Rework the Implicit CallStack solver to handle local lets.

We can't just solve CallStack constraints indiscriminately when they
occur in the RHS of a let-binder. The top-level given CallStack (if
any) will not be in scope, so I've re-worked the CallStack solver as
follows:

1. CallStacks are treated like regular IPs unless one of the following
   two rules apply.

2. In a function call, we push the call-site onto a NEW wanted
   CallStack, which GHC will solve as a regular IP (either directly from a
   given, or by quantifying over it in a local let).

3. If, after the constraint solver is done, any wanted CallStacks
   remain, we default them to the empty CallStack. This rule exists mainly
   to clean up after rule 2 in a top-level binder with no given CallStack.

In rule (2) we have to be careful to emit the new wanted with an
IPOccOrigin instead of an OccurrenceOf origin, so rule (2) doesn't fire
again. This is a bit shady but I've updated the Note to explain the
trick.

Test Plan: validate

Reviewers: simonpj, austin, bgamari, hvr

Reviewed By: simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1422

GHC Trac Issues: #10845

1 files changed, 5 insertions, 1 deletions

diff --git a/libraries/base/GHC/Stack.hs b/libraries/base/GHC/Stack.hs
index f6fe41f06f..8f57239a84 100644
--- a/libraries/base/GHC/Stack.hs
+++ b/libraries/base/GHC/Stack.hs
@@ -23,7 +23,10 @@ module GHC.Stack (
     errorWithStackTrace,
 
     -- * Implicit parameter call stacks
-    SrcLoc(..), CallStack(..),
+    CallStack, getCallStack, pushCallStack, prettyCallStack,
+
+    -- * Source locations
+    SrcLoc(..), prettySrcLoc,
 
     -- * Internals
     CostCentreStack,
@@ -40,6 +43,7 @@ module GHC.Stack (
   ) where
 
 import GHC.Stack.CCS
+import GHC.Stack.Types
 import GHC.IO
 import GHC.Base
 import GHC.List