ICE: `failed to resolve instance for <i32 as Baz>::c: Err(ErrorGuaranteed(()))`

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -Zmir-opt-level=5 -Zvalidate-mir
trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {}

trait Bar: Foo {
    fn w(&self) -> i32 {
        21
    }
}

trait Baz: Bar {}

impl Baz for i32 {}

fn main() {
    let baz: &dyn Baz = &1;

    let foo: &dyn Foo = baz;
}

original code

original:

//@ run-pass

#![feature(trait_upcasting)]

trait Foo: PartialEq<i32> + std::fmt::Debug + Send + Sync {
    fn a(&self) -> i32 {
        10
    }

    fn z(&self) -> i32 {
        11
    }

    fn y(&self) -> i32 {
        12
    }
}

trait Bar: Foo {
    fn b(&self) -> i32 {
        20
    }

    fn w(&self) -> i32 {
        21
    }
}

trait Baz: Bar {
    fn c(&self) -> i32 {
        30
    }
}

impl Foo for i32 {
    fn a(&self) -> i32 {
        100
    }
}

impl Bar for i32 {
    fn b(&self) -> i32 {
        200
    }
}

impl Baz for i32 {
    fn c(&<() as Identity>::Selff) -> i32 {
        300
    }
}

fn main() {
    let baz: &dyn Baz = &1;
    let _: &dyn std::fmt::Debug = baz;
    assert_eq!(*baz, 1);
    assert_eq!(baz.a(), 100);
    assert_eq!(baz.b(), 200);
    assert_eq!(baz.c(), 300);
    assert_eq!(baz.z(), 11);
    assert_eq!(baz.y(), 12);
    assert_eq!(baz.w(), 21);

    let bar: &dyn Bar = baz;
    let _: &dyn std::fmt::Debug = bar;
    assert_eq!(*bar, 1);
    assert_eq!(bar.a(), 100);
    assert_eq!(bar.b(), 200);
    assert_eq!(bar.z(), 11);
    assert_eq!(bar.y(), 12);
    assert_eq!(bar.w(), 21);

    let foo: &dyn Foo = baz;
    let _: &dyn std::fmt::Debug = foo;
    assert_eq!(*foo, 1);
    assert_eq!(foo.a(), 100);
    assert_eq!(foo.z(), 11);
    assert_eq!(foo.y(), 12);

    let foo: &dyn Foo = bar;
    let _: &dyn std::fmt::Debug = foo;
    assert_eq!(*foo, 1);
    assert_eq!(foo.a(), 100);
    assert_eq!(foo.z(), 11);
    assert_eq!(foo.y(), 12);
}

Version information

rustc 1.86.0-nightly (9e48dfe5e 2025-02-01)
binary: rustc
commit-hash: 9e48dfe5e0d5ec1b064b5d25504948ede0ec9b53
commit-date: 2025-02-01
host: x86_64-unknown-linux-gnu
release: 1.86.0-nightly
LLVM version: 19.1.7

Possibly related line of code:

rust/compiler/rustc_middle/src/ty/instance.rs

Lines 597 to 609 in 9e48dfe

	shrunk,
	was_written,
	path,
	type_length,
	});
	} else {
	span_bug!(
	span_or_local_def_span(),
	"failed to resolve instance for {}",
	tcx.def_path_str_with_args(def_id, args)
	)
	}
	}

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zmir-opt-level=5 -Zvalidate-mir

Program output

error[E0277]: the trait bound `i32: Bar` is not satisfied
  --> /tmp/icemaker_global_tempdir.l3aOyLMWnNj0/rustc_testrunner_tmpdir_reporting.66VcXZBzeNme/mvce.rs:11:14
   |
11 | impl Baz for i32 {}
   |              ^^^ the trait `Bar` is not implemented for `i32`
   |
help: this trait has no implementations, consider adding one
  --> /tmp/icemaker_global_tempdir.l3aOyLMWnNj0/rustc_testrunner_tmpdir_reporting.66VcXZBzeNme/mvce.rs:3:1
   |
3  | trait Bar: Foo {
   | ^^^^^^^^^^^^^^
note: required by a bound in `Baz`
  --> /tmp/icemaker_global_tempdir.l3aOyLMWnNj0/rustc_testrunner_tmpdir_reporting.66VcXZBzeNme/mvce.rs:9:12
   |
9  | trait Baz: Bar {}
   |            ^^^ required by this bound in `Baz`

error[E0658]: cannot cast `dyn Baz` to `dyn Foo`, trait upcasting coercion is experimental
  --> /tmp/icemaker_global_tempdir.l3aOyLMWnNj0/rustc_testrunner_tmpdir_reporting.66VcXZBzeNme/mvce.rs:16:25
   |
16 |     let foo: &dyn Foo = baz;
   |                         ^^^
   |
   = note: see issue #65991 <https://github.com/rust-lang/rust/issues/65991> for more information
   = help: add `#![feature(trait_upcasting)]` to the crate attributes to enable
   = note: this compiler was built on 2025-02-01; consider upgrading it if it is out of date
   = note: required when coercing `&dyn Baz` into `&dyn Foo`

warning: unused variable: `foo`
  --> /tmp/icemaker_global_tempdir.l3aOyLMWnNj0/rustc_testrunner_tmpdir_reporting.66VcXZBzeNme/mvce.rs:16:9
   |
16 |     let foo: &dyn Foo = baz;
   |         ^^^ help: if this is intentional, prefix it with an underscore: `_foo`
   |
   = note: `#[warn(unused_variables)]` on by default

error: internal compiler error: compiler/rustc_middle/src/ty/instance.rs:603:21: failed to resolve instance for <i32 as Bar>::w
 --> /tmp/icemaker_global_tempdir.l3aOyLMWnNj0/rustc_testrunner_tmpdir_reporting.66VcXZBzeNme/mvce.rs:4:5
  |
4 |     fn w(&self) -> i32 {
  |     ^^^^^^^^^^^^^^^^^^


thread 'rustc' panicked at compiler/rustc_middle/src/ty/instance.rs:603:21:
Box<dyn Any>
stack backtrace:
   0:     0x7dc2d5a29bb0 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::h3b2667ac0fa900ee
   1:     0x7dc2d6213926 - core::fmt::write::h1e20c49f4105e8cb
   2:     0x7dc2d7552191 - std::io::Write::write_fmt::hcd547efb96ce3ce8
   3:     0x7dc2d5a29a12 - std::sys::backtrace::BacktraceLock::print::h01ae5c2f80bbe6a2
   4:     0x7dc2d5a2be92 - std::panicking::default_hook::{{closure}}::h38590813ec9a77f4
   5:     0x7dc2d5a2bd1a - std::panicking::default_hook::he0005f0d1786b50d
   6:     0x7dc2d4b85069 - std[a0d1a3d0cea39bfd]::panicking::update_hook::<alloc[7f35e0ab9d09dc5c]::boxed::Box<rustc_driver_impl[2c04243663399a5d]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7dc2d5a2c9d3 - std::panicking::rust_panic_with_hook::h2d8aa917e3df8004
   8:     0x7dc2d4bbfdc1 - std[a0d1a3d0cea39bfd]::panicking::begin_panic::<rustc_errors[ea6c319454a86de8]::ExplicitBug>::{closure#0}
   9:     0x7dc2d4bb4ca6 - std[a0d1a3d0cea39bfd]::sys::backtrace::__rust_end_short_backtrace::<std[a0d1a3d0cea39bfd]::panicking::begin_panic<rustc_errors[ea6c319454a86de8]::ExplicitBug>::{closure#0}, !>
  10:     0x7dc2d4bb4c8d - std[a0d1a3d0cea39bfd]::panicking::begin_panic::<rustc_errors[ea6c319454a86de8]::ExplicitBug>
  11:     0x7dc2d4bc9d11 - <rustc_errors[ea6c319454a86de8]::diagnostic::BugAbort as rustc_errors[ea6c319454a86de8]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7dc2d512df4c - <rustc_errors[ea6c319454a86de8]::DiagCtxtHandle>::span_bug::<rustc_span[b8e136aab0dfb1f0]::span_encoding::Span, alloc[7f35e0ab9d09dc5c]::string::String>
  13:     0x7dc2d51af667 - rustc_middle[1fcb647e943a03f5]::util::bug::opt_span_bug_fmt::<rustc_span[b8e136aab0dfb1f0]::span_encoding::Span>::{closure#0}
  14:     0x7dc2d51985da - rustc_middle[1fcb647e943a03f5]::ty::context::tls::with_opt::<rustc_middle[1fcb647e943a03f5]::util::bug::opt_span_bug_fmt<rustc_span[b8e136aab0dfb1f0]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  15:     0x7dc2d519846b - rustc_middle[1fcb647e943a03f5]::ty::context::tls::with_context_opt::<rustc_middle[1fcb647e943a03f5]::ty::context::tls::with_opt<rustc_middle[1fcb647e943a03f5]::util::bug::opt_span_bug_fmt<rustc_span[b8e136aab0dfb1f0]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  16:     0x7dc2d3b11667 - rustc_middle[1fcb647e943a03f5]::util::bug::span_bug_fmt::<rustc_span[b8e136aab0dfb1f0]::span_encoding::Span>
  17:     0x7dc2d6937ad1 - <rustc_middle[1fcb647e943a03f5]::ty::instance::Instance>::expect_resolve
  18:     0x7dc2d6e87095 - <rustc_middle[1fcb647e943a03f5]::ty::instance::Instance>::expect_resolve_for_vtable
  19:     0x7dc2d69ea8fe - rustc_trait_selection[5ad581e41908d580]::traits::vtable::vtable_entries::{closure#0}
  20:     0x7dc2d716317d - rustc_trait_selection[5ad581e41908d580]::traits::vtable::vtable_entries
  21:     0x7dc2d7162ea0 - rustc_query_impl[ecbf26680a1b845f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ecbf26680a1b845f]::query_impl::vtable_entries::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1fcb647e943a03f5]::query::erase::Erased<[u8; 16usize]>>
  22:     0x7dc2d7162e5f - <rustc_query_impl[ecbf26680a1b845f]::query_impl::vtable_entries::dynamic_query::{closure#2} as core[efe334e59b562b4]::ops::function::FnOnce<(rustc_middle[1fcb647e943a03f5]::ty::context::TyCtxt, rustc_type_ir[ec807296d52f2592]::predicate::TraitRef<rustc_middle[1fcb647e943a03f5]::ty::context::TyCtxt>)>>::call_once
  23:     0x7dc2d7161c27 - rustc_query_system[4b092dbe7cee5ac2]::query::plumbing::try_execute_query::<rustc_query_impl[ecbf26680a1b845f]::DynamicConfig<rustc_query_system[4b092dbe7cee5ac2]::query::caches::DefaultCache<rustc_type_ir[ec807296d52f2592]::predicate::TraitRef<rustc_middle[1fcb647e943a03f5]::ty::context::TyCtxt>, rustc_middle[1fcb647e943a03f5]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[ecbf26680a1b845f]::plumbing::QueryCtxt, false>
  24:     0x7dc2d716192f - rustc_query_impl[ecbf26680a1b845f]::query_impl::vtable_entries::get_query_non_incr::__rust_end_short_backtrace
  25:     0x7dc2d52e1d93 - <rustc_const_eval[84a83e4b3b990e9e]::interpret::eval_context::InterpCx<rustc_const_eval[84a83e4b3b990e9e]::const_eval::dummy_machine::DummyMachine>>::vtable_entries
  26:     0x7dc2d7c62e51 - <rustc_const_eval[84a83e4b3b990e9e]::interpret::eval_context::InterpCx<rustc_const_eval[84a83e4b3b990e9e]::const_eval::dummy_machine::DummyMachine>>::unsize_into_ptr.cold
  27:     0x7dc2d6d44f97 - <rustc_mir_transform[519422bbe054fe1f]::gvn::VnState>::insert
  28:     0x7dc2d6d3e9ab - <rustc_mir_transform[519422bbe054fe1f]::gvn::VnState>::simplify_rvalue::{closure#0}
  29:     0x7dc2d3a3fb4d - <rustc_mir_transform[519422bbe054fe1f]::gvn::GVN as rustc_mir_transform[519422bbe054fe1f]::pass_manager::MirPass>::run_pass
  30:     0x7dc2d62044a2 - rustc_mir_transform[519422bbe054fe1f]::pass_manager::run_passes_inner
  31:     0x7dc2d675f336 - rustc_mir_transform[519422bbe054fe1f]::optimized_mir
  32:     0x7dc2d675eb27 - rustc_query_impl[ecbf26680a1b845f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ecbf26680a1b845f]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1fcb647e943a03f5]::query::erase::Erased<[u8; 8usize]>>
  33:     0x7dc2d648a3df - rustc_query_system[4b092dbe7cee5ac2]::query::plumbing::try_execute_query::<rustc_query_impl[ecbf26680a1b845f]::DynamicConfig<rustc_query_system[4b092dbe7cee5ac2]::query::caches::DefIdCache<rustc_middle[1fcb647e943a03f5]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[ecbf26680a1b845f]::plumbing::QueryCtxt, false>
  34:     0x7dc2d64897df - rustc_query_impl[ecbf26680a1b845f]::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
  35:     0x7dc2d30cd644 - <rustc_middle[1fcb647e943a03f5]::ty::context::TyCtxt>::instance_mir
  36:     0x7dc2d65a6ac1 - rustc_interface[2a4b200aeca1f4ad]::passes::run_required_analyses
  37:     0x7dc2d6d1021e - rustc_interface[2a4b200aeca1f4ad]::passes::analysis
  38:     0x7dc2d6d101ef - rustc_query_impl[ecbf26680a1b845f]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[ecbf26680a1b845f]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1fcb647e943a03f5]::query::erase::Erased<[u8; 0usize]>>
  39:     0x7dc2d71d9895 - rustc_query_system[4b092dbe7cee5ac2]::query::plumbing::try_execute_query::<rustc_query_impl[ecbf26680a1b845f]::DynamicConfig<rustc_query_system[4b092dbe7cee5ac2]::query::caches::SingleCache<rustc_middle[1fcb647e943a03f5]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[ecbf26680a1b845f]::plumbing::QueryCtxt, false>
  40:     0x7dc2d71d95ce - rustc_query_impl[ecbf26680a1b845f]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  41:     0x7dc2d71a8d2b - rustc_interface[2a4b200aeca1f4ad]::passes::create_and_enter_global_ctxt::<core[efe334e59b562b4]::option::Option<rustc_interface[2a4b200aeca1f4ad]::queries::Linker>, rustc_driver_impl[2c04243663399a5d]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  42:     0x7dc2d718a733 - rustc_interface[2a4b200aeca1f4ad]::interface::run_compiler::<(), rustc_driver_impl[2c04243663399a5d]::run_compiler::{closure#0}>::{closure#1}
  43:     0x7dc2d70d0935 - std[a0d1a3d0cea39bfd]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[2a4b200aeca1f4ad]::util::run_in_thread_with_globals<rustc_interface[2a4b200aeca1f4ad]::util::run_in_thread_pool_with_globals<rustc_interface[2a4b200aeca1f4ad]::interface::run_compiler<(), rustc_driver_impl[2c04243663399a5d]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  44:     0x7dc2d70d0619 - <<std[a0d1a3d0cea39bfd]::thread::Builder>::spawn_unchecked_<rustc_interface[2a4b200aeca1f4ad]::util::run_in_thread_with_globals<rustc_interface[2a4b200aeca1f4ad]::util::run_in_thread_pool_with_globals<rustc_interface[2a4b200aeca1f4ad]::interface::run_compiler<(), rustc_driver_impl[2c04243663399a5d]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[efe334e59b562b4]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  45:     0x7dc2d70cfdaf - std::sys::pal::unix::thread::Thread::new::thread_start::h1c3d68b5dbee7695
  46:     0x7dc2d12a339d - <unknown>
  47:     0x7dc2d132849c - <unknown>
  48:                0x0 - <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: rustc 1.86.0-nightly (9e48dfe5e 2025-02-01) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z mir-opt-level=5 -Z validate-mir -Z dump-mir-dir=dir

query stack during panic:
#0 [vtable_entries] finding all vtable entries for trait `Baz`
#1 [optimized_mir] optimizing MIR for `main`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 3 previous errors; 1 warning emitted

Some errors have detailed explanations: E0277, E0658.
For more information about an error, try `rustc --explain E0277`.

@rustbot label +F-trait_upcasting +-Zvalidate-mir

[matthiaskrgr]

bisects to #135318

[matthiaskrgr]

-Zmir-enable-passes=+GVN -Zvalidate-mir are the relevant flags

[matthiaskrgr]

//@ compile-flags: -Zmir-enable-passes=+GVN -Zvalidate-mir
#![feature(trait_upcasting)]
trait Foo: PartialEq<i32> {}

trait Bar: Foo {
    fn w(&self) -> i32 {
        todo!();
    }
}

trait Baz: Bar {}

impl Baz for i32 {}
impl Foo for i32 {}
//impl Bar for i32 {}  // uncommenting this fixes the ICE



fn main() {
    let baz: &dyn Baz = &1;
    let foo: &dyn Foo = baz;
}

[matthiaskrgr]

woops, I messed up the minimisation, here's a snipped that retains the ErrorGuaranteed

//@ compile-flags -Zvalidate-mir -Zmir-enable-passes=+GVN
#![feature(trait_upcasting)]

trait A {}
trait B: A {
    fn c(&self);
}
impl B for i32 {
    fn c(self) {
        todo!();
    }
}

fn main() {
    let baz: &dyn B = &1;
    let bar: &dyn A = baz;
}

[compiler-errors]

Pretty sure this always was an ICE with debug assertions. All that #135318 likely did was remove the debug assertion gating.

[matthiaskrgr]

Hmm...
I know that I'v been seeing various "failed to resolve instance" ICEs for some time now so that particular code point was already reachable before without debug assertions.
Looking at https://github.com/rust-lang/rust/pull/135318/files there were some un-debug-assertion-isation in vtables() (which does not show up in the backtrace) and in unsize_into_ptr() so it's probably this one?