "OccString: " ++ "'" ++ getOccString dcName ++ "'"
+        dc <- ioLookupDataCon hscEnv dcName
+        putStrLn $ "DataCon: "   ++ showPpr dflags dc
     _ -> return ()