Newtype as the key of a HashMap/BTreeMap is not supported

[TonalidadeHidrica]

A newtype - a named single-tuple type - cannot be used as the key of a HashMap or a BTreeMap.

playground

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

#[derive(PartialEq, Eq, Hash, Debug, Deserialize)]
struct NewType(String);

fn main() -> anyhow::Result<()> {
    let map = toml::from_str::<HashMap<NewType, u32>>(r#"
    x = 1
    y = 2
    "#)?;
    println!("{map:?}");
    Ok(())
}

Error: TOML parse error at line 2, column 5
  |
2 |     x = 1
  |     ^
invalid type: string "x", expected tuple struct NewType

This is contrary to serde_json.

playground

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

#[derive(PartialEq, Eq, Hash, Debug, Deserialize)]
struct NewType(String);

fn main() -> anyhow::Result<()> {
    let map = serde_json::from_str::<HashMap<NewType, u32>>(r#"
    {"x": 1, "y": 2}
    "#)?;
    println!("{map:?}");
    Ok(())
}

{NewType("x"): 1, NewType("y"): 2}

I ran into this issue in my project, and I think that this should definitely be supported.

[m4tx]

Ran into the same issue today.

[m4tx]

Actually - this might be somewhat expected, as NewType does not seem to support deserialization from &str by default.

The solution is to modify the newtype so that it behaves just like the underlying type:

#[derive(PartialEq, Eq, Hash, Debug, Deserialize)]
#[serde(transparent)]
struct NewType(String);

With the addition of #[serde(transparent)], the code posted in the issue seems to be working just fine.