Rust error E0789: `rustc_allowed_through_unstable_modules` attribute must be paired with a `stable` attribute

The compiler team's explanation

This error code is internal to the compiler and will not be emitted with normal Rust code.

The internal rustc_allowed_through_unstable_modules attribute must be used on an item with a stable attribute.

Erroneous code example:

// NOTE: both of these attributes are perma-unstable and should *never* be
//       used outside of the compiler and standard library.
#![feature(rustc_attrs)]
#![feature(staged_api)]
#![allow(internal_features)]

#![unstable(feature = "foo_module", reason = "...", issue = "123")]

#[rustc_allowed_through_unstable_modules = "deprecation message"]
// #[stable(feature = "foo", since = "1.0")]
struct Foo;
// ^^^ error: `rustc_allowed_through_unstable_modules` attribute must be
//            paired with a `stable` attribute

Typically when an item is marked with a stable attribute, the modules that enclose the item must also be marked with stable attributes, otherwise the item becomes *de facto* unstable. #[rustc_allowed_through_unstable_modules] is a workaround which allows an item to "escape" its unstable parent modules. This error occurs when an item is marked with #[rustc_allowed_through_unstable_modules] but no supplementary stable attribute exists. See #99288 for an example of #[rustc_allowed_through_unstable_modules] in use.

From rustc --explain E0789 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.