Fix trailing space in empty signature (#2443)

* Make sure not to break out-of-margin emtpy-sig

CHANGES.md

@@ -4,6 +4,12 @@ Items marked with an asterisk (\*) are changes that are likely to format
 existing code differently from the previous release when using the default
 profile. This started with version 0.26.0.
 
+## unreleased
+
+### Fixed
+
+- Remove trailing space inside a wrapping empty signature (#2443, @Julow)
+
 ## 0.26.1 (2023-09-15)
 
 ### Changed

lib/Fmt_ast.ml

@@ -3463,7 +3463,7 @@ and fmt_module_type c ?(rec_ = false) ({ast= mty; _} as xmty) =
       { opn= None
       ; pro= Some (before $ str "sig" $ fmt_if empty " ")
       ; psp= fmt_if (not empty) "@;<1000 2>"
-      ; bdy= within $ fmt_signature c ctx s
+      ; bdy= (within $ if empty then noop else fmt_signature c ctx s)
       ; cls= noop
       ; esp= fmt_if (not empty) "@;<1000 0>"
       ; epi=

test/passing/dune.inc

@@ -3897,6 +3897,24 @@
  (package ocamlformat)
  (action (diff tests/module_type.ml.err module_type.ml.stderr)))
 
+(rule
+ (deps tests/.ocamlformat )
+ (package ocamlformat)
+ (action
+  (with-stdout-to module_type.mli.stdout
+   (with-stderr-to module_type.mli.stderr
+     (run %{bin:ocamlformat} --margin-check %{dep:tests/module_type.mli})))))
+
+(rule
+ (alias runtest)
+ (package ocamlformat)
+ (action (diff tests/module_type.mli.ref module_type.mli.stdout)))
+
+(rule
+ (alias runtest)
+ (package ocamlformat)
+ (action (diff tests/module_type.mli.err module_type.mli.stderr)))
+
 (rule
  (deps tests/.ocamlformat )
  (package ocamlformat)

test/passing/tests/module_type.mli

@@ -0,0 +1,4 @@
+(* Wrapping an empty sig *)
+module Foo
+    (A : FOO)
+    (B : FOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO) : sig end

test/passing/tests/module_type.mli.err

@@ -0,0 +1 @@
+Warning: tests/module_type.mli:3 exceeds the margin

test/passing/tests/module_type.mli.ref

@@ -0,0 +1,4 @@
+(* Wrapping an empty sig *)
+module Foo
+    (A : FOO)
+    (B : FOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO) : sig end