Fix _this_ on non-let-bindings

CHANGELOG.md

@@ -9,6 +9,11 @@
 
 - Runtime builders take a `description` optional argument.
 
+### Fixed
+
+- `_this_` infix on non-let-binding expressions throws a syntax error instead of being a no-op.
+  - As a design decision aligned with [#51](https://github.com/lukstafi/ppx_minidebug/issues/51), we output the error instead of processing the expression, i.e. we don't ignore the `_this_` infix.
+
 ## [1.5.1] -- 2024-07-07
 
 ### Changed

ppx_minidebug.ml

@@ -646,8 +646,8 @@ let unpack_runtime toplevel_opt_arg exp =
         [%e exp]]
 
 let has_runtime_arg = function
-  | { toplevel_opt_arg = Nested | Toplevel_no_arg; _ } -> false
-  | _ -> true
+  | { toplevel_opt_arg = PrintBox | Generic; _ } -> true
+  | _ -> false
 
 let loc_to_name loc =
   let fname = Filename.basename loc.loc_start.pos_fname |> Filename.remove_extension in
@@ -1343,7 +1343,10 @@ let debug_this_expander context payload =
       (* This is the [let%debug_this ... in] use-case: do not debug the whole body. *)
       let bindings = List.map (debug_binding context callback) bindings in
       { payload with pexp_desc = Pexp_let (recflag, bindings, body) }
-  | expr -> expr
+  | expr ->
+    A.pexp_extension ~loc:expr.pexp_loc
+      @@ Location.error_extensionf ~loc:expr.pexp_loc
+           "ppx_minidebug: to avoid confusion, _this_ indicator is only allowed on let-bindings"
 
 let debug_expander context payload = traverse_expression#expression context payload
 

test/dune

@@ -60,6 +60,14 @@
  (action
   (run ./%{pp} --impl %{input} -o %{targets})))
 
+(rule
+ (targets test_debug_show_error_this.actual.ml)
+ (deps
+  (:pp pp.exe)
+  (:input test_debug_show_error_this.ml))
+ (action
+  (run ./%{pp} --impl %{input} -o %{targets})))
+
 (rule
  (alias runtest)
  (action
@@ -99,6 +107,13 @@
    test_debug_log_prefixed.expected.ml
    test_debug_log_prefixed.actual.ml)))
 
+(rule
+ (alias runtest)
+ (action
+  (diff
+   test_debug_show_error_this.expected.ml
+   test_debug_show_error_this.actual.ml)))
+
 (executable
  (name test_debug_sexp)
  (modules test_debug_sexp)

test/test_debug_show_error_this.expected.ml

@@ -0,0 +1,4 @@
+let () =
+  [%ocaml.error
+    "ppx_minidebug: to avoid confusion, _this_ indicator is only allowed on let-bindings"];
+  ()

test/test_debug_show_error_this.ml

@@ -0,0 +1,9 @@
+(* module Debug_runtime =
+   (val Minidebug_runtime.debug_flushing ~filename:"debugger_show_flushing" ()) *)
+
+let () =
+  [%debug_this_show
+    [%log_entry
+      "The end";
+      [%log (42 : int)]]];
+  ()