Store StackTrace and StackSnapshot in Backtraces

Instead of decoding the stack traces when collecting the `Backtraces`,
defer this decoding until actually showing the `Backtraces`.
This allows users to customise how `Backtraces` are displayed by
using a custom implementation of `displayExceptionWithInfo`, overwriting
the default implementation for `Backtraces` (`displayBacktraces`).

(cherry picked from commit c91e265)

Author: fendor

libraries/ghc-internal/src/GHC/Internal/Exception/Backtrace.hs

@@ -11,9 +11,9 @@ import GHC.Internal.IORef
 import GHC.Internal.IO.Unsafe (unsafePerformIO)
 import GHC.Internal.Exception.Context
 import GHC.Internal.Ptr
+import GHC.Internal.Data.Maybe (fromMaybe)
 import GHC.Internal.Stack.Types as GHC.Stack (CallStack)
 import qualified GHC.Internal.Stack as HCS
-import qualified GHC.Internal.ExecutionStack as ExecStack
 import qualified GHC.Internal.ExecutionStack.Internal as ExecStack
 import qualified GHC.Internal.Stack.CloneStack as CloneStack
 import qualified GHC.Internal.Stack.CCS as CCS
@@ -91,8 +91,8 @@ data Backtraces =
     Backtraces {
         btrCostCentre :: Maybe (Ptr CCS.CostCentreStack),
         btrHasCallStack :: Maybe HCS.CallStack,
-        btrExecutionStack :: Maybe [ExecStack.Location],
-        btrIpe :: Maybe [CloneStack.StackEntry]
+        btrExecutionStack :: Maybe ExecStack.StackTrace,
+        btrIpe :: Maybe CloneStack.StackSnapshot
     }
 
 -- | Render a set of backtraces to a human-readable string.
@@ -109,8 +109,10 @@ displayBacktraces bts = concat
 
     -- The unsafePerformIO here is safe as we don't currently unload cost-centres.
     displayCc   = unlines . map (indent 2) . unsafePerformIO . CCS.ccsToStrings
-    displayExec = unlines . map (indent 2 . flip ExecStack.showLocation "")
-    displayIpe  = unlines . map (indent 2 . CloneStack.prettyStackEntry)
+    displayExec = unlines . map (indent 2 . flip ExecStack.showLocation "") . fromMaybe [] . ExecStack.stackFrames
+    -- The unsafePerformIO here is safe as 'StackSnapshot' makes sure neither the stack frames nor
+    -- references closures can be garbage collected.
+    displayIpe  = unlines . map (indent 2 . CloneStack.prettyStackEntry) . unsafePerformIO . CloneStack.decode
     displayHsc  = unlines . map (indent 2 . prettyCallSite) . HCS.getCallStack
       where prettyCallSite (f, loc) = f ++ ", called at " ++ HCS.prettySrcLoc loc
 
@@ -140,12 +142,11 @@ collectBacktraces' enabled = HCS.withFrozenCallStack $ do
         Just `fmap` CCS.getCurrentCCS ()
 
     exec <- collect ExecutionBacktrace $ do
-        ExecStack.getStackTrace
+        ExecStack.collectStackTrace
 
     ipe <- collect IPEBacktrace $ do
         stack <- CloneStack.cloneMyStack
-        stackEntries <- CloneStack.decode stack
-        return (Just stackEntries)
+        return (Just stack)
 
     hcs <- collect HasCallStackBacktrace $ do
         return (Just ?callStack)