Selectable's check_for_backend ignores deserialize_as attribute

[Ten0]

This causes it to generate a _check_field_compatibility that verifies that the final target type matches the schema, instead of the type it will actually be deserialized in, causing unnecessary compilation errors until check_for_backend is disabled.

[weiznich]

Yes that's a bug in this code:

diesel/diesel_derives/src/selectable.rs

Line 53 in 3590184

let field_ty = to_field_ty_bound(&f.ty);

We need to call ty_for_deserialize() there instead of directly using f.ty.

[weiznich]

Closed by #3721