flambda-backend: Don't substitute into exclaves in simplif.ml (#1448)

Co-authored-by: Mark Shinwell <mshinwell@pm.me>

Author: lukemaurer

lambda/simplif.ml

@@ -492,7 +492,10 @@ let simplify_lets lam =
   | Lregion (l, _) ->
       count bv l
   | Lexclave l ->
-      count bv l
+      (* Not safe in general to move code into an exclave, so block
+         single-use optimizations by treating them the same as lambdas
+         and loops *)
+      count Ident.Map.empty l
 
   and count_default bv sw = match sw.sw_failaction with
   | None -> ()

testsuite/tests/typing-local/exclave.ml

@@ -192,3 +192,22 @@ val bar : 'a -> string = <fun>
 - : string = "Some of 5"
 |}]
 
+(* Ensure that Alias bindings are not substituted by Simplif (PR1448) *)
+type 'a glob = Glob of ('a[@global])
+
+let[@inline never] return_local a = [%local] (Glob a)
+
+let f () =
+  let (Glob x) = return_local 1 in
+  [%exclave]
+    (let (_ : _) = return_local 99 in
+     assert (x = 1))
+;;
+f ();;
+[%%expect{|
+type 'a glob = Glob of global_ 'a
+val return_local : 'a -> local_ 'a glob = <fun>
+val f : unit -> local_ unit = <fun>
+- : unit = ()
+|}]
+