Minimized reproducer:

producer.rs (compiled with rustc producer.rs --crate-type=lib):

#![feature(type_alias_impl_trait)]

pub const K: Opaque = 12;

pub type Opaque = impl Trait;

pub trait Trait {}
impl Trait for i32 {}

consumer.rs (documented with rustdoc consumer.rs --edition=2021 --extern=producer=libproducer.rlib):

pub use producer::*;

Rendered output in doc/consumer/constant.K.html (notice the transmute part):

pub const K: impl Trait = 12; // {transmute(0x0000000c): producer::Opaque}

@rustbot label C-bug T-rustdoc F-type_alias_impl_trait A-cross-crate-reexports