Lint to detect non `#[repr(C)]` type in `#[repr(C)]` struct

What it does

Rust doesn't have a stable ABI, to still allow the exchange of data with external code an ABI can be defined per type. This is done with the #[repr(abi)] attribute. However, (AFAIK), the attribute only affects the type itself, meaning that the user has to ensure that all fields in the type also have #[repr(abi)].

The lint should warn, if a struct has an #[repr(abi)] attribute, but uses a type which doesn't have #[repr(abi)]. Generics should probably be ignored, as we can't check for those ABIs.

Lint Name

field_without_repr

Category

restriction

Advantage

Safe communication with other code

Drawbacks

1. For external types, it's hard to impossible to create a suggestion.
2. There are reasons to have #[repr(C)] on a type, but not on each field. This will therefore create FPs
3. Maybe a lot of lint triggers

These drawbacks are fine if the lint is in the lint is in the restriction category

Example

// This is fine
#[repr(C)]
struct Magic {
    data: u32
}

// This can cause problems
#[repr(C)]
struct Wizzard {
    // `String` is not `#[repr(C)]`
    name: String,
}

No files, tests, or entry points are named in the issue. Start by reviewing the proposed field_without_repr behavior, including generic fields and external types; the work is done when the restriction lint reliably identifies non-#[repr(abi)] fields in #[repr(abi)] structs without requiring suggestions.