The target of derive(CoercePointee) macro has inadmissible specification for a meaningful use.

Erroneous code examples:

The target data is not a struct.

#![feature(coerce_pointee)]
use std::marker::CoercePointee;
#[derive(CoercePointee)]
enum NotStruct<'a, T: ?Sized> {
    Variant(&'a T),
}

The target data has a layout that is not transparent, or repr(transparent) in other words.

#![feature(coerce_pointee)]
use std::marker::CoercePointee;
#[derive(CoercePointee)]
struct NotTransparent<'a, #[pointee] T: ?Sized> {
    ptr: &'a T,
}

The target data has no data field.

#![feature(coerce_pointee)]
use std::marker::CoercePointee;
#[derive(CoercePointee)]
#[repr(transparent)]
struct NoField<'a, #[pointee] T: ?Sized> {}

The target data is not generic over any data, or has no generic type parameter.

#![feature(coerce_pointee)]
use std::marker::CoercePointee;
#[derive(CoercePointee)]
#[repr(transparent)]
struct NoGeneric<'a>(&'a u8);

The target data has multiple generic type parameters, but none is designated as a pointee for coercion.

#![feature(coerce_pointee)]
use std::marker::CoercePointee;
#[derive(CoercePointee)]
#[repr(transparent)]
struct AmbiguousPointee<'a, T1: ?Sized, T2: ?Sized> {
    a: (&'a T1, &'a T2),
}

The target data has multiple generic type parameters that are designated as pointees for coercion.

#![feature(coerce_pointee)]
use std::marker::CoercePointee;
#[derive(CoercePointee)]
#[repr(transparent)]
struct TooManyPointees<
    'a,
    #[pointee] A: ?Sized,
    #[pointee] B: ?Sized>
((&'a A, &'a B));

The type parameter that is designated as a pointee is not marked ?Sized.

#![feature(coerce_pointee)]
use std::marker::CoercePointee;
#[derive(CoercePointee)]
#[repr(transparent)]
struct NoMaybeSized<'a, #[pointee] T> {
    ptr: &'a T,
}

In summary, the CoercePointee macro demands the type to be a struct that is generic over at least one type or over more types, one of which is marked with #[pointee], and has at least one data field and adopts a repr(transparent) layout. The only generic type or the type marked with #[pointee] has to be also marked as ?Sized.