Add a performance test for #2762

tests/ghc-regress/perf/space_leaks/T2762.hs

+
+module Main (main) where
+
+import T2762A
+
+main :: IO ()
+main = do
+          let content1 = concat (replicate 1000000 "1x") ++ "0"
+          let i1 = fst $ input content1
+          view i1
+
+          let content2 = concat (replicate 1000001 "1y") ++ "0"
+          let i2 = fst $ input content2
+          view i2
+
+view :: [Char] -> IO ()
+view [] = return ()
+view (i : is) = i `seq` view is

tests/ghc-regress/perf/space_leaks/T2762A.hs

+
+module T2762A (input) where
+
+class InputOutput a where
+    input :: String -> (a, String)
+
+instance InputOutput Char where
+    input (x : bs) = (x, bs)
+
+instance InputOutput a => InputOutput [a] where
+    input ('0':bs) = ([], bs)
+    input ('1':bs) = case input bs of
+                     (x, bs') ->
+                         case input bs' of
+                         ~(xs, bs'') -> (x : xs, bs'')

tests/ghc-regress/perf/space_leaks/all.T

@@ -29,3 +29,11 @@ test('T4334',
       omit_ways(['profasm','profthreaded'])
       ],
      compile_and_run, [''])
+
+test('T2762',
+     [# peak_megabytes_allocated is 2 with 7.0.2.
+      # Was 57 with 6.12.3.
+      stats_num_field('peak_megabytes_allocated', 1, 3),
+      only_ways(['normal']),
+      extra_clean(['T2762A.hi', 'T2762A.o'])],
+     compile_and_run, ['-O'])