Lint when `serde(flatten)` causes a name collision or "shadowed" field

What it does

When using [serde(flatten)] with derive(serde::Deserialize), if there's a name collision between fields on the "outer" struct and the flattened struct, then, during deserialization, only the field on the "outer" struct is set.

Playground example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=046c25618923ffdb3194ce7cadaecb0f

Advantage

Avoid accidentally using default values in deserialization

Drawbacks

The obvious one is that this is, as far as I can tell, overly specific, applying only to serde(flatten).

Possibly serde_derive itself could be augmented to make this a hard error.

Example

struct Outer {
    #[serde(flatten)]
    inner: Inner,

    data: String,
}

struct Inner {
    data: String
    ...
}

Could be written as:

struct Outer {
    #[serde(flatten)]
    inner: Inner,
}

// < Inner would not change >

or

struct Outer {
    #[serde(flatten)]
    inner: Inner,

    data: String,
}

struct Inner {
    ...
}

The issue names no repository file or test. Start by reproducing the linked Rust Playground example with serde(flatten) and derive(serde::Deserialize), then inspect how Clippy lints are structured. Done means a lint detects the outer and flattened-field name collision and avoids false positives for the alternatives described.