"missing optimized MIR" due to privacy violation, via associated type equality constraint in TAIT

This bug is a variation of https://github.com/rust-lang/rust/issues/151284. cc @petrochenkov

I tried this code:

src/dep/lib.rs

#![feature(type_alias_impl_trait)]

struct Priv;
struct Middle;

// Priv -> Middle
pub trait Chain {
    type AssocChain: GetUnreachable;
}
impl Chain for Priv {
    type AssocChain = Middle;
}

// Middle -> Unreachable
pub trait GetUnreachable {
    type AssocUnreachable;
}
impl GetUnreachable for Middle {
    type AssocUnreachable = m::Unreachable;
}

type Opaque = impl Chain<AssocChain: GetUnreachable<AssocUnreachable = m::Unreachable>>;
#[define_opaque(Opaque)]
const _: Opaque = Priv;

// This trait is used to mention Opaque without mentioning Priv
pub trait GetOpaque<T> {}
impl GetOpaque<Opaque> for i32 {}

mod m {
    pub struct Unreachable;
    impl Unreachable {
        #[expect(dead_code)]
        pub fn exploit() {
            println!("huh");
        }
    }
}

src/main.rs

use dep::{Chain, GetOpaque, GetUnreachable};

fn main() {
    <i32 as Access<_>>::Goal::exploit();
}

trait Access<T> {
    type Goal;
}

impl<T: Chain, U: GetOpaque<T>> Access<T> for U {
    type Goal = <<T as Chain>::AssocChain as GetUnreachable>::AssocUnreachable;
}

Compiling the above code resulted in the following error:

error: missing optimized MIR for `dep::m::Unreachable::exploit` in the crate `dep`
   |
note: missing optimized MIR for this item (was the crate `dep` compiled with `--emit=metadata`?)
  --> dep\src\lib.rs:34:9
   |
34 |         pub fn exploit() {
   |         ^^^^^^^^^^^^^^^^

I assume that this error isn't supposed to happen for normal cargo usage.

The use of the only-one-trait-impl rule to access a private type is from https://github.com/rust-lang/rust/issues/151115

Meta

rustc --version --verbose:

rustc 1.95.0-nightly (a293cc4af 2026-01-30)
binary: rustc
commit-hash: a293cc4af8b26701c42738381c0c6f9d2ba881e0
commit-date: 2026-01-30
host: x86_64-pc-windows-msvc
release: 1.95.0-nightly
LLVM version: 22.1.0

Start by compiling the two-file reproducer in src/dep/lib.rs and src/main.rs with the reported nightly toolchain, confirming the missing optimized MIR error. Then trace the compiler handling for type_alias_impl_trait, associated type equality constraints, and privacy across the dependency boundary. Done means the reproducer compiles normally without exposing the private type or producing the missing-MIR error.