Possible `deserialize_into()` method?

[frankmcsherry]

I'm new here, and perhaps this is old news, but I'd love to have a method

    fn deserialize_into<'a,T>(bytes: &[u8], target: T) -> Result<T>
    where 
        T: Deserialize<'a>;

with the point being that any allocations already present in target can be re-used. So, if I am deserializing a Vec<String> I might reasonably have a Vec<String> already on hand (perhaps I've just previously deserialized and processed such a thing) with a bunch of allocations ready to be populated.

This is a bit like Clone's method clone_from(&mut self, other: &T) which has some decent performance properties. Also, it is a bit analogous to Recycler (edit: specifically the recreate method) which hasn't been used in forever, but which gives about a 2x performance bump in Vec<Vec<String>> cloning.

Anyhow, maybe this is a non-goal, but it's something I'm looking at right now as a clear perf speedbump in some code (I have owned T types being dropped because I don't know how to hand the resources to bincode to use for deserialization). Also very possible that there is a clever pattern that lets bincode do this and I don't know it, in which case I apologize!

[dtolnay]

This is waiting on a bincode release, but you could implement that function today as:

// [dependencies]
// bincode = { git = "https://github.com/TyOverby/bincode" }

use serde::de::{Deserialize, DeserializeSeed, Deserializer};

fn deserialize_into<'de, T>(bytes: &'de [u8], mut target: T) -> bincode::Result<T>
where 
    T: Deserialize<'de>,
{
    struct InPlace<'a, T>(&'a mut T);

    impl<'a, 'de, T> DeserializeSeed<'de> for InPlace<'a, T>
    where
        T: Deserialize<'de>,
    {
        type Value = ();

        fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
        where
            D: Deserializer<'de>,
        {
            T::deserialize_in_place(deserializer, self.0)
        }
    }

    bincode::config().deserialize_seed(InPlace(&mut target), bytes)?;
    Ok(target)
}

[frankmcsherry]

Thank you! Let me get my head around this and I'll re-open if I can't figure out what is going on. :D