Grotesque hack on Unsafe.Coerce

The hack is described in detail in Note [Meta-hack for coerce].
It's a consequence of this, perfectly legitimate, commit:

  commit 7a1480c7c590d4d2fa7a105a4eebf299e921e056
  Author: Simon Peyton Jones <simonpj@microsoft.com>
  Date:   Tue Jan 22 22:43:22 2013 +0000

      Allow eta-reduction of eval'd functions if of arity 1

      See Note [Eta reduction of an eval'd function] in CoreUtils.
      This doesn't fix Trac #7542, but that was the ticket that
      pointed out this infelicity.

   compiler/coreSyn/CoreUtils.lhs |   24 ++++++++++++++++++++++--

1 files changed, 24 insertions, 1 deletions

diff --git a/libraries/base/Unsafe/Coerce.hs b/libraries/base/Unsafe/Coerce.hs
index 50f52edc4e..a802ba319b 100644
--- a/libraries/base/Unsafe/Coerce.hs
+++ b/libraries/base/Unsafe/Coerce.hs
@@ -31,10 +31,33 @@
 
 module Unsafe.Coerce (unsafeCoerce) where
 
+
 #if defined(__GLASGOW_HASKELL__)
 import GHC.Prim (unsafeCoerce#)
+
+local_id :: a -> a
+local_id x = x   -- See Note [Mega-hack for coerce]
+
+{- Note [Meta-hack for coerce]
+
+If we just say
+  unsafeCoerce x = unsafeCoercs# x
+then the simple-optimiser that the desugarer runs will eta-reduce to
+  unsafeCoerce :: forall (a:*) (b:*). a -> b
+  unsafeCoercs = unsafeCoerce#
+And that, sadly, is ill-typed because unsafeCoercs# has OpenKind type variables
+And rightly so, becuase we shouldn't be calling unsafeCoerce# in a higher
+order way; it has a compulsory unfolding 
+   unsafeCoerce# a b x = x |> UnsafeCo a b
+and we really rely on it being inlined pronto. But the simple-optimiser doesn't.
+The identity function local_id delays the eta reduction just long enough
+for unsafeCoerce# to get inlined.
+
+Sigh. This is horrible, but then so is unsafeCoerce.
+-}
+
 unsafeCoerce :: a -> b
-unsafeCoerce x = unsafeCoerce# x
+unsafeCoerce x = local_id (unsafeCoerce# x)
   -- See Note [Unsafe coerce magic] in basicTypes/MkId
   -- NB: Do not eta-reduce this definition, else the type checker 
   -- give usafeCoerce the same (dangerous) type as unsafeCoerce#