Self does not work in a field with serialize_with

[dtolnay]

Closely related to #1506, #1565, #1830.

use serde::{Serialize, Serializer};

#[derive(Serialize)]
struct Struct {
    #[serde(serialize_with = "vec_first_element")]
    vec: Vec<Self>,  // works if written as Vec<Struct>
}

fn vec_first_element<S>(vec: &Vec<Struct>, serializer: S) -> Result<S::Ok, S::Error>
where
    S: Serializer,
{
    vec.first().serialize(serializer)
}

error[E0308]: mismatched types
 --> src/main.rs:3:10
  |
3 | #[derive(Serialize)]
  |          ^^^^^^^^^ expected struct `__SerializeWith`, found struct `Struct`
  |
  = note: expected reference `&Vec<__SerializeWith<'_>>`
             found reference `&Vec<Struct>`
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0308]: mismatched types
 --> src/main.rs:3:10
  |
3 | #[derive(Serialize)]
  |          ^^^^^^^^^ expected struct `Struct`, found struct `__SerializeWith`
  |
  = note: expected reference `&Vec<Struct>`
             found reference `&'__a Vec<__SerializeWith<'__a>>`
  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)