Add optimisation to skip variable aliasings

This particularly of effect to the code introduced by closure conversion.

compiler/shared_ast/optimizations.ml

@@ -207,8 +207,9 @@ let rec optimize_expr :
       in
       EMatch { e = arg; cases; name = n1 }
     | EApp { f = EAbs { binder; _ }, _; args }
-      when binder_vars_used_at_most_once binder ->
-      (* beta reduction when variables not used. *)
+      when binder_vars_used_at_most_once binder
+           || List.for_all (function EVar _, _ -> true | _ -> false) args ->
+      (* beta reduction when variables not used, and for variable aliases *)
       Mark.remove (Bindlib.msubst binder (List.map fst args |> Array.of_list))
     | EStructAccess { name; field; e = EStruct { name = name1; fields }, _ }
       when StructName.equal name name1 ->

tests/test_func/good/closure_conversion.catala_en

@@ -64,13 +64,7 @@ let scope S (S_in: S_in {x_in: bool}): S {z: integer} =
       (handle_default_opt
          []
          (λ (_: unit) → true)
-         (λ (_: unit) →
-          ESome
-            (let code_and_env :
-                 ((closure_env, integer) → integer * closure_env) =
-               f
-             in
-             code_and_env.0 code_and_env.1 -1)))
+         (λ (_: unit) → ESome (f.0 f.1 -1)))
     with
     | ENone _ → raise NoValueProvided
     | ESome arg → arg

tests/test_func/good/closure_through_scope.catala_en

@@ -64,13 +64,7 @@ let scope T (T_in: T_in): T {y: integer} =
       (handle_default_opt
          []
          (λ (_: unit) → true)
-         (λ (_: unit) →
-          ESome
-            (let code_and_env :
-                 ((closure_env, integer) → integer * closure_env) =
-               s.f
-             in
-             code_and_env.0 code_and_env.1 2)))
+         (λ (_: unit) → ESome (s.f.0 s.f.1 2)))
     with
     | ENone _ → raise NoValueProvided
     | ESome arg → arg

tests/test_func/good/scope_call_func_struct_closure.catala_en

@@ -158,10 +158,7 @@ let scope Foo
   let set b : bool =
     match
       (handle_default_opt
-         [
-           let code_and_env : ((any, unit) → eoption bool * any) = b in
-           code_and_env.0 code_and_env.1 ()
-         ]
+         [b.0 b.1 ()]
          (λ (_: unit) → true)
          (λ (_: unit) →
           ESome