Adding flags: -ffull-guard-reasoning and too-many-guards

Introduction of two new flags, for more precise control over the new
pattern match checker's behaviour when reasoning about guards. This is
supposed to address #11195 (and maybe more performance bugs related to
the NP-Hardness of coverage checking).

Expected behaviour:

  * When `-ffull-guard-reasoning` is on, run the new pattern match
    checker in its full power

  * When `-ffull-guard-reasoning` is off (the default), for every
    match, check a metric to see whether pattern match checking for it
    has high probability of being non performant (at the the moment we
    check whether the number of guards is over 20 but I would like to
    use a more precise measure in the future). If the probability is
    high:

    - Oversimplify the guards (less expressive but more performant)
      and run the checker, and

    - Issue a warning about the simplification that happened.

A new flag `-Wtoo-many-guards/-Wno-too-many-guards` suppresses the
warning about the simplification (useful when combined with -Werror).

Test Plan: validate

Reviewers: goldfire, austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D1676

GHC Trac Issues: #11195

1 files changed, 13 insertions, 0 deletions

diff --git a/utils/mkUserGuidePart/Options/Warnings.hs b/utils/mkUserGuidePart/Options/Warnings.hs
index e56e04115c..c775af4f6c 100644
--- a/utils/mkUserGuidePart/Options/Warnings.hs
+++ b/utils/mkUserGuidePart/Options/Warnings.hs
@@ -222,6 +222,11 @@ warningsOptions =
          , flagType = DynamicFlag
          , flagReverse = "-Wno-tabs"
          }
+  , flag { flagName = "-Wtoo-many-guards"
+         , flagDescription = "warn when a match has too many guards"
+         , flagType = DynamicFlag
+         , flagReverse = "-Wno-too-many-guards"
+         }
   , flag { flagName = "-Wtype-defaults"
          , flagDescription = "warn when defaulting happens"
          , flagType = DynamicFlag
@@ -364,4 +369,12 @@ warningsOptions =
          , flagType = DynamicFlag
          , flagReverse = "-Wno-deriving-typeable"
          }
+  , flag { flagName = "-ffull-guard-reasoning"
+         , flagDescription =
+           "enable the full reasoning of the pattern match checker "++
+           "concerning guards, for more precise exhaustiveness/coverage "++
+           "warnings"
+         , flagType = DynamicFlag
+         , flagReverse = "-fno-full-guard-reasoning"
+         }
   ]