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

[theemathas]

This bug is a variation of #151284. cc @petrochenkov

I tried this code:

src/dep/lib.rs

struct Priv;

pub trait Super {
    type AssocSuper: GetUnreachable;
}
#[expect(private_bounds)]
pub trait Sub: Super<AssocSuper = Priv> {}

// This Dummy type is only used in call_handler
struct Dummy;
impl Super for Dummy {
    type AssocSuper = Priv;
}
impl Sub for Dummy {}

pub trait SubHandler {
    fn handle<T: Sub>();
}
pub fn call_handler<T: SubHandler>() {
    <T as SubHandler>::handle::<Dummy>();
}

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, Sub, SubHandler, Super, call_handler};

fn main() {
    call_handler::<Handler>();
}

struct Handler;
impl SubHandler for Handler {
    fn handle<T: Sub>() {
        <T as Access>::AccessAssoc::generic::<i32>();
    }
}

// Without this indirection, Handler::handle notices that
// it's mentioning dep::Priv.
trait Access: Super {
    type AccessAssoc;
}
impl<T: Super> Access for T {
    type AccessAssoc = <<T as Super>::AssocSuper 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:30:9
   |
30 |         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 1.95.0-nightly (eda76d9d1 2026-01-21)
binary: rustc
commit-hash: eda76d9d1d133effbf7facb28168fd78d75fd434
commit-date: 2026-01-21
host: aarch64-apple-darwin
release: 1.95.0-nightly
LLVM version: 21.1.8

[petrochenkov]

Added this test case to #151168 as well.