Cannot deserialize with Struct without fields

[zhongzc]

#[derive(Debug, Serialize, Deserialize)]
struct A {
    b: Option<B>,
}
#[derive(Debug, Serialize, Deserialize)]
struct B {}

let a = A { b: Some(B {}) };
let de: A = Config::try_from(&a).unwrap().try_deserialize().unwrap();
assert!(de.b.is_some()); // Failed

[matthiasbeyer]

How would you specify that in a configuration file?

[zhongzc]

How would you specify that in a configuration file?

#[derive(Debug, Serialize, Deserialize, PartialEq)]
struct A {
    b: Option<B>,
}
#[derive(Debug, Serialize, Deserialize, PartialEq)]
struct B {}

impl Default for A {
    fn default() -> Self {
        Self { b: Some(B {}) }
    }
}

let a = A::default();
let toml_str = toml::to_string(&a).unwrap();
println!("toml str: {}", toml_str);

// let de_from_toml = toml::from_str::<A>(&toml_str).unwrap();
// assert_eq!(a, de_from_toml); // Passed

let de_from_toml: A = Config::builder()
    .add_source(File::from_str(&toml_str, FileFormat::Toml))
    .build()
    .unwrap()
    .try_deserialize()
    .unwrap();
assert_eq!(a, de_from_toml); // Passed

let de_from_default_object: A = Config::builder()
    .add_source(Config::try_from(&a).unwrap())
    .build()
    .unwrap()
    .try_deserialize()
    .unwrap();
assert_eq!(a, de_from_default_object); // Failed

Output:

toml str: [b]

assertion failed: `(left == right)`
  left: `A { b: Some(B) }`,
 right: `A { b: None }`

[matthiasbeyer]

And again I ask: How would you specify a instance of A { b: Some(B) } in a configuration file?

[zhongzc]

Just like the previous example, for toml file, I specify it like this:

[b]

[matthiasbeyer]

Ah, I missed that bit. Hnm, indeed that's an issue. Thanks for reporting!

[matthiasbeyer]

I opened #463 with a testcase for this issue. If you have an idea solving that problem, you're of course very much welcome!

[zhongzc]

The scenarios that cause B to be lost are related to iterators. An example that better reflects the situation I found:

#[derive(Debug, Serialize, Deserialize, PartialEq)]
struct A {
    b_option: Option<B>,
    b_vec: Vec<B>,
    b_set: HashSet<B>,
    b_map: HashMap<String, B>,
}
#[derive(Debug, Serialize, Deserialize, PartialEq, Eq, Hash)]
struct B {}

let a = A {
    b_option: Some(B {}),
    b_vec: vec![B {}],
    b_set: HashSet::from([B {}]),
    b_map: HashMap::from([("".to_owned(), B {})]),
};

// Serialize that `a` with `ConfigSerializer` will produce an empty config
let config = Config::try_from(&a).unwrap();

[polarathene]

TL;DR: Looks like this can be resolved easily enough with the below bolded fixes.

NOTE: Depending on when this is acted on, the referenced snippets below may change due to https://github.com/mehcode/config-rs/pull/465 (suggested fixes remain compatible, other referenced snippets may be refactored)

---

Note that there is two ways to define a unit-like struct (technically more?):

// Officially documented way:
// https://doc.rust-lang.org/book/ch05-01-defining-structs.html#unit-like-structs-without-any-fields
// https://learning-rust.github.io/docs/structs/
struct B;

// RFC:
// https://rust-lang.github.io/rfcs/0218-empty-struct-with-braces.html
// Previously not supported: https://github.com/rust-lang/rfcs/pull/147#issuecomment-47589168
// Accepted 2015: https://github.com/rust-lang/rfcs/pull/218#issuecomment-91558974
// Stabilized 2016: https://github.com/rust-lang/rust/issues/29720#issuecomment-189523437
struct B {}

These are roughly the same but have some differences.

If you omit the Option<>, and always have the unit struct:

• struct B {} will fail with try_deserialize() due to error: value: missing field 'b'. Does not happen for struct B; (successful with struct).
• toml::to_string(a) will fail for struct B; with: Error { inner: UnsupportedType(Some("B")) }. This also happens with the same error when wrapped in an Option<>.

---

struct B; would hit this route:

https://github.com/mehcode/config-rs/blob/6946069755e4bc75af9b7ca678da66c055a0af16/src/ser.rs#L165-L182

Even though it does reach serialize_some() you can see that it takes the same path as serialize_none(), as value.serialize(self) will resolve to the serialize_unit_struct() method, which like serialize_none() redirects to serialize_unit() which normalizes to the Nil type, aka None.

Fix: Instead of self.serialize_unit(), a unit struct is more likely to map to a Table/Map like other structs. So an empty struct seems more appropriate?:

// Call this instead:
self.serialize_primitive(Value::from(crate::value::Table::new()))

EDIT: This fails to deserialize the unit struct properly with error:

It requires adding an additional deserialize method for Value in de.rs instead of relying on unit_struct in serde::forward_to_deserialize_any!:

#[inline]
fn deserialize_unit_struct<V: de::Visitor<'de>>(self, _name: &'static str, visitor: V) -> Result<V::Value> {
    visitor.visit_unit()
}

---

struct B {} instead hits the same serialize_some() route to start, but value.serialize(self) recognizes it as a struct thus calls serialize_struct():

https://github.com/mehcode/config-rs/blob/6946069755e4bc75af9b7ca678da66c055a0af16/src/ser.rs#L243-L249

• The _name is the struct B, and len is 0.
• The returned result for both may look like different types but they are type aliases to Self?:

https://github.com/mehcode/config-rs/blob/6946069755e4bc75af9b7ca678da66c055a0af16/src/ser.rs#L85-L94

This is also None presumably because there is no logic involved that serializes to the config-rs internal Value type (which serialize_unit() did for struct B; earlier by calling serialize_primitive()):

https://github.com/mehcode/config-rs/blob/6946069755e4bc75af9b7ca678da66c055a0af16/src/ser.rs#L15-L22

https://github.com/mehcode/config-rs/blob/6946069755e4bc75af9b7ca678da66c055a0af16/src/ser.rs#L31-L32

Fix: A similar fix here is to also create a table, presumably only when len is 0. This should not replace the existing call, due to conflicting return type. Modify serialize_struct():

// Struct is empty (unit struct with empty braces):
if len == 0 {
    self.serialize_primitive(Value::from(crate::value::Table::new()))?;
}
// Keep the original call:
self.serialize_map(Some(len))

---

In both cases, I assume the expected serialization is what I've observed the deserialized format to Value is, an empty variant of Table (Map<String, Value>).

Applying either fix for that as suggested above now serializes the unit struct type option successfully, which will properly deserialize the option (since ValueKind::Nil would resolve to None but any other ValueKind variant like Table resolves to Some):

https://github.com/mehcode/config-rs/blob/6946069755e4bc75af9b7ca678da66c055a0af16/src/de.rs#L132-L142

---

Deserialization isn't an issue without the Option wrapping type, since Nil seems to resolve implicitly via the unit_struct -> unit derived methods generated here (deserialize_unit_struct() that calls self.deserialize_unit(visitor) that calls visitor.visit_unit()):

https://github.com/mehcode/config-rs/blob/6946069755e4bc75af9b7ca678da66c055a0af16/src/de.rs#L167-L171

The value prior to calling visitor.visit_unit() is Value of ValueKind::Nil:

Value {
    origin: None,
    kind: Nil,
}

I assume directly after this call it's converted to the unit struct, but I'm not sure how that works 😅