Remove the space before a comma in singular_string

This is done by specifying suffix punctuation to chomp with WithVars.
If we delete the space on line 64 without also doing the rest of this change, the result is that the comma is not emitted at all.

PiperOrigin-RevId: 601516987

src/google/protobuf/compiler/rust/accessors/singular_string.cc

@@ -29,17 +29,18 @@ void SingularString::InMsgImpl(Context& ctx, const FieldDescriptor& field,
           {"setter_thunk", ThunkName(ctx, field, "set")},
           {"clearer_thunk", ThunkName(ctx, field, "clear")},
           {"proxied_type", RsTypePath(ctx, field)},
-          {"transform_view",
-           [&] {
-             if (field.type() == FieldDescriptor::TYPE_STRING) {
-               ctx.Emit(R"rs(
-                // SAFETY: The runtime doesn't require ProtoStr to be UTF-8.
-                unsafe { $pb$::ProtoStr::from_utf8_unchecked(view) }
-              )rs");
-             } else {
-               ctx.Emit("view");
-             }
-           }},
+          io::Printer::Sub("transform_view",
+                           [&] {
+                             if (field.type() == FieldDescriptor::TYPE_STRING) {
+                               ctx.Emit(R"rs(
+              // SAFETY: The runtime doesn't require ProtoStr to be UTF-8.
+              unsafe { $pb$::ProtoStr::from_utf8_unchecked(view) }
+            )rs");
+                             } else {
+                               ctx.Emit("view");
+                             }
+                           })
+              .WithSuffix(""),  // This lets `$transform_view$,` work.
           {"transform_field_entry",
            [&] {
              if (field.type() == FieldDescriptor::TYPE_STRING) {
@@ -62,7 +63,7 @@ void SingularString::InMsgImpl(Context& ctx, const FieldDescriptor& field,
             pub fn $field$_opt($view_self$) -> $pb$::Optional<&$view_lifetime$ $proxied_type$> {
                 let view = unsafe { $getter_thunk$(self.raw_msg()).as_ref() };
                 $pb$::Optional::new(
-                  $transform_view$ ,
+                  $transform_view$,
                   unsafe { $hazzer_thunk$(self.raw_msg()) }
                 )
               }