ICE: `there are no vtables for unsized types`

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(object_safe_for_dispatch)]
struct Vtable(dyn Cap);

trait Cap<'a> {}

union Transmute {
    t: u64,
    u: &'static Vtable,
}

const G: &Copy = unsafe { Transmute { t: 1 }.u };

original:

struct Vtable(dyn Cap);

trait Cap<'a> {}

union Transmute {
    t: u64,  
    u: &'static Vtable,
}

const G: &Copy = unsafe { Transmute { t: 1 }.u };

Version information

rustc 1.83.0-nightly (f7b4c72c8 2024-09-17)
binary: rustc
commit-hash: f7b4c72c8ffb320616bda27f70bb8e0a341673fb
commit-date: 2024-09-17
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

Command: /home/matthias/.rustup/toolchains/master/bin/rustc -Zcrate-attr=feature(object_safe_for_dispatch)

Program output

``` error[E0106]: missing lifetime specifier --> /tmp/icemaker_global_tempdir.pvcMdeDPSWCA/rustc_testrunner_tmpdir_reporting.xxjpTONUWZGm/mvce.rs:1:19 | 1 | struct Vtable(dyn Cap); | ^^^ expected named lifetime parameter | = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html help: consider making the bound lifetime-generic with a new `'a` lifetime | 1 | struct Vtable(dyn for<'a> Cap<'a>); | +++++++ ++++ help: consider introducing a named lifetime parameter | 1 | struct Vtable<'a>(dyn Cap<'a>); | ++++ ++++ error[E0601]: `main` function not found in crate `mvce` --> /tmp/icemaker_global_tempdir.pvcMdeDPSWCA/rustc_testrunner_tmpdir_reporting.xxjpTONUWZGm/mvce.rs:10:50 | 10 | const G: &Copy = unsafe { Transmute { t: 1 }.u }; | ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.pvcMdeDPSWCA/rustc_testrunner_tmpdir_reporting.xxjpTONUWZGm/mvce.rs` warning: trait objects without an explicit `dyn` are deprecated --> /tmp/icemaker_global_tempdir.pvcMdeDPSWCA/rustc_testrunner_tmpdir_reporting.xxjpTONUWZGm/mvce.rs:10:11 | 10 | const G: &Copy = unsafe { Transmute { t: 1 }.u }; | ^^^^ | = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021! = note: for more information, see = note: `#[warn(bare_trait_objects)]` on by default help: if this is an object-safe trait, use `dyn` | 10 | const G: &dyn Copy = unsafe { Transmute { t: 1 }.u }; | +++ error[E0038]: the trait `Copy` cannot be made into an object --> /tmp/icemaker_global_tempdir.pvcMdeDPSWCA/rustc_testrunner_tmpdir_reporting.xxjpTONUWZGm/mvce.rs:10:27 | 10 | const G: &Copy = unsafe { Transmute { t: 1 }.u }; | ^^^^^^^^^^^^^^^^^^^^ `Copy` cannot be made into an object | = note: the trait cannot be made into an object because it requires `Self: Sized` = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit = note: required for the cast from `&'static Vtable` to `&'static (dyn Copy + 'static)` thread 'rustc' panicked at compiler/rustc_const_eval/src/interpret/traits.rs:45:9: there are no vtables for unsized types stack backtrace: 0: 0x79812a267e2a - ::fmt::h51109d568367e76a 1: 0x79812aa03717 - core::fmt::write::h7e8f0b1f95e6be45 2: 0x79812b8f3ab3 - std::io::Write::write_fmt::h229d78786f5fe690 3: 0x79812a267c82 - std::sys::backtrace::BacktraceLock::print::hf70613765fd0f061 4: 0x79812a26a401 - std::panicking::default_hook::{{closure}}::hf82216f80d30e016 5: 0x79812a26a234 - std::panicking::default_hook::hf7a9ea4a129f2d92 6: 0x79812936fdbf - std[3a9e0bc7de817fee]::panicking::update_hook::>::{closure#0} 7: 0x79812a26ab28 - std::panicking::rust_panic_with_hook::h24ea4287235e7f5a 8: 0x79812a26a8c3 - std::panicking::begin_panic_handler::{{closure}}::h1c06db2f98bda4d1 9: 0x79812a2682d9 - std::sys::backtrace::__rust_end_short_backtrace::h1affcde0392f0bed 10: 0x79812a26a5c4 - rust_begin_unwind 11: 0x798127094c03 - core::panicking::panic_fmt::hc8bfb6421cce059b 12: 0x79812c55d0ae - >::get_vtable_size_and_align.llvm.18396178888626651702.cold 13: 0x79812bcc0e7e - >::size_and_align_of.llvm.18396178888626651702.warm 14: 0x79812b0e4751 - >::check_safe_pointer 15: 0x79812b0eae4e - as rustc_const_eval[316a1db182d8521a]::interpret::visitor::ValueVisitor>::visit_value 16: 0x7981283d4170 - rustc_const_eval[316a1db182d8521a]::const_eval::eval_queries::eval_to_allocation_raw_provider 17: 0x79812b0d63f6 - rustc_query_impl[652b508c0e1a12a1]::plumbing::__rust_begin_short_backtrace::> 18: 0x79812b0d5c12 - rustc_query_system[8d4b4cf30ddeb20c]::query::plumbing::try_execute_query::, rustc_middle[1950792dfd3a6e88]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[652b508c0e1a12a1]::plumbing::QueryCtxt, false> 19: 0x79812b0d57ef - rustc_query_impl[652b508c0e1a12a1]::query_impl::eval_to_allocation_raw::get_query_non_incr::__rust_end_short_backtrace 20: 0x79812b0d7661 - rustc_const_eval[316a1db182d8521a]::const_eval::eval_queries::eval_to_const_value_raw_provider 21: 0x79812b0d7476 - rustc_query_impl[652b508c0e1a12a1]::plumbing::__rust_begin_short_backtrace::> 22: 0x79812b0d5bd5 - rustc_query_system[8d4b4cf30ddeb20c]::query::plumbing::try_execute_query::, rustc_middle[1950792dfd3a6e88]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[652b508c0e1a12a1]::plumbing::QueryCtxt, false> 23: 0x79812b0d56f3 - rustc_query_impl[652b508c0e1a12a1]::query_impl::eval_to_const_value_raw::get_query_non_incr::__rust_end_short_backtrace 24: 0x79812abc8c7d - ::par_body_owners::::{closure#0} 25: 0x79812abc6506 - rustc_hir_analysis[6094934d5bdd113d]::check_crate 26: 0x79812abc2e45 - rustc_interface[a2113eed4df3235f]::passes::run_required_analyses 27: 0x79812b73d79e - rustc_interface[a2113eed4df3235f]::passes::analysis 28: 0x79812b73d771 - rustc_query_impl[652b508c0e1a12a1]::plumbing::__rust_begin_short_backtrace::> 29: 0x79812b8d7f6e - rustc_query_system[8d4b4cf30ddeb20c]::query::plumbing::try_execute_query::>, false, false, false>, rustc_query_impl[652b508c0e1a12a1]::plumbing::QueryCtxt, false> 30: 0x79812b8d7ccf - rustc_query_impl[652b508c0e1a12a1]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace 31: 0x79812b72721e - rustc_interface[a2113eed4df3235f]::interface::run_compiler::, rustc_driver_impl[df1321a44fb92b9b]::run_compiler::{closure#0}>::{closure#1} 32: 0x79812b7ec010 - std[3a9e0bc7de817fee]::sys::backtrace::__rust_begin_short_backtrace::, rustc_driver_impl[df1321a44fb92b9b]::run_compiler::{closure#0}>::{closure#1}, core[5cb66491b2f8db54]::result::Result<(), rustc_span[e2d2343c41afc08b]::ErrorGuaranteed>>::{closure#0}, core[5cb66491b2f8db54]::result::Result<(), rustc_span[e2d2343c41afc08b]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5cb66491b2f8db54]::result::Result<(), rustc_span[e2d2343c41afc08b]::ErrorGuaranteed>> 33: 0x79812b7ec67a - <::spawn_unchecked_, rustc_driver_impl[df1321a44fb92b9b]::run_compiler::{closure#0}>::{closure#1}, core[5cb66491b2f8db54]::result::Result<(), rustc_span[e2d2343c41afc08b]::ErrorGuaranteed>>::{closure#0}, core[5cb66491b2f8db54]::result::Result<(), rustc_span[e2d2343c41afc08b]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5cb66491b2f8db54]::result::Result<(), rustc_span[e2d2343c41afc08b]::ErrorGuaranteed>>::{closure#1} as core[5cb66491b2f8db54]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} 34: 0x79812b7eca6b - std::sys::pal::unix::thread::Thread::new::thread_start::hc93f82c3bb4603d0 35: 0x79812ce7539d - 36: 0x79812cefa49c - 37: 0x0 - 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: rustc 1.83.0-nightly (f7b4c72c8 2024-09-17) running on x86_64-unknown-linux-gnu note: compiler flags: -Z crate-attr=feature(object_safe_for_dispatch) -Z dump-mir-dir=dir query stack during panic: #0 [eval_to_allocation_raw] const-evaluating + checking `G` #1 [eval_to_const_value_raw] simplifying constant for the type system `G` end of query stack error: aborting due to 3 previous errors; 1 warning emitted Some errors have detailed explanations: E0038, E0106, E0601. For more information about an error, try `rustc --explain E0038`. ```

@rustbot label +F-object_safe_for_dispatch

[matthiaskrgr]

126996

[theemathas]

Isn't this the same as https://github.com/rust-lang/rust/issues/127742#issuecomment-2227610840 ?

[matthiaskrgr]

nope