rts: More const correct-ness fixes

In addition to more const-correctness fixes this patch fixes an
infelicity of the previous const-correctness patch (995cf0f356) which
left `UNTAG_CLOSURE` taking a `const StgClosure` pointer parameter
but returning a non-const pointer. Here we restore the original type
signature of `UNTAG_CLOSURE` and add a new function
`UNTAG_CONST_CLOSURE` which takes and returns a const `StgClosure`
pointer and uses that wherever possible.

Test Plan: Validate on Linux, OS X and Windows

Reviewers: Phyx, hsyl20, bgamari, austin, simonmar, trofi

Reviewed By: simonmar, trofi

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2231

1 files changed, 1 insertions, 1 deletions

diff --git a/rts/STM.c b/rts/STM.c
index 7437491824..9cd0833550 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -353,7 +353,7 @@ static void unlock_inv(StgAtomicInvariant *inv) {
 
 static StgBool watcher_is_tso(StgTVarWatchQueue *q) {
   StgClosure *c = q -> closure;
-  StgInfoTable *info = get_itbl(c);
+  const StgInfoTable *info = get_itbl(c);
   return (info -> type) == TSO;
 }