Fix missing parentheses around let..in with attribute (#2564)

On a side note, parentheses around let..in shouldn't have
`~fits_break:false`, but changing that would cause large diffs.

CHANGES.md

@@ -52,8 +52,9 @@ profile. This started with version 0.26.0.
 - Fix comments around underscore in record patterns (#2540, @Julow)
 - Fix dropped comments before `begin .. end` in a match case (#2541, @Julow)
 - Fix closing `*)` in doc-comments exceeding the margin (#2550, @Julow)
-- Fix invalid syntax geneated for begin..end attributes (#2551, @Julow)
+- Fix invalid syntax generated for begin..end attributes (#2551, @Julow)
   The attribute is moved from `begin .. end [@attr]` to `begin [@attr] .. end`.
+- Fix missing parentheses around `let .. in [@attr]` (#2564, @Julow)
 - Display `a##b` instead of `a ## b` and similarly for operators that start with # (#2580, @v-gb)
 
 ### Changes

lib/Fmt_ast.ml

@@ -4476,7 +4476,6 @@ and fmt_structure_item c ~last:last_item ~semisemi {ctx= parent_ctx; ast= si}
 
 and fmt_let c ~rec_flag ~bindings ~parens ~fmt_atrs ~fmt_expr ~loc_in
     ~body_loc ~has_attr ~indent_after_in =
-  let parens = parens || has_attr in
   let fmt_in indent =
     match c.conf.fmt_opts.break_before_in.v with
     | `Fit_or_vertical -> break 1 (-indent) $ str "in"
@@ -4495,13 +4494,14 @@ and fmt_let c ~rec_flag ~bindings ~parens ~fmt_atrs ~fmt_expr ~loc_in
         | `Compact -> space_break )
   in
   let blank_line_after_in = sequence_blank_line c loc_in body_loc in
-  Params.Exp.wrap c.conf ~parens:(parens || has_attr) ~fits_breaks:false
-    (vbox 0
-       ( hvbox 0 (list_fl bindings fmt_binding)
-       $ ( if blank_line_after_in then str "\n" $ cut_break
-           else break 1000 indent_after_in )
-       $ hvbox 0 fmt_expr ) )
-  $ fmt_atrs
+  Params.Exp.wrap c.conf ~parens ~fits_breaks:false
+    ( Params.Exp.wrap c.conf ~parens:has_attr ~fits_breaks:false
+        (vbox 0
+           ( hvbox 0 (list_fl bindings fmt_binding)
+           $ ( if blank_line_after_in then str "\n" $ cut_break
+               else break 1000 indent_after_in )
+           $ hvbox 0 fmt_expr ) )
+    $ fmt_atrs )
 
 and fmt_value_constraint c vc_opt =
   let fmt_sep x =

test/passing/tests/lazy.ml

@@ -10,3 +10,9 @@ let () =
 let _ = lazy (a.b <- 1)
 
 let _ = match x with (lazy (Some _ as x)), x -> x
+
+let _ =
+  lazy
+    ((let () = () in
+      () )
+    [@attr] )