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

This bug was found while investigating #146470, based on the snippet in https://github.com/rust-lang/rust/pull/146470#issuecomment-3755229532. cc @petrochenkov

I tried this code:

src/dep/lib.rs

pub trait ToPriv {
    type AssocPriv;
}

pub trait PubTr {
    #[expect(private_bounds)]
    type Assoc: ToPriv<AssocPriv = Priv>;
}

struct Dummy;
struct DummyToPriv;
impl PubTr for Dummy {
    type Assoc = DummyToPriv;
}
impl ToPriv for DummyToPriv {
    type AssocPriv = Priv;
}

pub fn get_dummy() -> impl PubTr {
    Dummy
}

struct Priv;

pub trait GetUnreachable {
    type Assoc;
}

mod m {
    pub struct Unreachable;

    impl Unreachable {
        #[expect(dead_code)]
        pub fn generic<T>() {}
    }

    impl crate::GetUnreachable for crate::Priv {
        type Assoc = Unreachable;
    }
}

src/main.rs

use dep::{GetUnreachable, PubTr, ToPriv, get_dummy};

fn main() {
    wut(get_dummy());
}

fn wut<T: PubTr>(_: T) {
    <T as Access>::AccessAssoc::generic::<i32>();
}

trait Access: PubTr {
    type AccessAssoc;
}

impl<T: PubTr> Access for T {
    type AccessAssoc = <<T::Assoc as ToPriv>::AssocPriv as GetUnreachable>::Assoc;
}

Compiling the above code resulted in the following error:

error: missing optimized MIR for `dep::m::Unreachable::generic::<i32>` 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 generic<T>() {}
   |         ^^^^^^^^^^^^^^^^^^^

error: could not compile `foo` (bin "foo") due to 1 previous error

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

Meta

rustc --version --verbose:

rustc 1.94.0-nightly (31cd367b9 2026-01-08)
binary: rustc
commit-hash: 31cd367b9ca1ce359268e7adf4ea540408c0ad85
commit-date: 2026-01-08
host: x86_64-pc-windows-msvc
release: 1.94.0-nightly
LLVM version: 21.1.8

Reproduce the issue with the examples in src/dep/lib.rs and src/main.rs using the reported nightly toolchain. Start by tracing the associated-type equality and privacy handling that leads to the cross-crate call. Done means normal Cargo compilation no longer reports missing optimized MIR for dep:Ⓜ️:Unreachable::generic.