deserialize_untagged_verbose_error

In serde, using the untagged representation of an enum has one big disadvantage when deserializing: The error message returned in case of failure is very unspecific and does not explain why deserializing the different variants failed. There have been attempts to integrate a more verbose handling into serde in the past, but so far, no consensus has been reached.

This crate offers a macro DeserializeUntaggedVerboseError which can be applied to any macro where each variant is a tuple struct with a single field. It behaves in the same way as a combination of the Deserialize with the untagged attribute. However, in case of a deserialization failure, it collects all errors into an UntaggedEnumDeError, providing detailed information why deserializing each variant failed. The following snippet shows a side-by-side comparison with the native serde error message:

use deserialize_untagged_verbose_error::DeserializeUntaggedVerboseError;
use serde::Deserialize;
use indoc::indoc;

// Just here to provide a payload to test against - but the macro works with
// any serde-supported format
use serde_yaml;

// Random structs used as variant of the enum
#[derive(Debug, Deserialize, PartialEq)]
#[allow(dead_code)]
struct Point {
    x: f64,
    y: f64,
}
#[derive(Debug, Deserialize, PartialEq)]
#[allow(dead_code)]
struct Message {
    epochtime: usize,
    content: String,
}

// Standard Serde approach
#[derive(Debug, Deserialize, PartialEq)]
#[serde(untagged)]
#[allow(dead_code)]
enum VarSerde {
    Message(Message),
    Point(Point),
    Value(f64),
}

// Using the macro provided by this crate
#[derive(Debug, DeserializeUntaggedVerboseError, PartialEq)]
#[allow(dead_code)]
enum VarVerboseErr {
    Message(Message),
    Point(Point),
    Value(f64),
}

let invalid_str = indoc! {"
---
name: Serde
"};

// Deserializing "invalid_str" fails, because it does not match any variant of
// VarSerde / VarVerboseErr
let err_serde = serde_yaml::from_str::<VarSerde>(invalid_str).unwrap_err();
let err_verbose = serde_yaml::from_str::<VarVerboseErr>(invalid_str).unwrap_err();

// Compare the error messages:
assert_eq!(
    err_serde.to_string(),
    "data did not match any variant of untagged enum VarSerde"
);
assert_eq!(
    err_verbose.to_string(),
    indoc! {"
    Failed to deserialize the untagged enum VarVerboseErr:
    - Could not deserialize as Message: missing field `epochtime`.
    - Could not deserialize as Point: missing field `x`.
    - Could not deserialize as Value: invalid type: map, expected f64.
    "}
);

// For valid inputs, both variants behave identical
let valid_str = indoc! {"
---
x: 1
y: 2
"};

let v1 = serde_yaml::from_str::<VarSerde>(valid_str).unwrap();
match v1 {
    VarSerde::Point(pt) => {
        assert_eq!(pt.x, 1.0);
        assert_eq!(pt.y, 2.0);
    },
    _ => panic!("Test failed")
}

let v2 = serde_yaml::from_str::<VarVerboseErr>(valid_str).unwrap();
match v2 {
    VarVerboseErr::Point(pt) => {
        assert_eq!(pt.x, 1.0);
        assert_eq!(pt.y, 2.0);
    },
    _ => panic!("Test failed")
}

Implementation and limitations

For the example shown above, applying DeserializeUntaggedVerboseError to VarVerboseErr generates roughly the following code:

impl<'de> serde::de::Deserialize<'de> for VarDeUnVeEr {
    fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>
    where
        __D: serde::de::Deserializer<'de>,
    {
        let __content =
            <serde::__private::de::Content as serde::Deserialize>::deserialize(__deserializer)?;
        let __deserializer =
            serde::__private::de::ContentRefDeserializer::<__D::Error>::new(&__content);
        use serde::de::Error;
        let mut __errors: [::std::mem::MaybeUninit<(&'static str, __D::Error)>; 3usize] =
            [const { ::std::mem::MaybeUninit::uninit() }; 3usize];
        let mut __counter: usize = 0;
        match Message::deserialize(__deserializer) {
            Ok(__var) => return Ok(VarDeUnVeEr::Message(__var)),
            Err(__error) => {
                let __elem = &mut __errors[__counter];
                __elem.write((stringify!(Message), __error));
                __counter += 1;
            }
        }
        match Point::deserialize(__deserializer) {
            Ok(__var) => return Ok(VarDeUnVeEr::Point(__var)),
            Err(__error) => {
                let __elem = &mut __errors[__counter];
                __elem.write((stringify!(Point), __error));
                __counter += 1;
            }
        }
        match f64::deserialize(__deserializer) {
            Ok(__var) => return Ok(VarDeUnVeEr::Value(__var)),
            Err(__error) => {
                let __elem = &mut __errors[__counter];
                __elem.write((stringify!(Value), __error));
                __counter += 1;
            }
        }
        let __errors_init: [(&'static str, __D::Error); 3usize] = unsafe {
            [
                std::ptr::read(&__errors[0]).assume_init(),
                std::ptr::read(&__errors[1]).assume_init(),
                std::ptr::read(&__errors[2]).assume_init(),
            ]
        };
        return Err(__D::Error::custom(
            deserialize_untagged_verbose_error::UntaggedEnumDeError {
                enum_name: stringify!(VarDeUnVeEr),
                errors: __errors_init,
            },
        ));
    }
}

This has the following implications:

• The macro only works for enums where all variants have a single field.

// This example compiles
#[derive(Debug, DeserializeUntaggedVerboseError)]
enum VarVerboseErr {
    Message(Message),
    Point(Point),
    Value(f64),
}

// This one does not
#[derive(Debug, DeserializeUntaggedVerboseError)]
enum Example {
    None, // Variants without fields are not allowed
    Point(f64, f64), // Variants with more than one field are not allowed
    Value { x: i64 }, // Struct variants are not allowed
}

• All errors which occur when trying to deserialize the different variants need to be collected into an array which is part of UntaggedEnumDeError. Even though this array is allocated on the stack, this still leads to slight performance losses compared to the combination of Deserialize and untagged.

Alternatives to this crate

serde-untagged provides a much more general solution which works for all possible enum variants (not just tuple structs with one field). In exchange, it requires writing a lot of boilerplate and also does not provide a verbose error where the failure for each variant is explained.

Documentation

The full API documentation is available at https://docs.rs/deserialize_untagged_verbose_error/0.1.5/deserialize_untagged_verbose_error/.