Use RTSStats instead. This will be removed in GHC 8.4.1

[RyanGlScott]

Compiling criterion on GHC 8.2.1 warns of impending doom:

[ 1 of 13] Compiling Criterion.Types  ( Criterion/Types.hs, dist/build/Criterion/Types.o )
[ 2 of 13] Compiling Criterion.Monad.Internal ( Criterion/Monad/Internal.hs, dist/build/Criterion/Monad/Internal.o )
[ 3 of 13] Compiling Criterion.Measurement ( Criterion/Measurement.hs, dist/build/Criterion/Measurement.o )

Criterion/Measurement.hs:51:25: warning: [-Wdeprecations]
    In the use of type constructor or class ‘GCStats’
    (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
   |
51 | getGCStats :: IO (Maybe GCStats)
   |                         ^^^^^^^

Criterion/Measurement.hs:53:16: warning: [-Wdeprecations]
    In the use of ‘getGCStats’ (imported from GHC.Stats):
    Deprecated: "Use getRTSStats instead.  This will be removed in GHC 8.4.1"
   |
53 |   (Just `fmap` Stats.getGCStats) `Exc.catch` \(_::Exc.SomeException) ->
   |                ^^^^^^^^^^^^^^^^

Criterion/Measurement.hs:203:23: warning: [-Wdeprecations]
    In the use of type constructor or class ‘GCStats’
    (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
203 | applyGCStats :: Maybe GCStats
    |                       ^^^^^^^

Criterion/Measurement.hs:205:23: warning: [-Wdeprecations]
    In the use of type constructor or class ‘GCStats’
    (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
205 |              -> Maybe GCStats
    |                       ^^^^^^^

Criterion/Measurement.hs:211:35: warning: [-Wdeprecations]
    In the use of ‘bytesAllocated’ (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
211 |     measAllocated          = diff bytesAllocated
    |                                   ^^^^^^^^^^^^^^

Criterion/Measurement.hs:212:35: warning: [-Wdeprecations]
    In the use of ‘numGcs’ (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
212 |   , measNumGcs             = diff numGcs
    |                                   ^^^^^^

Criterion/Measurement.hs:213:35: warning: [-Wdeprecations]
    In the use of ‘bytesCopied’ (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
213 |   , measBytesCopied        = diff bytesCopied
    |                                   ^^^^^^^^^^^

Criterion/Measurement.hs:214:35: warning: [-Wdeprecations]
    In the use of ‘mutatorWallSeconds’ (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
214 |   , measMutatorWallSeconds = diff mutatorWallSeconds
    |                                   ^^^^^^^^^^^^^^^^^^

Criterion/Measurement.hs:215:35: warning: [-Wdeprecations]
    In the use of ‘mutatorCpuSeconds’ (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
215 |   , measMutatorCpuSeconds  = diff mutatorCpuSeconds
    |                                   ^^^^^^^^^^^^^^^^^

Criterion/Measurement.hs:216:35: warning: [-Wdeprecations]
    In the use of ‘gcWallSeconds’ (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
216 |   , measGcWallSeconds      = diff gcWallSeconds
    |                                   ^^^^^^^^^^^^^

Criterion/Measurement.hs:217:35: warning: [-Wdeprecations]
    In the use of ‘gcCpuSeconds’ (imported from GHC.Stats):
    Deprecated: "Use RTSStats instead.  This will be removed in GHC 8.4.1"
    |
217 |   , measGcCpuSeconds       = diff gcCpuSeconds
    |                                   ^^^^^^^^^^^^
[ 4 of 13] Compiling Criterion.Monad  ( Criterion/Monad.hs, dist/build/Criterion/Monad.o )
[ 5 of 13] Compiling Criterion.IO.Printf ( Criterion/IO/Printf.hs, dist/build/Criterion/IO/Printf.o )
[ 6 of 13] Compiling Criterion.Analysis ( Criterion/Analysis.hs, dist/build/Criterion/Analysis.o )
[ 7 of 13] Compiling Paths_criterion  ( dist/build/autogen/Paths_criterion.hs, dist/build/Paths_criterion.o )
[ 8 of 13] Compiling Criterion.Report ( Criterion/Report.hs, dist/build/Criterion/Report.o )
[ 9 of 13] Compiling Criterion.Main.Options ( Criterion/Main/Options.hs, dist/build/Criterion/Main/Options.o )
[10 of 13] Compiling Criterion.IO     ( Criterion/IO.hs, dist/build/Criterion/IO.o )
[11 of 13] Compiling Criterion.Internal ( Criterion/Internal.hs, dist/build/Criterion/Internal.o )
[12 of 13] Compiling Criterion.Main   ( Criterion/Main.hs, dist/build/Criterion/Main.o )
[13 of 13] Compiling Criterion        ( Criterion.hs, dist/build/Criterion.o )

We should find a way to migrate over to RTSStats on GHC 8.2.1 or later, as the warning suggests.

[RyanGlScott]

One minor complication with this plan is that criterion directly exposes a function getGCStats :: IO GCStats, which is obviously not sustainable given GCStats' planned obsolescence. I believe we should deprecate our getGCStats as well in favor of something like getMeasured :: IO Measured, and make sure that Measured provides a backwards compatible view into RTS stats.