Remove "Ord FastString" instance

FastStrings can be compared in 2 ways: by Unique or lexically. We don't
want to bless one particular way with an "Ord" instance because it leads
to bugs (#18562) or to suboptimal code (e.g. using lexical comparison
while a Unique comparison would suffice).

UTF-8 encoding has the advantage that sorting strings by their encoded
bytes also sorts them by their Unicode code points, without having to
decode the actual code points. BUT GHC uses Modified UTF-8 which
diverges from UTF-8 by encoding \0 as 0xC080 instead of 0x00 (to avoid
null bytes in the middle of a String so that the string can still be
null-terminated). This patch adds a new `utf8CompareShortByteString`
function that performs sorting by bytes but that also takes Modified
UTF-8 into account. It is much more performant than decoding the strings
into [Char] to perform comparisons (which we did in the previous patch).

Bump haddock submodule

1 files changed, 5 insertions, 0 deletions

diff --git a/compiler/GHC/Utils/Binary.hs b/compiler/GHC/Utils/Binary.hs
index d73939c53c..d4afa624cf 100644
--- a/compiler/GHC/Utils/Binary.hs
+++ b/compiler/GHC/Utils/Binary.hs
@@ -5,6 +5,8 @@
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE MultiWayIf #-}
 {-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
 {-# OPTIONS_GHC -O2 -funbox-strict-fields #-}
 -- We always optimise this, otherwise performance of a non-optimised
@@ -1207,6 +1209,9 @@ instance Binary FastString where
     case getUserData bh of
         UserData { ud_get_fs = get_fs } -> get_fs bh
 
+deriving instance Binary NonDetFastString
+deriving instance Binary LexicalFastString
+
 -- Here to avoid loop
 instance Binary LeftOrRight where
    put_ bh CLeft  = putByte bh 0