Make sure GHC.List.last is memory-efficient

by eta-expanding its definition so that GHC optmizes the foldl here.
Also make sure that other uses of last go via foldl as well, to allow
list fusion (tested in T9339). Fixes #10260.

libraries/base/GHC/List.hs

@@ -84,8 +84,15 @@ last [x]                =  x
 last (_:xs)             =  last xs
 last []                 =  errorEmptyList "last"
 #else
--- use foldl to allow fusion
-last = foldl (\_ x -> x) (errorEmptyList "last")
+-- Use foldl to make last a good consumer.
+-- This will compile to good code for the actual GHC.List.last.
+-- (At least as long it is eta-expaned, otherwise it does not, #10260.)
+last xs = foldl (\_ x -> x) lastError xs
+{-# INLINE last #-}
+-- The inline pragma is required to make GHC remember the implementation via
+-- foldl.
+lastError :: a
+lastError = errorEmptyList "last"
 #endif
 
 -- | Return all the elements of a list except the last one.