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

@@ -29,7 +29,7 @@ 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",
+          io::Printer::Sub("transform_view",
                            [&] {
                              if (field.type() == FieldDescriptor::TYPE_STRING) {
                                ctx.Emit(R"rs(
@@ -39,7 +39,8 @@ void SingularString::InMsgImpl(Context& ctx, const FieldDescriptor& field,
                              } else {
                                ctx.Emit("view");
                              }
-           }},
+                           })
+              .WithSuffix(""),  // This lets `$transform_view$,` work.
           {"transform_field_entry",
            [&] {
              if (field.type() == FieldDescriptor::TYPE_STRING) {