rust-lang/rust · error

expected `Session`

Error message

expected `Session`

What it means

lib.rs:1241: in a delayed-lint callback, the compiler downcasts a &dyn std::any::Any to rustc_session::Session via .expect("expected `Session`"). The invariant: the opaque value stored in the DelayedLint is always the compiler Session. A panic here means a delayed lint was registered with a non-Session Any value, a compiler-internal contract violation.

Source

Thrown at compiler/rustc_ast_lowering/src/lib.rs:1241

        target_hir_id: HirId,
        target: Target,
    ) -> Vec<hir::Attribute> {
        let l = self.span_lowerer();
        self.attribute_parser.parse_attribute_list(
            attrs,
            target_span,
            target,
            OmitDoc::Lower,
            |s| l.lower(s),
            |lint_id, span, kind| {
                self.delayed_lints.push(DelayedLint {
                    lint_id,
                    id: target_hir_id,
                    span,
                    callback: Box::new(move |dcx, level, sess: &dyn std::any::Any| {
                        let sess = sess
                            .downcast_ref::<rustc_session::Session>()
                            .expect("expected `Session`");
                        (kind.0)(dcx, level, sess)
                    }),
                });
            },
        )
    }

    fn alias_attrs(&mut self, id: HirId, target_id: HirId) {
        assert_eq!(id.owner, self.current_hir_id_owner);
        assert_eq!(target_id.owner, self.current_hir_id_owner);
        if let Some(&a) = self.attrs.get(&target_id.local_id) {
            assert!(!a.is_empty());
            self.attrs.insert(id.local_id, a);
        }
    }

    fn lower_delim_args(&self, args: &DelimArgs) -> DelimArgs {
        args.clone()

View on GitHub (pinned to 7088e4b63a)

Solutions

1. File a rustc ICE report; this is a compiler-internal contract violation.
2. Bisect the regression with cargo bisect-rustc.
3. Try stable vs nightly to see if it is a recent regression.
4. Reduce the triggering crate (often one that exercises rare lint paths) and attach it to the issue.

When it happens

Trigger: Internal compiler code that pushes a DelayedLint whose callback receives an Any that is not a Session; cannot be triggered by normal user source.

Common situations: A regression in the lint-delay mechanism during compiler development; not reachable from ordinary Rust programs.

Related errors

• alt layout should always work
• alt layout should have a niche like the regular one
• `Self` generic param is not found while expected
• must be at least one segment
• must contain self type as `SelfTy` propagation kind is speci

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/6f4bb4a963c96149. Report an issue: GitHub.