I have added a feature in my crate which adds serde support. However, I don't quite understand how to use it properly:
// #[derive(Debug, Serialize, Deserialize, Clone)] // goes to:
#[derive(Debug, Clone)]
#[cfg(feature = "serde_support")]
#[derive(Serialize, Deserialize)]
pub struct MyStruct;
This code treats everything below cfg(feature) as conditionally compiled, so without my serde_support feature my crate does not have MyStruct also.
I have tried to wrap it with braces but it gives another error:
Code:
#[derive(Debug, Clone)]
#[cfg(feature = "serde_support")] {
#[derive(Serialize, Deserialize)]
}
pub struct MyStruct;
Error:
error: expected item after attributes
--> mycrate/src/lib.rs:65:33
|
65 | #[cfg(feature = "serde_support")] {
| ^
So how to do this?
You can use the cfg_attr(a, b) attribute:
#[derive(Debug, Clone)]
#[cfg_attr(feature = "serde_support", derive(Serialize, Deserialize))]
pub struct MyStruct;
It's described in the Rust reference about "conditional compilation":
#[cfg_attr(a, b)]
item
Will be the same as #[b] item if a is set by cfg, and item otherwise.
Related
I know that there is a similar question here, but I've not been able to make it fit my use case.
I have a Model struct that's nested into other structs. The model can have two different types of Config objects, a ModelConfig or a SeedConfig. They are nearly identical save for a few fields. As it stands now, I need two concrete implementations of Model (SeedModel and ModelModel) in order to change the config field, resulting in duplication of all the methods and trait implementations.
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
pub struct MetaModel {
pub model: Model
}
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
pub struct Model {
pub name: String,
pub config: Option<ModelConfig>
}
What I've tried:
Using Generics: This pushes the generic type up the chain and results in very complex definitions and areas where I don't have the context to create the parent struct (i.e. the MetaModel definition has no access to the Model definition at creation).
This eventually results in a the type parameter C is not constrained by the impl trait, self type, or predicates unconstrained type parameter error
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
pub struct MetaModel<C> {
pub model: Model<C>
}
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
pub struct Model<C> {
pub name: String,
pub config: Option<C>
}
Trait Objects: This doesn't work because serde cannot serialize trait objects
pub trait Config {}
pub struct ModelConfig;
impl Config for ModelConfig {}
pub struct SeedConfig;
impl Config for SeedConfig {}
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
pub struct Model {
pub name: String,
pub config: Option<Box<dyn Config>
}
What I'd like to do:
impl OtherTrait for Model {
type Value = Model;
fn build_model(&self, m: DerivedMeta) -> Result<Self::Value, &'static str> {
Ok(Model {
// Either a SeedConfig or a ModelConfig
})
}
}
What I would do is use a combination of #[serde(flatten)] and #[serde(untagged)]:
use serde::{Serialize, Deserialize};
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
struct Config {
members: i32,
shared_by: String,
both: i64,
#[serde(flatten)]
specific: SpecificConfig,
}
#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
#[serde(untagged)]
enum SpecificConfig {
SeedConfig {
some_members: i16,
unique_to: String,
seed_config: u64,
},
ModelConfig {
other_members: i8,
not_shared_with_above: u32,
},
}
Serde explanation of flatten:
Flatten the contents of this field into the container it is defined in.
This removes one level of structure between the serialized representation and the Rust data structure representation.
Serde explanation of untagged:
There is no explicit tag identifying which variant the data contains. Serde will try to match the data against each variant in order and the first one that deserializes successfully is the one returned.
By combining these two, we get the following behavior:
flatten allows all shared fields and specific fields to be on the same level in the config
untagged allows us to avoid adding an explicit tag in the config
all shared properties are directly accessible
only specific properties require matching the specific enum
I have a struct defined like the following (other fields removed for brevity):
use chrono::{DateTime, Utc};
use serde::{Deserialize, Serialize};
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Test {
#[serde(skip_serializing_if = "Option::is_none")]
#[serde(with = "bson::serde_helpers::chrono_datetime_as_bson_datetime")]
pub time_period: Option<Vec<DateTime<Utc>>>
}
And I'm using the following in my Cargo.toml:
[dependencies]
bson = { version = "^2.4", default-features = false, features = [ "chrono-0_4" ] }
chrono = "^0.4"
serde = { version = "^1.0", default-features = false, features = [ "derive" ] }
But the serde derivations throw error because it expects a DateTime object:
error[E0308]: mismatched types
--> src/main.rs:4:39
|
4 | #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
| ^^^^^^^^^
| |
| expected struct `chrono::DateTime`, found enum `std::option::Option`
| arguments to this function are incorrect
|
= note: expected reference `&chrono::DateTime<Utc>`
found reference `&'__a std::option::Option<Vec<chrono::DateTime<Utc>>>`
note: function defined here
--> /home/kmdreko/.cargo/registry/src/github.com-1ecc6299db9ec823/bson-2.4.0/src/serde_helpers.rs:296:12
|
296 | pub fn serialize<S: Serializer>(
| ^^^^^^^^^
= note: this error originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0308]: mismatched types
--> src/main.rs:4:50
|
4 | #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
| ^^^^^^^^^^^ expected enum `std::option::Option`, found struct `chrono::DateTime`
|
= note: expected enum `std::option::Option<Vec<chrono::DateTime<Utc>>>`
found struct `chrono::DateTime<Utc>`
= note: this error originates in the macro `try` (in Nightly builds, run with -Z macro-backtrace for more info)
Any ideas how to serialize optional vector of DateTime objects?
Serde's with attribute can only be used if your function/module expects the exact type of the field. That is not possible to change and sometimes you see *_opt functions/modules to provide support for Options, but never for arbitrary nesting.
The bson crate has a feature to use serde_with to work around that. You need to enable the serde_with feature of bson and import serde_with v1 into your code. Note the extra default attribute to serde. That is unnecessary for v2 of serde_with, but bson is still on v1.
#[serde_with::serde_as]
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct Test {
#[serde(skip_serializing_if = "Option::is_none", default)]
#[serde_as(as = "Option<Vec<bson::DateTime>>")]
pub time_period: Option<Vec<DateTime<Utc>>>
}
[dependencies]
bson = { version = "^2.4", default-features = false, features = [ "chrono-0_4", "serde_with" ] }
chrono = "^0.4"
serde = { version = "^1.0", default-features = false, features = [ "derive" ] }
serde_with = "1"
Run on Rustexplorer
Why doesn't the OP's code work
The bson::serde_helpers::chrono_datetime_as_bson_datetime module contains helper functions that (de)serializes a single chrono::DateTime object, so it doesn't work with the field of type Option<Vec<DateTime<Utc>>>.
Solution using nested struct
To (de)serialize such data, you can define a struct TestInner that contains the inner DateTime<Utc> object, along with the #[serde(transparent)] container attribute so that it's (de)serialized in the same way as a single DateTime<Utc> object. For example, the struct Test in the question can be changed into something like this.
#[derive(Debug, Serialize, Deserialize)]
pub struct Test {
#[serde(skip_serializing_if = "Option::is_none")]
pub time_period: Option<Vec<TestInner>>,
}
#[derive(Debug, Serialize, Deserialize)]
#[serde(transparent)]
pub struct TestInner {
#[serde(with = "bson::serde_helpers::chrono_datetime_as_bson_datetime")]
datetime: DateTime<Utc>,
}
I am trying to declare a struct that contains another struct of any given type that can be Deserialize and Serialize.
#[derive(Debug, Serialize, Deserialize)]
pub struct Foo<T: Deserialize + Serialize> {
pub data: T,
}
Rust playground.
For that, I have tried to use trait bounds, using traits such as DeserializeOwned or Deserialize. Both have failed at compilation time with the following errors:
error[E0283]: type annotations required: cannot resolve `T: serde::Deserialize<'de>`
--> src/main.rs:9:28
|
9 | #[derive(Debug, Serialize, Deserialize)]
| ^^^^^^^^^^^
|
= note: required by `serde::Deserialize`
error[E0637]: `&` without an explicit lifetime name cannot be used here
--> src/main.rs:10:19
|
10 | pub struct Foo<T: Deserialize + Serialize> {
| ^^^^^^^^^^^ explicit lifetime name needed here
I faced errors trying to add a lifetime since I am not using storing a reference but a value.
What is the most idiomatic way of declaring this type of struct?
Just don't place the bounds on the type:
use serde::{Deserialize, Serialize}; // 1.0.91
#[derive(Debug, Serialize, Deserialize)]
pub struct Foo<T> {
pub data: T,
}
#[derive(Debug, Serialize, Deserialize)]
pub struct Boo {
pub a: i64,
}
fn main() {
let data = Boo { a: 1 };
let wrap = Foo { data };
println!("{:?}", wrap);
}
Then, place the bounds on methods where you need that behavior:
fn use_it<T>(foo: Foo<T>)
where
Foo<T>: Serialize,
{
// ...
}
I found a solution thanks to a member of the Rust Discord who referred me to the following Github issue. The trick is not to use trait bounds but attribute bounds.
#[derive(Debug, Serialize, Deserialize)]
pub struct Foo<T> {
#[serde(bound(
serialize = "T: Serialize",
deserialize = "T: Deserialize<'de>",
))]
pub data: T,
}
I'm trying to create a custom struct using the UUID struct from Rocket as a field type. I want it to be serialized using Serde in order to convert it to JSON easily.
When trying to do this, I get an error:
error[E0277]: the trait bound `rocket_contrib::UUID:
model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::Serialize` is not
satisfied
--> src/service/document.rs:4:10
|
4 | #[derive(Serialize, Deserialize)]
| ^^^^^^^^^ the trait
`model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::Serialize` is not
implemented for `rocket_contrib::UUID`
|
= note: required by `model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::ser::SerializeStruct::serialize_field`
error[E0277]: the trait bound `rocket_contrib::UUID:
model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::Deserialize<'_>` is not satisfied
--> src/service/document.rs:4:21
|
4 | #[derive(Serialize, Deserialize)]
| ^^^^^^^^^^^ the trait
`model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::Deserialize<'_>` is not implemented for `rocket_contrib::UUID`
|
= note: required by `model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::de::SeqAccess::next_element`
error[E0277]: the trait bound `rocket_contrib::UUID: model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::Deserialize<'_>` is not satisfied
--> src/service/document.rs:4:21
|
4 | #[derive(Serialize, Deserialize)]
| ^^^^^^^^^^^ the trait `model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::Deserialize<'_>` is not implemented for `rocket_contrib::UUID`
|
= note: required by `model::event::_IMPL_DESERIALIZE_FOR_Event::_serde::de::MapAccess::next_value`
My struct:
#[derive(Serialize, Deserialize)]
pub struct Document {
id: UUID,
user_id: UUID,
created: i64,
updated: i64,
text: String
}
My imports:
[dependencies]
rocket = "0.3.17"
rocket_codegen = "0.3.17"
serde_derive = "1.0.80"
serde = "1.0.80"
chrono = "0.4"
[dependencies.rocket_contrib]
version = "0.3.17"
default-features = false
features = ["json", "uuid", "serde"]
Endpoint where I use the struct:
#[get("/document/<id>")]
pub fn get_document(id: UUID) -> status::Accepted<Json<Document>> {
status::Accepted(Some(Json(document::get_document(id))))
}
I've checked all dependencies, and I have the serde feature enabled in rocket_contrib. I've run out of ideas what to check next.
rocket_contrib::UUID does not implement Serialize:
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug)]
pub struct UUID(uuid_ext::Uuid);
If the type doesn't implement Serialize, you can't make it.
As mcarton points out:
you still can implement Serialize for your type, you simply can't derive it and will have to implement it by hand.
That could look something like:
#[derive(Serialize, Deserialize)]
pub struct Document {
#[serde(with = "my_uuid")]
id: UUID,
#[serde(with = "my_uuid")]
user_id: UUID,
created: i64,
updated: i64,
text: String,
}
mod my_uuid {
use rocket_contrib::UUID;
use serde::{de::Error, Deserialize, Deserializer, Serialize, Serializer};
use std::str::FromStr;
pub fn serialize<S>(val: &UUID, serializer: S) -> Result<S::Ok, S::Error>
where
S: Serializer,
{
val.to_string().serialize(serializer)
}
pub fn deserialize<'de, D>(deserializer: D) -> Result<UUID, D::Error>
where
D: Deserializer<'de>,
{
let val: &str = Deserialize::deserialize(deserializer)?;
UUID::from_str(val).map_err(D::Error::custom)
}
}
See also:
How to transform fields during serialization using Serde?
How to transform fields during deserialization using Serde?
Is there a way for me to use #[derive] on a struct or enum from a library without editing the actual library's source code?
How do I implement a trait I don't own for a type I don't own?
Add Serialize attribute to type from third-party lib
I have the following code:
extern crate serde;
#[macro_use]
extern crate serde_derive;
#[derive(Debug, Default, Serialize, Deserialize)]
pub struct Base {
bold: Option<bool>,
}
#[derive(Debug, Serialize, Deserialize)]
#[serde(untagged)]
pub enum Component {
String {
#[serde(flatten)] base: Base,
text: String,
},
}
This gives me a compiler error:
error[E0425]: cannot find value `__collect` in this scope
--> src/main.rs:10:28
|
10 | #[derive(Debug, Serialize, Deserialize)]
| ^^^^^^^^^^^ did you mean `__content`?
error[E0277]: the trait bound `<__S as serde::Serializer>::SerializeStruct: serde::ser::SerializeMap` is not satisfied
--> src/main.rs:10:17
|
10 | #[derive(Debug, Serialize, Deserialize)]
| ^^^^^^^^^ the trait `serde::ser::SerializeMap` is not implemented for `<__S as serde::Serializer>::SerializeStruct`
|
= help: consider adding a `where <__S as serde::Serializer>::SerializeStruct: serde::ser::SerializeMap` bound
= note: required because of the requirements on the impl of `serde::Serializer` for `serde::private::ser::FlatMapSerializer<'_, <__S as serde::Serializer>::SerializeStruct>`
= note: required by `serde::Serialize::serialize`
If I change my code to this, it compiles fine:
extern crate serde;
#[macro_use]
extern crate serde_derive;
#[derive(Debug, Default, Serialize, Deserialize)]
pub struct Base {
bold: Option<bool>,
}
#[derive(Debug, Serialize, Deserialize)]
pub struct Striii {
#[serde(flatten)]
base: Base,
text: String,
}
#[derive(Debug, Serialize, Deserialize)]
#[serde(untagged)]
pub enum Component {
String(Striii),
}
I don't understand the compiler error or why it shouldn't work. Does Serde have no support for field attributes inside enum struct variants?
This is a known issue with Serde for both serialization and deserialization. There are no listed workarounds.