Types used in remote derive are dead code

[dtolnay]

Remote derive structs exist only to tell serde_derive what are the fields of the real struct. As such, no code uses the fields of the remote derive struct.

#[macro_use]
extern crate serde_derive;

extern crate serde;

#[derive(Serialize)]
#[serde(remote = "S")]
struct SDef {
    a: u32,
}

struct S {
    a: u32,
}

#[derive(Serialize)]
struct W {
    #[serde(with = "SDef")]
    s: S,
}

fn main() {}

warning: field is never used: `a`
 --> src/main.rs:9:5
  |
9 |     a: u32,
  |     ^^^^^^
  |
  = note: #[warn(dead_code)] on by default

Let's emit some silly code along with the derive to use each field so the user doesn't need to put #[allow(dead_code)] on every remote derive struct.

[dtolnay]

Things to be careful about:

• Should work for non-public structs.
• Should work for structs with fields of non-public type.
• Should work for structs with generic type parameters.
• Should work for structs with type parameters bound by a non-public trait.
• Should work for structs with lifetime parameters.
• All of the above but for enums.