This is a lint for code like

struct S;
mod m {
    type A = S;
}

when it comes from a derive macro.

The code should obviously not compile because S is not in scope at its point of use, but before #51952 it compiled if the code was produced by a derive macro, and some popular crates like actix_derive, diesel_derives and palette_derive exploited this in the wild.

This deprecation lint was added in #51952 as warn-by-default.