Simplify: Take care with eta reduction in recursive RHSs (#21652)

Similar to the fix to #20836 in CorePrep, we now track the set of enclosing
recursive binders in the SimplEnv and SimpleOptEnv.
See Note [Eta reduction in recursive RHSs] for details.

I also updated Note [Arity robustness] with the insights Simon and I had in a
call discussing the issue.

Fixes #21652.

Unfortunately, we get a 5% ghc/alloc regression in T16577. That is due to
additional eta reduction in GHC.Read.choose1 and the resulting ANF-isation
of a large list literal at the top-level that didn't happen before (presumably
because it was too interesting to float to the top-level). There's not much we
can do about that.

Metric Increase:
    T16577

6 files changed, 24 insertions, 13 deletions

diff --git a/testsuite/tests/arityanal/should_compile/Arity03.stderr b/testsuite/tests/arityanal/should_compile/Arity03.stderr
index f41fc1552c..652fcde173 100644
--- a/testsuite/tests/arityanal/should_compile/Arity03.stderr
+++ b/testsuite/tests/arityanal/should_compile/Arity03.stderr
@@ -26,12 +26,7 @@ fac = \ (x :: Int) -> case x of { GHC.Types.I# ww -> case F3.$wfac ww of ww1 { _
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 f3 :: Int -> Int
-[GblId,
- Arity=1,
- Str=<1!P(1L)>,
- Cpr=1,
- Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=True)
-         Tmpl= fac}]
+[GblId, Arity=1, Str=<1!P(1L)>, Cpr=1, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)}]
 f3 = fac
 
 
diff --git a/testsuite/tests/arityanal/should_run/Makefile b/testsuite/tests/arityanal/should_run/Makefile
new file mode 100644
index 0000000000..9101fbd40a
--- /dev/null
+++ b/testsuite/tests/arityanal/should_run/Makefile
@@ -0,0 +1,3 @@
+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
diff --git a/testsuite/tests/arityanal/should_run/T21652.hs b/testsuite/tests/arityanal/should_run/T21652.hs
new file mode 100644
index 0000000000..16fda0aeaa
--- /dev/null
+++ b/testsuite/tests/arityanal/should_run/T21652.hs
@@ -0,0 +1,10 @@
+import GHC.Exts
+
+f, g :: a -> a
+f = g
+g x = f x
+{-# NOINLINE f #-}
+{-# NOINLINE g #-}
+
+-- should print done, not <<loop>>
+main = lazy g `seq` putStrLn "done"
diff --git a/testsuite/tests/arityanal/should_run/T21652.stdout b/testsuite/tests/arityanal/should_run/T21652.stdout
new file mode 100644
index 0000000000..19f86f493a
--- /dev/null
+++ b/testsuite/tests/arityanal/should_run/T21652.stdout
@@ -0,0 +1 @@
+done
diff --git a/testsuite/tests/arityanal/should_run/all.T b/testsuite/tests/arityanal/should_run/all.T
new file mode 100644
index 0000000000..a6b06fbb15
--- /dev/null
+++ b/testsuite/tests/arityanal/should_run/all.T
@@ -0,0 +1,2 @@
+# Regression tests
+test('T21652', [ only_ways(['optasm']) ], compile_and_run, [''])
diff --git a/testsuite/tests/deSugar/should_compile/T19969.stderr b/testsuite/tests/deSugar/should_compile/T19969.stderr
index 5e23785472..3ded6f27a4 100644
--- a/testsuite/tests/deSugar/should_compile/T19969.stderr
+++ b/testsuite/tests/deSugar/should_compile/T19969.stderr
@@ -1,7 +1,7 @@
 
 ==================== Tidy Core ====================
 Result size of Tidy Core
-  = {terms: 12, types: 18, coercions: 0, joins: 0/0}
+  = {terms: 8, types: 14, coercions: 0, joins: 0/0}
 
 Rec {
 -- RHS size: {terms: 3, types: 2, coercions: 0, joins: 0/0}
@@ -10,7 +10,7 @@ f [Occ=LoopBreaker] :: [Int] -> [Int]
 f = \ (x :: [Int]) -> f x
 end Rec }
 
--- RHS size: {terms: 3, types: 2, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 g [InlPrag=INLINE (sat-args=1)] :: [Int] -> [Int]
 [GblId,
  Arity=1,
@@ -19,10 +19,10 @@ g [InlPrag=INLINE (sat-args=1)] :: [Int] -> [Int]
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=False,boring_ok=True)
-         Tmpl= \ (x [Occ=Once1] :: [Int]) -> f x}]
-g = \ (x :: [Int]) -> f x
+         Tmpl= f}]
+g = f
 
--- RHS size: {terms: 3, types: 2, coercions: 0, joins: 0/0}
+-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 h [InlPrag=INLINE (sat-args=1)] :: [Int] -> [Int]
 [GblId,
  Arity=1,
@@ -31,8 +31,8 @@ h [InlPrag=INLINE (sat-args=1)] :: [Int] -> [Int]
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=False,boring_ok=True)
-         Tmpl= \ (x [Occ=Once1] :: [Int]) -> f x}]
-h = \ (x :: [Int]) -> f x
+         Tmpl= f}]
+h = f