Add -ddump-ds-preopt

This allows you to see the output immediately after desugaring
but before any optimisation.

I've wanted this for some time, but I was triggered into action
by Trac #13032 comment:9.

Interestingly, the change means that with -dcore-lint we will
now Lint the output before the very simple optimiser;
and this showed up Trac #14749.  But that's not the fault
of -ddump-ds-preopt!

5 files changed, 13 insertions, 8 deletions

diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
index e866f0d3ec..e83f8392d2 100644
--- a/compiler/coreSyn/CoreLint.hs
+++ b/compiler/coreSyn/CoreLint.hs
@@ -275,7 +275,7 @@ coreDumpFlag CoreDoSpecialising       = Just Opt_D_dump_spec
 coreDumpFlag CoreDoSpecConstr         = Just Opt_D_dump_spec
 coreDumpFlag CoreCSE                  = Just Opt_D_dump_cse
 coreDumpFlag CoreDoVectorisation      = Just Opt_D_dump_vect
-coreDumpFlag CoreDesugar              = Just Opt_D_dump_ds
+coreDumpFlag CoreDesugar              = Just Opt_D_dump_ds_preopt
 coreDumpFlag CoreDesugarOpt           = Just Opt_D_dump_ds
 coreDumpFlag CoreTidy                 = Just Opt_D_dump_simpl
 coreDumpFlag CorePrep                 = Just Opt_D_dump_prep
diff --git a/compiler/deSugar/Desugar.hs b/compiler/deSugar/Desugar.hs
index 518f02f1a0..05d322680c 100644
--- a/compiler/deSugar/Desugar.hs
+++ b/compiler/deSugar/Desugar.hs
@@ -160,10 +160,7 @@ deSugar hsc_env
         -- You might think it doesn't matter, but the simplifier brings all top-level
         -- things into the in-scope set before simplifying; so we get no unfolding for F#!
 
-#if defined(DEBUG)
-          -- Debug only as pre-simple-optimisation program may be really big
         ; endPassIO hsc_env print_unqual CoreDesugar final_pgm rules_for_imps
-#endif
         ; (ds_binds, ds_rules_for_imps, ds_vects)
             <- simpleOptPgm dflags mod final_pgm rules_for_imps vects0
                          -- The simpleOptPgm gets rid of type
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index cf889ecd72..6cea932bf5 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -347,6 +347,7 @@ data DumpFlag
    | Opt_D_dump_core_stats
    | Opt_D_dump_deriv
    | Opt_D_dump_ds
+   | Opt_D_dump_ds_preopt
    | Opt_D_dump_foreign
    | Opt_D_dump_inlinings
    | Opt_D_dump_rule_firings
@@ -3058,6 +3059,8 @@ dynamic_flags_deps = [
         (setDumpFlag Opt_D_dump_deriv)
   , make_ord_flag defGhcFlag "ddump-ds"
         (setDumpFlag Opt_D_dump_ds)
+  , make_ord_flag defGhcFlag "ddump-ds-preopt"
+        (setDumpFlag Opt_D_dump_ds_preopt)
   , make_ord_flag defGhcFlag "ddump-foreign"
         (setDumpFlag Opt_D_dump_foreign)
   , make_ord_flag defGhcFlag "ddump-inlinings"
diff --git a/docs/users_guide/debugging.rst b/docs/users_guide/debugging.rst
index 399791925c..48222ae467 100644
--- a/docs/users_guide/debugging.rst
+++ b/docs/users_guide/debugging.rst
@@ -212,11 +212,16 @@ subexpression elimination pass.
     Print a one-line summary of the size of the Core program at the end
     of the optimisation pipeline.
 
-.. ghc-flag:: -ddump-ds
-    :shortdesc: Dump desugarer output
+.. ghc-flag:: -ddump-ds -ddump-ds-preopt
+    :shortdesc: Dump desugarer output.
     :type: dynamic
 
-    Dump desugarer output
+    Dump desugarer output. `-ddump-ds` dumps the output after the very simple
+    optimiser has run (which discards a lot of clutter and
+    hence is a sensible default.  `-ddump-ds-preopt` shows
+    the output after desugaring but before the very simple
+    optimiser.
+
 
 .. ghc-flag:: -ddump-simpl-iterations
     :shortdesc: Dump output from each simplifier iteration
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 795e1730a9..49683b7db4 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -566,7 +566,7 @@ test('T13651a', normal, compile, [''])
 test('T13680', normal, compile, [''])
 test('T13785', normal, compile, [''])
 test('T13804', normal, compile, [''])
-test('T13822', normal, compile, [''])
+test('T13822', expect_broken(14749), compile, [''])
 test('T13848', normal, compile, [''])
 test('T13871', normal, compile, [''])
 test('T13879', normal, compile, [''])