Extract definition of DsM into GHC.HsToCore.Types

`DsM` was previously defined in `GHC.Tc.Types`, along with `TcM`. But
`GHC.Tc.Types` is in the set of transitive dependencies of `GHC.Parser`,
a set which we aim to minimise. Test case `CountParserDeps` checks for
that.

Having `DsM` in that set means the parser also depends on the innards of
the pattern-match checker in `GHC.HsToCore.PmCheck.Types`, which is the
reason we have that module in the first place.

In the previous commit, we represented the `TyState` by an `InertSet`,
but that pulls the constraint solver as well as 250 more modules into
the set of dependencies, triggering failure of `CountParserDeps`.
Clearly, we want to evolve the pattern-match checker (and the desugarer)
without being concerned by this test, so this patch includes a small
refactor that puts `DsM` into its own module.

1 files changed, 77 insertions, 0 deletions

diff --git a/compiler/GHC/HsToCore/Types.hs b/compiler/GHC/HsToCore/Types.hs
new file mode 100644
index 0000000000..26f6a6af79
--- /dev/null
+++ b/compiler/GHC/HsToCore/Types.hs
@@ -0,0 +1,77 @@
+-- | Various types used during desugaring.
+module GHC.HsToCore.Types (
+        DsM, DsLclEnv(..), DsGblEnv(..),
+        DsMetaEnv, DsMetaVal(..), CompleteMatches
+    ) where
+
+import Data.IORef
+
+import GHC.Types.CostCentre.State
+import GHC.Types.Name.Env
+import GHC.Types.SrcLoc
+import GHC.Types.Var
+import GHC.Hs (HsExpr, GhcTc)
+import GHC.Tc.Types (TcRnIf, IfGblEnv, IfLclEnv, CompleteMatches)
+import {-# SOURCE #-} GHC.HsToCore.PmCheck.Types (Nablas)
+import GHC.Core.FamInstEnv
+import GHC.Utils.Error
+import GHC.Utils.Outputable as Outputable
+import GHC.Unit.Module
+
+{-
+************************************************************************
+*                                                                      *
+                Desugarer monad
+*                                                                      *
+************************************************************************
+
+Now the mondo monad magic (yes, @DsM@ is a silly name)---carry around
+a @UniqueSupply@ and some annotations, which
+presumably include source-file location information:
+-}
+
+-- | Global read-only context and state of the desugarer.
+-- The statefulness is implemented through 'IORef's.
+data DsGblEnv
+  = DsGblEnv
+  { ds_mod          :: Module             -- For SCC profiling
+  , ds_fam_inst_env :: FamInstEnv         -- Like tcg_fam_inst_env
+  , ds_unqual  :: PrintUnqualified
+  , ds_msgs    :: IORef Messages          -- Warning messages
+  , ds_if_env  :: (IfGblEnv, IfLclEnv)    -- Used for looking up global,
+                                          -- possibly-imported things
+  , ds_complete_matches :: CompleteMatches
+     -- Additional complete pattern matches
+  , ds_cc_st   :: IORef CostCentreState
+     -- Tracking indices for cost centre annotations
+  }
+
+instance ContainsModule DsGblEnv where
+  extractModule = ds_mod
+
+-- | Local state of the desugarer, extended as we lexically descend
+data DsLclEnv
+  = DsLclEnv
+  { dsl_meta    :: DsMetaEnv   -- ^ Template Haskell bindings
+  , dsl_loc     :: RealSrcSpan -- ^ To put in pattern-matching error msgs
+  , dsl_nablas  :: Nablas
+  -- ^ See Note [Note [Long-distance information] in "GHC.HsToCore.PmCheck".
+  -- The set of reaching values Nablas is augmented as we walk inwards, refined
+  -- through each pattern match in turn
+  }
+
+-- Inside [| |] brackets, the desugarer looks
+-- up variables in the DsMetaEnv
+type DsMetaEnv = NameEnv DsMetaVal
+
+data DsMetaVal
+  = DsBound Id         -- Bound by a pattern inside the [| |].
+                       -- Will be dynamically alpha renamed.
+                       -- The Id has type THSyntax.Var
+
+  | DsSplice (HsExpr GhcTc) -- These bindings are introduced by
+                            -- the PendingSplices on a HsBracketOut
+
+-- | Desugaring monad. See also 'TcM'.
+type DsM = TcRnIf DsGblEnv DsLclEnv
+