Deserialization doesn't work on hashmap if both generic parameters are constrainted with DeserializeAs types

[Pzixel]

Consider following code:

use serde::{Deserialize, Deserializer, Serialize};
use serde_with::serde_as;
use serde_with::DeserializeAs;
use std::collections::HashMap;

#[derive(Serialize, Deserialize)]
struct Test(i32);

impl<'de> DeserializeAs<'de, i32> for Test {
    fn deserialize_as<D>(deserializer: D) -> Result<i32, D::Error>
    where
        D: Deserializer<'de>,
    {
        let result = Self::deserialize(deserializer)?;
        Ok(result.0)
    }
}

#[serde_as]
#[derive(Deserialize)]
struct Foo {
    #[serde_as(deserialize_as = "HashMap<i32, Test>")]
    pub bar: HashMap<i32, i32>,
}

It fails with cryptic message:

error[E0277]: the trait bound `HashMap<i32, Test>: DeserializeAs<'_, _>` is not satisfied
   --> src/lib.rs:19:1
    |
19  | #[serde_as]
    | ^^^^^^^^^^^ the trait `DeserializeAs<'_, _>` is not implemented for `HashMap<i32, Test>`
    |
    = help: the following other types implement trait `DeserializeAs<'de, T>`:
              <HashMap<KAs, VAs> as DeserializeAs<'de, BTreeSet<(K, V)>>>
              <HashMap<KAs, VAs> as DeserializeAs<'de, BinaryHeap<(K, V)>>>
              <HashMap<KAs, VAs> as DeserializeAs<'de, HashSet<(K, V)>>>
              <HashMap<KAs, VAs> as DeserializeAs<'de, LinkedList<(K, V)>>>
              <HashMap<KAs, VAs> as DeserializeAs<'de, Vec<(K, V)>>>
              <HashMap<KAs, VAs> as DeserializeAs<'de, VecDeque<(K, V)>>>
              <HashMap<KAs, VAs> as DeserializeAs<'de, [(K, V); N]>>
              <HashMap<KAs, VAs> as DeserializeAs<'de, std::option::Option<(K, V)>>>
              <HashMap<KU, VU, S> as DeserializeAs<'de, HashMap<K, V, S>>>
note: required by a bound in `As::<T>::deserialize`
   --> /Users/pzixel/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_with-1.14.0/src/lib.rs:442:12
    |
442 |         T: DeserializeAs<'de, I>,
    |            ^^^^^^^^^^^^^^^^^^^^^ required by this bound in `As::<T>::deserialize`
    = note: this error originates in the attribute macro `serde_as` (in Nightly builds, run with -Z macro-backtrace for more info)

Note the line <HashMap<KU, VU, S> as DeserializeAs<'de, HashMap<K, V, S>>>.
So if we constraint both hashmap args with Test:

#[serde_as]
#[derive(Deserialize)]
struct Foo {
    #[serde_as(deserialize_as = "HashMap<Test, Test>")]
    pub bar: HashMap<i32, i32>,
}

Then it gets compiled as expected. For now I implemented a thin wrapper that is only used to fool this constraint but I think it's overly conservative. Users definitely should be able to deserialize HashMap<SomeStandardType, SomeWrapperType>

[Pzixel]

Seems like duplicate a to #202 . Would be very happy to have this implemented. Least effort solution may be implementing SerdeIdproxy type and documenting its usage in docs.

[jonasbb]

The correct attribute is #[serde_as(deserialize_as = "HashMap<_, Test>")], with _ indicating that you want to use the Deserialize implementation of the type.
Otherwise, yes that is a duplicate of #202. PRs are welcome. I am however not aware of any way the current trait system could be used to implement this.

I don't know what your suggested SerdeId is supposed to do. Is it identical to serde_with::Same?

[Pzixel]

Yes, I believe Same is what I was looking for. I wasn't also aware of _ syntax, I think docs would benefit from explicitly stating what is it and how to use it. Now when you explained it I can see them in arrays section but it's not particularly obvious so I think having a specific section about it would be nice.