Support flatten as a container attribute

[SpriteOvO]

Let's say I have a internal private struct struct P, and I also have multiple public structs

(All these structs are #[derive(Deserialize)], I omitted it from the code below)

struct P { ... }

struct A { p: P, ... }
struct B { p: P, ... }
struct C { p: P, ... }

Since P is a private struct, I want to hide the field p from the deserialization of these public structs, so we have to write the code in this way

struct P { ... }

struct A { #[serde(flatten)] p: P, ... }
struct B { #[serde(flatten)] p: P, ... }
struct C { #[serde(flatten)] p: P, ... }

#[serde(flatten)] is being written over and over again, considering that if there are a lot of such public structs, if we forget to write the field attr somewhere, it will expose this unintended field p to the user. This is hard to discover unless we write tests for each public struct.

So if we support flatten as a container attribute, we can get rid of that worry

#[serde(flatten)]
struct P { ... }

struct A { p: P, ... }
struct B { p: P, ... }
struct C { p: P, ... }

[dtolnay]

This is not something that can be implemented in most formats. By the time any code in P's Serialize impl runs, the field name "p" would already have been emitted. Similarly P's Deserialize impl would only come into play if a "p" field is present.