Refactor GHCi UI to fix #11606, #12091, #15721, #16096

Instead of parsing and executing a statement or declaration directly we
now parse them first and then execute in a separate step. This gives us
the flexibility to inspect the parsed declaration before execution.
Using this we now inspect parsed declarations, and if it's a single
declaration of form `x = y` we execute it as `let x = y` instead, fixing
a ton of problems caused by poor declaration support in GHCi.

To avoid any users of the modules I left `execStmt` and `runDecls`
unchanged and added `execStmt'` and `runDecls'` which work on parsed
statements/declarations.

1 files changed, 4 insertions, 15 deletions

diff --git a/compiler/deSugar/Desugar.hs b/compiler/deSugar/Desugar.hs
index aa24ee0a5d..aa9748ee35 100644
--- a/compiler/deSugar/Desugar.hs
+++ b/compiler/deSugar/Desugar.hs
@@ -149,8 +149,7 @@ deSugar hsc_env
           keep_alive <- readIORef keep_var
         ; let (rules_for_locals, rules_for_imps) = partition isLocalRule all_rules
               final_prs = addExportFlagsAndRules target export_set keep_alive
-                                                 mod rules_for_locals
-                                                 (fromOL all_prs)
+                                                 rules_for_locals (fromOL all_prs)
 
               final_pgm = combineEvBinds ds_ev_binds final_prs
         -- Notice that we put the whole lot in a big Rec, even the foreign binds
@@ -284,9 +283,9 @@ deSugarExpr hsc_env tc_expr = do {
 -}
 
 addExportFlagsAndRules
-    :: HscTarget -> NameSet -> NameSet -> Module -> [CoreRule]
+    :: HscTarget -> NameSet -> NameSet -> [CoreRule]
     -> [(Id, t)] -> [(Id, t)]
-addExportFlagsAndRules target exports keep_alive mod rules prs
+addExportFlagsAndRules target exports keep_alive rules prs
   = mapFst add_one prs
   where
     add_one bndr = add_rules name (add_export name bndr)
@@ -319,20 +318,10 @@ addExportFlagsAndRules target exports keep_alive mod rules prs
         -- simplification), and retain them all in the TypeEnv so they are
         -- available from the command line.
         --
-        -- Most of the time, this can be accomplished by use of
-        -- targetRetainsAllBindings, which returns True if the target is
-        -- HscInteractive. However, there are cases when one can use GHCi with
-        -- a target other than HscInteractive (e.g., with the -fobject-code
-        -- flag enabled, as in #12091). In such scenarios,
-        -- targetRetainsAllBindings can return False, so we must fall back on
-        -- isInteractiveModule to be doubly sure we export entities defined in
-        -- a GHCi session.
-        --
         -- isExternalName separates the user-defined top-level names from those
         -- introduced by the type checker.
     is_exported :: Name -> Bool
-    is_exported | targetRetainsAllBindings target
-                  || isInteractiveModule mod      = isExternalName
+    is_exported | targetRetainsAllBindings target = isExternalName
                 | otherwise                       = (`elemNameSet` exports)
 
 {-