[ICE]: `should have resolved vars before calling` when transmuting generic types

[theemathas]

Code

#![feature(transmutability)]

fn foo<T, U>(x: T) -> U {
    unsafe {
        std::mem::TransmuteFrom::transmute(x)
    }
}

Possibly related to #153006

Meta

Reproducible on the playground with version 1.96.0-nightly (2026-03-10 0c68443b0a0469e4211a)

Error output

thread 'rustc' (29) panicked at /rustc-dev/3b1b0ef4d80d3117924d91352c8b6ca528708b3c/compiler/rustc_trait_selection/src/traits/structural_normalize.rs:35:9:
should have resolved vars before calling

Backtrace

stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: <rustc_infer::infer::at::At as rustc_trait_selection::traits::structural_normalize::StructurallyNormalizeExt>::structurally_normalize_term::<rustc_infer::traits::engine::ScrubbedTraitError>
   3: <rustc_trait_selection::traits::engine::ObligationCtxt>::structurally_normalize_const
   4: <rustc_trait_selection::error_reporting::TypeErrCtxt>::select_transmute_obligation_for_reporting
   5: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_selection_error
   6: <rustc_trait_selection::error_reporting::TypeErrCtxt>::report_fulfillment_errors
   7: rustc_hir_typeck::typeck_with_inspect::{closure#0}
      [... omitted 1 frame ...]
   8: <rustc_middle::ty::context::TyCtxt>::par_hir_body_owners::<rustc_hir_analysis::check_crate::{closure#2}>::{closure#0}
   9: rustc_hir_analysis::check_crate
  10: rustc_interface::passes::analysis
  11: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::SingleCache<rustc_middle::query::erase::ErasedData<[u8; 0]>>, false>
  12: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: the compiler unexpectedly panicked. This is a bug

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/playground/rustc-ice-2026-03-12T03_48_51-27.txt` to your bug report

note: rustc 1.96.0-nightly (3b1b0ef4d 2026-03-11) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [typeck] type-checking `foo`
#1 [analysis] running analysis passes on crate `playground`
end of query stack

[theemathas]

searched nightlies: from nightly-2026-02-17 to nightly-2026-03-11
regressed nightly: nightly-2026-02-20
searched commit range: c043085...7f99507
regressed commit: fbd6934

bisected with cargo-bisect-rustc v0.6.11

Host triple: aarch64-apple-darwin
Reproduce with:

cargo bisect-rustc --regress=ice

---

Regressed in the rollup in #152825, I'm guessing due to #151703. cc @zedddie, @jdonszelmann

[zedddie]

Interesting, I will look into it tomorrow