String recursion error after upgrading from 0.26.0 to 0.27.1

[JenChampagne]

I'm getting a really weird error after upgrading from 0.26.0 to 0.27.1. I've tried to put together a reproduce case, but I'm having difficulty doing so, but I'll give as many details as I can.

The error I get is as follows:

error[E0275]: overflow evaluating the requirement `&mut std::string::String: std::fmt::Write`
  |
  = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`lib`)
  = note: required for `&mut &mut std::string::String` to implement `std::fmt::Write`
  = note: 127 redundant requirements hidden
  = note: required for `&mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut std::string::String` to implement `std::fmt::Write`
  = note: required for `se::element::ElementSerializer<'_, &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut std::string::String>` to implement `serde::Serializer`

For more information about this error, try `rustc --explain E0275`.
error: could not compile `project` due to previous error

I make use of AsRef<T> and Cow<'_, str> in several cases, but I'm not sure that it is the core of the issue since I'm not able to make a reproduce case. I don't understand where the recursion of &mut is coming from. Nothing in my code is explicitly recursive as far as I am aware, and cargo expand does not highlight anything that stands out.

Does the above provide any clues that might point me in the direction of where to look in my code? I will try to again to reproduce, but the repository having the issue is a private one.

[Mingun]

This is something about serializer. Serializer was rewritten in 0.27.0 and now writes to fmt::Write instead of io::Write. My speculation: probably you try to serialize to a string and there somewhere the unnecessary &mut.

[Clazex]

Met similar problem in both 0.27.0 & 0.27.1.
The problem does not persist with quick-xml removed or downgraded to 0.26.0.

Build log with -Z macro-backtrace:

error: reached the recursion limit while instantiating `some_type::_::_serde::ser::impls::<impl Serialize for &some_type::_::_serde::__private::ser::content::Content>::serialize::<se::element::ElementSerializer<'_, &mut &mut ...>>`
   --> <REDACTED>\quick-xml-0.27.1\src\se\element.rs:211:9
    |
211 | /         value.serialize(ElementSerializer {
212 | |             ser: self.ser.new_seq_element_serializer(),
213 | |             key: self.key,
214 | |         })?;
    | |__________^
    |
note: `some_type::_::_serde::ser::impls::<impl Serialize for &'a T>::serialize` defined here
   --> <REDACTED>\serde-1.0.152\src\ser\impls.rs:423:13
    |
415 | /  macro_rules! deref_impl {
416 | |      (
417 | |          $(#[doc = $doc:tt])*
418 | |          <$($desc:tt)+
...   |
423 | |/             fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
424 | ||             where
425 | ||                 S: Serializer,
    | ||______________________________^
...   |
430 | |      };
431 | |  }
    | |__- in this expansion of `deref_impl!`
432 |
433 |    deref_impl!(<'a, T: ?Sized> Serialize for &'a T where T: Serialize);
    |    ------------------------------------------------------------------- in this macro invocation
    = note: the full type name has been written to '<REDACTED>.long-type.txt'

long-type.txt:

some_type::_::_serde::ser::impls::<impl Serialize for &some_type::_::_serde::__private::ser::content::Content>::serialize::<se::element::ElementSerializer<'_, &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut &mut std::string::String>>

There is no &mut in all types in the crate. And increasing recursion limit does not help.

[Mingun]

Could you provide the minimal reproducible example of this? What version of serde you are using? cargo tree should show it

[Clazex]

here

[Mingun]

I shortened the example to:

use serde::Serialize;

#[derive(Serialize)]
pub enum Enum {
    Variant {},
}

#[derive(Serialize)]
pub struct Struct {
    #[serde(flatten)]
    flatten: Enum,
}

pub fn to_xml(data: &Struct) -> Result<String, quick_xml::DeError> {
    quick_xml::se::to_string(&data)
}

Interesting, that removing pub from to_xml fixes the problem, so probably this is a bug in rustc

[Clazex]

I guess &mut come from here because if I do quick_xml::se::Serializer::new(std::ffi::OsString::new()), the long-type still ends with String

[Clazex]

Update: This panics for Unsupported("cannot serialize map without defined root tag")

fn main() {
    use serde::ser::Serializer;
    quick_xml::se::Serializer::new(String::new())
        .serialize_map(None).unwrap();
}

However the derived Serialize impl for struct or variant that has flattened struct/enum fields always starts with serialize_map call (checked with cargo-expand)
So the new serializer cannot deal with flattening at all 😢

[Mingun]

So the new serializer cannot deal with flattening at all cry

No, it is because it was created with new instead of with_root. Probably I should include correct method to the error message. But you're right, flattening works poorly for XML, it is better to avoid it, especially for deserialization. In many cases it will not work due to its design.