Serde_with KeyValueMap usage

help
1

I'm trying to replicate the behavior described on serde_with's user guide of using one of a struct fields' as a key with the toml crate but to no avail.

The following source code:

use std::error;

use serde::{Deserialize, Serialize};
use serde_with::{serde_as, KeyValueMap};

#[derive(Debug, Deserialize, Serialize)]
struct Entry {
    #[serde(rename = "$key$")]
    should_be_key: String,
    field: String,
}

#[serde_as]
#[derive(Debug, Deserialize, Serialize)]
struct Entries {
    #[serde_as(as = "KeyValueMap<_>")]
    entries: Vec<Entry>,
}

fn main() -> Result<(), Box<dyn error::Error>> {
    let contents = toml::toml! {
        [[entries]]
        should_be_key = "keyname1"
        field = "field value of keyname1"

        [[entries]]
        should_be_key = "keyname2"
        field = "field value of keyname2"
    }
    .to_string();

    println!("{:#?}", contents);

    let entries: Entries = match toml::from_str(&contents) {
        Ok(r) => r,
        Err(e) => {
            eprintln!("{}", e);
            return Err("Failed to deserialize".into());
        }
    };

    println!("{:#?}", entries);

    Ok(())
}

Returns this error message:

[user@hostname:~/project-root]$ cargo run
TOML parse error at line 1, column 1
  |
1 | [[entries]]
  | ^^^^^^^^^^^
invalid type: sequence, expected a map

Error: "Failed to deserialize"

What could be causing this error? Perhaps the usage of TOML's array of tables?

2

No, this has nothing to do with TOML. You have not one but two errors in your own code:

  1. You are not using a newtype struct for the map, and
  2. you are simply giving the wrong structure to the deserializer. It's completely different from what they have in the example; why would you expect it to work? The deserializer should be fed a map of structs, but you are giving it an array of maps.

This works:

use std::error;

use serde::{Deserialize, Serialize};
use serde_with::{serde_as, KeyValueMap};

#[derive(Debug, Deserialize, Serialize)]
struct Entry {
    #[serde(rename = "$key$")]
    should_be_key: String,
    field: String,
}

#[serde_as]
#[derive(Debug, Deserialize, Serialize)]
struct Entries(
    #[serde_as(as = "KeyValueMap<_>")]
    Vec<Entry>,
);

fn main() -> Result<(), Box<dyn error::Error>> {
    let contents = toml::toml! {
        "keyname1" = {field = "field value of keyname1"}
        "keyname2" = {field = "field value of keyname2"}
    }
    .to_string();

    println!("{contents}");

    let entries: Entries = toml::from_str(&contents)?;

    println!("{entries:#?}");

    Ok(())
}
1 Like
3

Thanks for the explanation!, @H2CO3. I can now understand what I was
doing wrong.