span and capturing additional fields

[User65k]

I have to capture additional fields when deserializing.

Unfortunately, this is messing up the nice error messages / span info.
It always points to the first line.

use std::collections::HashMap;
use serde::Deserialize;

#[derive(Deserialize, Debug)]
struct Outer {
    item: u8,
    #[serde(flatten)]
    rest: HashMap<String, u8>,
}
pub fn main(){    
        let t_str = r#"#a bit of content
item = 7
i = false
"#;
        let res: Result<Outer, toml::de::Error> = toml::from_str(t_str);
        let got = res.unwrap_err();
        println!("{}", got);
}

TOML parse error at line 1, column 1
  |
1 | #a bit of content
  | ^^^^^^^^^^^^^^^^^
invalid type: boolean `false`, expected u8

Playground link with tests: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ca33eb1e091fb94d4313f361022c2208

[epage]

The call stack for an error during flatten looks something like

0: toml_edit::parser::errors::TomlError::custom                                                                                   [1187/4759]
          at /home/epage/src/personal/toml/crates/toml_edit/src/parser/errors.rs:52:24                                                       1: toml_edit::de::Error::custom                                                                                                                        at /home/epage/src/personal/toml/crates/toml_edit/src/de/mod.rs:36:20                                                              2: <toml_edit::de::Error as serde::de::Error>::custom                                                                                                  at /home/epage/src/personal/toml/crates/toml_edit/src/de/mod.rs:65:9
   serde::de::Error::invalid_type
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/de/mod.rs:209:17
3: serde::__private::de::content::ContentRefDeserializer<E>::invalid_type
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/private/de.rs:1750:13
4: serde::__private::de::content::ContentRefDeserializer<E>::deserialize_integer
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/private/de.rs:1766:26
   <serde::__private::de::content::ContentRefDeserializer<E> as serde::de::Deserializer>::deserialize_u8
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/private/de.rs:1907:13
5: serde::de::impls::<impl serde::de::Deserialize for u8>::deserialize
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/de/impls.rs:129:17
   <core::marker::PhantomData<T> as serde::de::DeserializeSeed>::deserialize
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/de/mod.rs:791:9
   <serde::__private::de::FlatMapAccess<E> as serde::de::MapAccess>::next_value_seed
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/private/de.rs:2886:28
   serde::de::MapAccess::next_entry_seed
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/de/mod.rs:1836:34
   serde::de::MapAccess::next_entry
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/de/mod.rs:1884:9
   <serde::de::impls::<impl serde::de::Deserialize for std::collections::hash::map::HashMap<K,V,S>>::deserialize::MapVisitor<K,V,S> as serde::de::Visitor>::visit_map
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/de/impls.rs:1393:61
   <serde::__private::de::FlatMapDeserializer<E> as serde::de::Deserializer>::deserialize_map
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/private/de.rs:2775:9
6: serde::de::impls::<impl serde::de::Deserialize for std::collections::hash::map::HashMap<K,V,S>>::deserialize
          at /home/epage/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/serde-1.0.152/src/de/impls.rs:1402:17
   <testsuite::de_errors::flatten_has_valid_span::_::<impl serde::de::Deserialize for testsuite::de_errors::flatten_has_valid_span::Flatten>::deserialize::__Visitor as serde::de::Visitor>::visit_map
          at tests/testsuite/de_errors.rs:487:14
   <toml_edit::de::table::TableDeserializer as serde::de::Deserializer>::deserialize_any
          at /home/epage/src/personal/toml/crates/toml_edit/src/de/table.rs:21:9
7: <toml_edit::de::value::ValueDeserializer as serde::de::Deserializer>::deserialize_any
          at /home/epage/src/personal/toml/crates/toml_edit/src/de/value.rs:75:38
8: <toml_edit::de::Deserializer as serde::de::Deserializer>::deserialize_any

[epage]

Normally you would expect the key/value processing to be in a call stack like this which is what could set the span. I suspect for flattens, serde is capturing what we deserialize and then replaying that, removing us from the call stack to set a more specific span.

So we set a span for the whole table with the problem but we aren't rendering multi-line spans correctly (#458)

[epage]

btw I have this written out as a test case

#[test]
fn flatten_has_valid_span() {
    #[derive(Deserialize, Debug)]
    struct Unflatten {
        #[allow(dead_code)]
        item: u8,
        #[allow(dead_code)]
        i: u8,
    }
    bad!(
        r#"#a bit of content
item = 7
i = false
"#,
        Unflatten,
        "\
TOML parse error at line 3, column 5
  |
3 | i = false
  |     ^^^^^
invalid type: boolean `false`, expected u8
"
    );
    #[derive(Deserialize, Debug)]
    struct Flatten {
        #[allow(dead_code)]
        item: u8,
        #[allow(dead_code)]
        #[serde(flatten)]
        rest: std::collections::HashMap<String, u8>,
    }
    bad!(
        r#"#a bit of content
item = 7
i = false
"#,
        Flatten,
        "\
TOML parse error at line 3, column 5
  |
3 | i = false
  |     ^^^^^
invalid type: boolean `false`, expected u8
"
    );
}

[epage]

Closing in favor of #589