rust-lang / rust-lang/rust

[ICE]: assumptions on binders: `could not find the supertrait vtable slot`

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-Zassumptions-on-binders C-bug I-ICE needs-triage T-compiler WG-trait-system-refactor
Dominant language
Rust
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Description

Code
//@compile-flags: -Zassumptions-on-binders  -Znext-solver=globally
trait Super<U> {
    fn a(&self) {
        let a: &dyn Sub = &();
        let b: &dyn Super<for<'a> fn(&'a ())> = a;
    }
}
impl<T> Super<T> for () {}
trait Sub: Super<fn(&'static ())> {}
impl Sub for () {}
fn main() {
    let a: &dyn Sub = &();
}
Meta

rustc --version --verbose:

rustc 1.98.0-nightly (4e391cf24 2026-06-12)
binary: rustc
commit-hash: 4e391cf2425cf96521af17ff460e9f220e9bca00
commit-date: 2026-06-12
host: x86_64-unknown-linux-gnu
release: 1.98.0-nightly
LLVM version: 22.1.6
Error output
warning: unused variable: `b`
 --> code.rs:4:13
  |
4 |         let b: &dyn Super<for<'a> fn(&'a ())> = a;
  |             ^ help: if this is intentional, prefix it with an underscore: `_b`
  |
  = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

warning: unused variable: `a`
  --> code.rs:11:9
   |
11 |     let a: &dyn Sub = &();
   |         ^ help: if this is intentional, prefix it with an underscore: `_a`

warning: method `a` is never used
 --> code.rs:2:8
  |
1 | trait Super<U> {
  |       ----- method in this trait
2 |     fn a(&self) {
  |        ^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: 3 warnings emitted
Backtrace 

...
warning: method `a` is never used
 --> code.rs:2:8
  |
1 | trait Super<U> {
  |       ----- method in this trait
2 |     fn a(&self) {
  |        ^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: 3 warnings emitted

note: no errors encountered even though delayed bugs were created

note: those delayed bugs will now be shown as internal compiler errors

error: internal compiler error: could not find the supertrait vtable slot for `dyn Sub` -> `dyn Super<for<'a> fn(&'a ())>`
  |
  = note: delayed at /rustc-dev/4e391cf2425cf96521af17ff460e9f220e9bca00/compiler/rustc_trait_selection/src/traits/vtable.rs:434:19
             0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
             1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
             2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
             3: <rustc_errors::DiagCtxtHandle>::delayed_bug::<alloc::string::String>
             4: rustc_trait_selection::traits::vtable::supertrait_vtable_slot
             5: rustc_query_impl::query_impl::supertrait_vtable_slot::invoke_provider_fn::__rust_begin_short_backtrace
             6: rustc_query_impl::execution::try_execute_query::<rustc_middle::query::caches::DefaultCache<(rustc_middle::ty::Ty, rustc_middle::ty::Ty), rustc_middle::query::erase::ErasedData<[u8; 16]>>, false>
             7: rustc_query_impl::query_impl::supertrait_vtable_slot::execute_query_non_incr::__rust_end_short_backtrace
             8: rustc_codegen_ssa::base::unsize_ptr::<rustc_codegen_llvm::builder::GenericBuilder<rustc_codegen_llvm::context::FullCx>>
             9: <rustc_codegen_ssa::mir::FunctionCx<rustc_codegen_llvm::builder::GenericBuilder<rustc_codegen_llvm::context::FullCx>>>::codegen_rvalue_operand
            10: rustc_codegen_ssa::mir::codegen_mir::<rustc_codegen_llvm::builder::GenericBuilder<rustc_codegen_llvm::context::FullCx>>
            11: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
            12: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::compile_codegen_unit
            13: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend, rustc_codegen_llvm::ModuleLlvm>
            14: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
            15: <rustc_interface::queries::Linker>::codegen_and_build_linker
            16: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
            17: std::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
            18: <std::thread::lifecycle::spawn_unchecked<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            19: <std::sys::thread::unix::Thread>::new::thread_start
            20: <unknown>
            21: <unknown>


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 `/home/matthias/vcs/github/CRED/rustc-ice-2026-06-13T10_56_31-4076222.txt` to your bug report

note: rustc 1.98.0-nightly (4e391cf24 2026-06-12) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z assumptions-on-binders -Z next-solver=globally

query stack during panic:
end of query stack

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First steps

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the provided example with a nightly compiler using -Zassumptions-on-binders and -Znext-solver=globally. Then inspect compiler/rustc_trait_selection/src/traits/vtable.rs around line 434 and the supertrait_vtable_slot path; done means the example no longer emits an internal compiler error.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Quiet
Clarity
Mostly clear
Newbie friendliness
42/100

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