Can't find a way to repeat value

[zoryamba]

Hi everyone. Have to say, that I'm pretty new to Rust, so my question may be quite amateur. I'm trying to implement serde for zipson format. You can find details here: https://www.npmjs.com/package/zipson There is repeat feature, þ char means repeat last item. So that |¨aaa¨þþ÷ should be parsed as ["aaa", "aaa", "aaa"].

The most obvious way is to make it in SeqAccess implementation, so i desided to store last_elem ref in my SeqAccess and return it in case I see þ char.

struct SeqAccess<'a, 'de: 'a> {
    de: &'a mut Deserializer<'de>,
    last_element: Value,
}

impl<'a, 'de: 'a> SeqAccess<'a, 'de> {
    fn new(de: &'a mut Deserializer<'de>) -> Self {
        SeqAccess {
            de,
            last_element: Value::Undefined
        }
    }
}

impl<'de, 'a> de::SeqAccess<'de> for SeqAccess<'a, 'de> {
    type Error = Error;

    fn next_element_seed<T>(&mut self, seed: T) -> std::result::Result<Option<T::Value>, Self::Error>
        where T: DeserializeSeed<'de>
    {
        match self.de.peek_char()? {
            ARRAY_END_TOKEN => {
                self.de.next_char()?;
                Ok(None)
            }
            ARRAY_REPEAT_TOKEN => {
                self.de.next_char()?;
                Ok(Some(self.last_element.clone()))
            }
            _ => Ok(Some(seed.deserialize(&mut *self.de)?))
        }
    }
}

Unfortunately I'm getting compillation error:

error[E0308]: mismatched types
   --> src/de.rs:661:25
    |
661 |                 Ok(Some(self.last_element.clone()))
    |                    ---- ^^^^^^^^^^^^^^^^^^^^^^^^^ expected associated type, found `Value`
    |                    |
    |                    arguments to this enum variant are incorrect
    |
    = note: expected associated type `<T as DeserializeSeed<'de>>::Value`
                          found enum `Value`
help: the type constructed contains `Value` due to the type of the argument passed
   --> src/de.rs:661:20
    |
661 |                 Ok(Some(self.last_element.clone()))
    |                    ^^^^^-------------------------^
    |                         |
    |                         this argument influences the type of `Some`
note: tuple variant defined here
   --> /Users/zoryamba/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/option.rs:571:5
    |
571 |     Some(#[stable(feature = "rust1", since = "1.0.0")] T),
    |     ^^^^
help: consider constraining the associated type `<T as DeserializeSeed<'de>>::Value` to `Value`
    |
652 |         where T: DeserializeSeed<'de, Value = Value>
    |                                     +++++++++++++++

Tried to constrain Value for DeserializeSeed:

    fn next_element_seed<T>(&mut self, seed: T) -> std::result::Result<Option<T::Value>, Self::Error>
        where T: DeserializeSeed<'de, Value = Value>

It also didn't help:

error[E0271]: type mismatch resolving `<T as DeserializeSeed<'de>>::Value == Value`
    --> src/de.rs:652:39
     |
652  |         where T: DeserializeSeed<'de, Value = Value>
     |                                       ^^^^^^^^^^^^^ expected associated type, found `Value`
     |
     = note: expected associated type `<T as DeserializeSeed<'de>>::Value`
                           found enum `Value`
note: the requirement `<T as DeserializeSeed<'de>>::Value == Value` appears on the `impl`'s method `next_element_seed` but not on the corresponding trait's method
    --> /Users/zoryamba/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.188/src/de/mod.rs:1712:8
     |
1702 | pub trait SeqAccess<'de> {
     |           --------- in this trait
...
1712 |     fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
     |        ^^^^^^^^^^^^^^^^^ this trait's method doesn't have the requirement `<T as DeserializeSeed<'de>>::Value == Value`

I tried to implement own DeseriealizeSeed, but it's deserialize method requires Deserializer trait:

fn deserialize<D>(self, deserializer: D) -> std::result::Result<Self::Value, D::Error>
        where D: de::Deserializer<'de>

So that i can't use my deserializer custom methods like next_char of peek_char.

So my question is: Is it possible to achieve what I want? And what is most convenient method to do that? Thank you!

[zoryamba]

As I understand is SeqAccess trait was generic itself, it would be possible to do such a trick:

pub trait SeqAccess<'de, V> {
    fn next_element_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
    where
        T: DeserializeSeed<'de Value = V>;
}

struct SeqAccess<'a, 'de: 'a, V> {
    de: &'a mut Deserializer<'de>,
    last_element: V,
}

Wouldn't that be useful?