Add special case for :Main module in `GHC.IfaceToCore.mk_top_id`

See Note [Root-main Id]

The `:Main` special binding is actually defined in the current module
(hence don't go looking for it externally) but the module name is rOOT_MAIN
rather than the current module so we need this special case.

There was already some similar logic in `GHC.Rename.Env` for
External Core, but now the "External Core" is in interface files it
needs to be moved here instead.

Fixes #22405

3 files changed, 25 insertions, 1 deletions

diff --git a/compiler/GHC/IfaceToCore.hs b/compiler/GHC/IfaceToCore.hs
index a2d63bb779..f4bb4057c9 100644
--- a/compiler/GHC/IfaceToCore.hs
+++ b/compiler/GHC/IfaceToCore.hs
@@ -123,6 +123,7 @@ import GHC.Driver.Env.KnotVars
 import GHC.Unit.Module.WholeCoreBindings
 import Data.IORef
 import Data.Foldable
+import GHC.Builtin.Names (ioTyConName, rOOT_MAIN)
 
 {-
 This module takes
@@ -930,7 +931,17 @@ tc_iface_binding i IfUseUnfoldingRhs = return (unfoldingTemplate $ realIdUnfoldi
 tc_iface_binding _ (IfRhs rhs) = tcIfaceExpr rhs
 
 mk_top_id :: IfaceTopBndrInfo -> IfL Id
-mk_top_id (IfGblTopBndr gbl_name) = tcIfaceExtId gbl_name
+mk_top_id (IfGblTopBndr gbl_name)
+  -- See Note [Root-main Id]
+  -- This special binding is actually defined in the current module
+  -- (hence don't go looking for it externally) but the module name is rOOT_MAIN
+  -- rather than the current module so we need this special case.
+  -- See some similar logic in `GHC.Rename.Env`.
+  | Just rOOT_MAIN == nameModule_maybe gbl_name
+    = do
+        ATyCon ioTyCon <- tcIfaceGlobal ioTyConName
+        return $ mkExportedVanillaId gbl_name (mkTyConApp ioTyCon [unitTy])
+  | otherwise = tcIfaceExtId gbl_name
 mk_top_id (IfLclTopBndr raw_name iface_type info details) = do
    name <- newIfaceName (mkVarOccFS raw_name)
    ty <- tcIfaceType iface_type
diff --git a/compiler/GHC/Rename/Env.hs b/compiler/GHC/Rename/Env.hs
index 3d3ded48f0..90c9f38faf 100644
--- a/compiler/GHC/Rename/Env.hs
+++ b/compiler/GHC/Rename/Env.hs
@@ -207,6 +207,11 @@ newTopSrcBinder (L loc rdr_name)
         -- the nice Exact name for the TyCon gets swizzled to an Orig name.
         -- Hence the badOrigBinding error message.
         --
+
+        -- MP 2022: I suspect this code path is never called for `rOOT_MAIN` anymore
+        -- because External Core has been removed but we instead have some similar logic for
+        -- serialising whole programs into interface files in GHC.IfaceToCore.mk_top_id.
+
         -- Except for the ":Main.main = ..." definition inserted into
         -- the Main module; ugh!
 
diff --git a/compiler/GHC/Tc/Module.hs b/compiler/GHC/Tc/Module.hs
index 76c5f0fb66..24a5d07090 100644
--- a/compiler/GHC/Tc/Module.hs
+++ b/compiler/GHC/Tc/Module.hs
@@ -2047,6 +2047,14 @@ This is unusual: it's a LocalId whose Name has a Module from another
 module. Tiresomely, we must filter it out again in GHC.Iface.Make, less we
 get two defns for 'main' in the interface file!
 
+When using `-fwrite-if-simplified-core` the root_main_id can end up in an interface file.
+When the interface is read back in we have to add a special case when creating the
+Id because otherwise we would go looking for the :Main module which obviously doesn't
+exist. For this logic see GHC.IfaceToCore.mk_top_id.
+
+There is also some similar (probably dead) logic in GHC.Rename.Env which says it
+was added for External Core which faced a similar issue.
+
 
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