ICE: called `Result::unwrap()` on an `Err` value: NoSolution

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(ptr_metadata)]
use std::{ops::FnMut, ptr::Pointee};

pub type EmplacerFn<'a, T> = dyn for<'b> FnMut(<T as Pointee>::Metadata) + 'a;

pub struct Emplacer<'a, T>(EmplacerFn<'a, T>);

impl<'a, T> Emplacer<'a, T> {
    pub unsafe fn from_fn<'b>(emplacer_fn: &'b mut EmplacerFn<'a, T>) -> &'b mut Self {
        unsafe { &mut *((emplacer_fn as *mut EmplacerFn<'a, T>) as *mut Self) }
    }
}

pub fn main() {}

original code

original:

#![feature(ptr_metadata)]
// This test is the result of minimizing the `emplacable` crate to reproduce
// <https://github.com/rust-lang/miri/issues/3541>.

use std::{ops::FnMut, ptr::Pointee};

pub type EmplacerFn<'a, T> = dyn for<'b> FnMut(<T as Pointee>::Metadata) + 'a;

#[repr(transparent)]
pub struct Emplacer<'a, T>(EmplacerFn<'a, T>)
where
    T: ?Sized;

impl<'a, T> Emplacer<'a, T>
where
    &'b mut Self: ?Sized,
{
    pub unsafe fn from_fn<'b>(emplacer_fn: &'b mut EmplacerFn<'a, T>) -> &'b mut Self {
        // This used to trigger:
        // constructing invalid value: wrong trait in wide pointer vtable: expected
        // `std::ops::FnMut(<[std::boxed::Box<i32>] as std::ptr::Pointee>::Metadata)`, but encountered
        // `std::ops::FnMut<(usize,)>`.
        unsafe { &mut *((emplacer_fn as *mut EmplacerFn<'a, T>) as *mut Self) }
    }
}

pub fn box_new_with<T>()
where
    T: ?Sized,
{
    let emplacer_closure = &mut |_meta| {
        unreachable!();
    };

    unsafe { Emplacer::<T>::from_fn(emplacer_closure) };
}

fn main() {
    box_new_with::<[Box<i32>]>();
}

Version information

rustc 1.82.0-nightly (edc4dc337 2024-08-03)
binary: rustc
commit-hash: edc4dc337b28a389dbd87b3825d7929381edeb8f
commit-date: 2024-08-03
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 19.1.0

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

thread 'rustc' panicked at compiler/rustc_borrowck/src/type_check/mod.rs:2379:38:
called `Result::unwrap()` on an `Err` value: NoSolution
stack backtrace:
   0:     0x7e8e559a114d - std::backtrace_rs::backtrace::libunwind::trace::h3396cafc0cf0d168
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1:     0x7e8e559a114d - std::backtrace_rs::backtrace::trace_unsynchronized::hbe65a7c762aaf69a
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7e8e559a114d - std::sys::backtrace::_print_fmt::hfdd3dddbd75728fc
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/sys/backtrace.rs:66:9
   3:     0x7e8e559a114d - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hfba02964013c3262
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/sys/backtrace.rs:39:26
   4:     0x7e8e559f168b - core::fmt::rt::Argument::fmt::h1cc3e67cc5e9d174
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/core/src/fmt/rt.rs:173:76
   5:     0x7e8e559f168b - core::fmt::write::hdb76d7e5bc6978cf
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/core/src/fmt/mod.rs:1178:21
   6:     0x7e8e55994ec3 - std::io::Write::write_fmt::h27ce112238514bf4
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/io/mod.rs:1823:15
   7:     0x7e8e559a3942 - std::sys::backtrace::BacktraceLock::print::h9feef95ad1ac9d67
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/sys/backtrace.rs:42:9
   8:     0x7e8e559a3942 - std::panicking::default_hook::{{closure}}::h33a1b00b93aac464
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/panicking.rs:266:22
   9:     0x7e8e559a35ae - std::panicking::default_hook::hfa713793aec3526f
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/panicking.rs:293:9
  10:     0x7e8e51ea6997 - std[e05a76fc5581e11]::panicking::update_hook::<alloc[76bfb925f7cccf2e]::boxed::Box<rustc_driver_impl[c2f8fa5a81365ad3]::install_ice_hook::{closure#0}>>::{closure#0}
  11:     0x7e8e559a4332 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h92beb97f09018d1f
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/alloc/src/boxed.rs:2164:9
  12:     0x7e8e559a4332 - std::panicking::rust_panic_with_hook::hb8e05073b20eee1b
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/panicking.rs:805:13
  13:     0x7e8e559a3fe7 - std::panicking::begin_panic_handler::{{closure}}::h8ac2d6811e12e338
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/panicking.rs:671:13
  14:     0x7e8e559a1639 - std::sys::backtrace::__rust_end_short_backtrace::h4e11dc802a78f1a1
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/sys/backtrace.rs:170:18
  15:     0x7e8e559a3c74 - rust_begin_unwind
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/panicking.rs:662:5
  16:     0x7e8e559edce3 - core::panicking::panic_fmt::h64d8681f23dadf6d
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/core/src/panicking.rs:74:14
  17:     0x7e8e559ee336 - core::result::unwrap_failed::hf97f3a13da3067be
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/core/src/result.rs:1679:5
  18:     0x7e8e536c2526 - <rustc_borrowck[7f0a2371269bdcfb]::type_check::TypeChecker>::typeck_mir
  19:     0x7e8e5456591a - rustc_borrowck[7f0a2371269bdcfb]::type_check::type_check
  20:     0x7e8e53506817 - rustc_borrowck[7f0a2371269bdcfb]::nll::compute_regions
  21:     0x7e8e544db80f - rustc_borrowck[7f0a2371269bdcfb]::do_mir_borrowck
  22:     0x7e8e544cbd47 - rustc_query_impl[59527bbfe333c2c8]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[59527bbfe333c2c8]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[5fc6768cbdcaf94e]::query::erase::Erased<[u8; 8usize]>>
  23:     0x7e8e535f4a45 - rustc_query_system[fb9c72285b8affb2]::query::plumbing::try_execute_query::<rustc_query_impl[59527bbfe333c2c8]::DynamicConfig<rustc_query_system[fb9c72285b8affb2]::query::caches::VecCache<rustc_span[6181c2fd5039252a]::def_id::LocalDefId, rustc_middle[5fc6768cbdcaf94e]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[59527bbfe333c2c8]::plumbing::QueryCtxt, false>
  24:     0x7e8e535f448d - rustc_query_impl[59527bbfe333c2c8]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  25:     0x7e8e53ee8eaa - rustc_interface[b1d83cf6a56376dd]::passes::analysis
  26:     0x7e8e53ee805b - rustc_query_impl[59527bbfe333c2c8]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[59527bbfe333c2c8]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[5fc6768cbdcaf94e]::query::erase::Erased<[u8; 1usize]>>
  27:     0x7e8e543a442e - rustc_query_system[fb9c72285b8affb2]::query::plumbing::try_execute_query::<rustc_query_impl[59527bbfe333c2c8]::DynamicConfig<rustc_query_system[fb9c72285b8affb2]::query::caches::SingleCache<rustc_middle[5fc6768cbdcaf94e]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[59527bbfe333c2c8]::plumbing::QueryCtxt, false>
  28:     0x7e8e543a418f - rustc_query_impl[59527bbfe333c2c8]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  29:     0x7e8e542b7063 - rustc_interface[b1d83cf6a56376dd]::interface::run_compiler::<core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>, rustc_driver_impl[c2f8fa5a81365ad3]::run_compiler::{closure#0}>::{closure#1}
  30:     0x7e8e54256289 - std[e05a76fc5581e11]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[b1d83cf6a56376dd]::util::run_in_thread_with_globals<rustc_interface[b1d83cf6a56376dd]::util::run_in_thread_pool_with_globals<rustc_interface[b1d83cf6a56376dd]::interface::run_compiler<core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>, rustc_driver_impl[c2f8fa5a81365ad3]::run_compiler::{closure#0}>::{closure#1}, core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>>::{closure#0}, core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>>
  31:     0x7e8e54256032 - <<std[e05a76fc5581e11]::thread::Builder>::spawn_unchecked_<rustc_interface[b1d83cf6a56376dd]::util::run_in_thread_with_globals<rustc_interface[b1d83cf6a56376dd]::util::run_in_thread_pool_with_globals<rustc_interface[b1d83cf6a56376dd]::interface::run_compiler<core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>, rustc_driver_impl[c2f8fa5a81365ad3]::run_compiler::{closure#0}>::{closure#1}, core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>>::{closure#0}, core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d16eec4b7581b6ce]::result::Result<(), rustc_span[6181c2fd5039252a]::ErrorGuaranteed>>::{closure#1} as core[d16eec4b7581b6ce]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  32:     0x7e8e559ae04b - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hdd26b728780d5edf
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/alloc/src/boxed.rs:2150:9
  33:     0x7e8e559ae04b - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h9f4e5f896641f315
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/alloc/src/boxed.rs:2150:9
  34:     0x7e8e559ae04b - std::sys::pal::unix::thread::Thread::new::thread_start::hf133519f4699d161
                               at /rustc/edc4dc337b28a389dbd87b3825d7929381edeb8f/library/std/src/sys/pal/unix/thread.rs:105:17
  35:     0x7e8e4e8a6ded - <unknown>
  36:     0x7e8e4e92a0dc - <unknown>
  37:                0x0 - <unknown>

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 1.82.0-nightly (edc4dc337 2024-08-03) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_borrowck] borrow-checking `<impl at /tmp/icemaker_global_tempdir.hs07VfNUIJu0/rustc_testrunner_tmpdir_reporting.PNoBQTfCLNlo/mvce.rs:7:1: 7:28>::from_fn`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 3 previous errors

Some errors have detailed explanations: E0601, E0658.
For more information about an error, try `rustc --explain E0601`.

@rustbot label +F-ptr_metadata

[matthiaskrgr]

bisects to #120248 🤔 cc @WaffleLapkin

[theemathas]

Minimized:

trait Trait {
    type Associated;
}

impl Trait for i32 {
    type Associated = i64;
}

trait Generic<T> {}

type TraitObject = dyn Generic<<i32 as Trait>::Associated>;

struct Wrap(TraitObject);

fn cast(x: *mut TraitObject) {
    x as *mut Wrap;
}

This compiles on stable. ICEs on beta.

Error output on beta

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at compiler/rustc_borrowck/src/type_check/mod.rs:2386:38:
called `Result::unwrap()` on an `Err` value: NoSolution
stack backtrace:
   0:     0x7f444bee13a5 - std::backtrace_rs::backtrace::libunwind::trace::hec2dc58314f3b5ff
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1:     0x7f444bee13a5 - std::backtrace_rs::backtrace::trace_unsynchronized::h331ee0b0169d7b05
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f444bee13a5 - std::sys::backtrace::_print_fmt::h29aa16f5d373cd01
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/sys/backtrace.rs:65:5
   3:     0x7f444bee13a5 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hee7def06ed53e721
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/sys/backtrace.rs:40:26
   4:     0x7f444bf30eab - core::fmt::rt::Argument::fmt::hc46773f831b98118
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/core/src/fmt/rt.rs:173:76
   5:     0x7f444bf30eab - core::fmt::write::hda013bd82d767381
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/core/src/fmt/mod.rs:1182:21
   6:     0x7f444bed5f2f - std::io::Write::write_fmt::hb37aee6d7f10fc9b
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/io/mod.rs:1827:15
   7:     0x7f444bee3b91 - std::sys::backtrace::BacktraceLock::print::h907db10e44632b3a
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/sys/backtrace.rs:43:9
   8:     0x7f444bee3b91 - std::panicking::default_hook::{{closure}}::hf6a2255341957303
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/panicking.rs:269:22
   9:     0x7f444bee386c - std::panicking::default_hook::h0b7d4412c1e361bd
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/panicking.rs:296:9
  10:     0x7f444f1f3a50 - std[efeefe75b6db041f]::panicking::update_hook::<alloc[759981cd4e4e801b]::boxed::Box<rustc_driver_impl[c3312e73a30af321]::install_ice_hook::{closure#0}>>::{closure#0}
  11:     0x7f444bee455f - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hdc9e661750688cd9
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/alloc/src/boxed.rs:2084:9
  12:     0x7f444bee455f - std::panicking::rust_panic_with_hook::h6fa5f6f38389e656
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/panicking.rs:808:13
  13:     0x7f444bee4187 - std::panicking::begin_panic_handler::{{closure}}::he96dd93ff17a0601
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/panicking.rs:674:13
  14:     0x7f444bee1869 - std::sys::backtrace::__rust_end_short_backtrace::h2f5b06618a35c92b
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/sys/backtrace.rs:168:18
  15:     0x7f444bee3e14 - rust_begin_unwind
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/panicking.rs:665:5
  16:     0x7f444bf2d463 - core::panicking::panic_fmt::h8c83251fb1f17232
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/core/src/panicking.rs:74:14
  17:     0x7f444bf2dab6 - core::result::unwrap_failed::ha33a948b4cc83713
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/core/src/result.rs:1679:5
  18:     0x7f445093b6ce - <rustc_borrowck[6480dc8642ee2324]::type_check::TypeChecker>::typeck_mir
  19:     0x7f44517c2c08 - rustc_borrowck[6480dc8642ee2324]::type_check::type_check
  20:     0x7f44508465df - rustc_borrowck[6480dc8642ee2324]::nll::compute_regions
  21:     0x7f445169ba1c - rustc_borrowck[6480dc8642ee2324]::do_mir_borrowck
  22:     0x7f445168ce38 - rustc_query_impl[31fd7ff07bfdf275]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[31fd7ff07bfdf275]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[2cecd0bcfd982753]::query::erase::Erased<[u8; 8usize]>>
  23:     0x7f4450a59468 - rustc_query_system[e7fb6eb8ab67ecdb]::query::plumbing::try_execute_query::<rustc_query_impl[31fd7ff07bfdf275]::DynamicConfig<rustc_query_system[e7fb6eb8ab67ecdb]::query::caches::VecCache<rustc_span[e52881666e69bc1d]::def_id::LocalDefId, rustc_middle[2cecd0bcfd982753]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[31fd7ff07bfdf275]::plumbing::QueryCtxt, false>
  24:     0x7f4450a58f15 - rustc_query_impl[31fd7ff07bfdf275]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  25:     0x7f44510ecf3b - rustc_interface[b6630b0d0d781590]::passes::analysis
  26:     0x7f44510ec867 - rustc_query_impl[31fd7ff07bfdf275]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[31fd7ff07bfdf275]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[2cecd0bcfd982753]::query::erase::Erased<[u8; 1usize]>>
  27:     0x7f445159ac22 - rustc_query_system[e7fb6eb8ab67ecdb]::query::plumbing::try_execute_query::<rustc_query_impl[31fd7ff07bfdf275]::DynamicConfig<rustc_query_system[e7fb6eb8ab67ecdb]::query::caches::SingleCache<rustc_middle[2cecd0bcfd982753]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[31fd7ff07bfdf275]::plumbing::QueryCtxt, false>
  28:     0x7f445159a9cf - rustc_query_impl[31fd7ff07bfdf275]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  29:     0x7f4451479016 - rustc_interface[b6630b0d0d781590]::interface::run_compiler::<core[c78a77099d0262f3]::result::Result<(), rustc_span[e52881666e69bc1d]::ErrorGuaranteed>, rustc_driver_impl[c3312e73a30af321]::run_compiler::{closure#0}>::{closure#1}
  30:     0x7f44513b851b - std[efeefe75b6db041f]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[b6630b0d0d781590]::util::run_in_thread_with_globals<rustc_interface[b6630b0d0d781590]::interface::run_compiler<core[c78a77099d0262f3]::result::Result<(), rustc_span[e52881666e69bc1d]::ErrorGuaranteed>, rustc_driver_impl[c3312e73a30af321]::run_compiler::{closure#0}>::{closure#1}, core[c78a77099d0262f3]::result::Result<(), rustc_span[e52881666e69bc1d]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[c78a77099d0262f3]::result::Result<(), rustc_span[e52881666e69bc1d]::ErrorGuaranteed>>
  31:     0x7f44513b82ea - <<std[efeefe75b6db041f]::thread::Builder>::spawn_unchecked_<rustc_interface[b6630b0d0d781590]::util::run_in_thread_with_globals<rustc_interface[b6630b0d0d781590]::interface::run_compiler<core[c78a77099d0262f3]::result::Result<(), rustc_span[e52881666e69bc1d]::ErrorGuaranteed>, rustc_driver_impl[c3312e73a30af321]::run_compiler::{closure#0}>::{closure#1}, core[c78a77099d0262f3]::result::Result<(), rustc_span[e52881666e69bc1d]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[c78a77099d0262f3]::result::Result<(), rustc_span[e52881666e69bc1d]::ErrorGuaranteed>>::{closure#1} as core[c78a77099d0262f3]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  32:     0x7f444beee5bb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h5e49ad344eded169
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/alloc/src/boxed.rs:2070:9
  33:     0x7f444beee5bb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hc344207bfeabc120
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/alloc/src/boxed.rs:2070:9
  34:     0x7f444beee5bb - std::sys::pal::unix::thread::Thread::new::thread_start::h30a261106ad4495b
                               at /rustc/08328a323ecd80b443a8fcc72c6b1071f48e233f/library/std/src/sys/pal/unix/thread.rs:108:17
  35:     0x7f444bdf8609 - start_thread
  36:     0x7f444bd1b353 - clone
  37:                0x0 - <unknown>

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: rustc 1.81.0-beta.2 (08328a323 2024-07-25) 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 [mir_borrowck] borrow-checking `cast`
#1 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `playground` (lib)

@rustbot label +regression-from-stable-to-beta

Of note, the opposite pointer cast fails to compile on stable, beta, and nightly. This seems wrong to me.

The opposite cast

trait Trait {
    type Associated;
}

impl Trait for i32 {
    type Associated = i64;
}

trait Generic<T> {}

type TraitObject = dyn Generic<<i32 as Trait>::Associated>;

struct Wrap(TraitObject);

fn cast(x: *mut Wrap) {
    x as *mut TraitObject;
}

Error:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `Wrap: Generic<i64>` is not satisfied
  --> src/lib.rs:16:5
   |
16 |     x as *mut TraitObject;
   |     ^ the trait `Generic<i64>` is not implemented for `Wrap`
   |
help: this trait has no implementations, consider adding one
  --> src/lib.rs:9:1
   |
9  | trait Generic<T> {}
   | ^^^^^^^^^^^^^^^^
   = note: required for the cast from `*mut Wrap` to `*mut (dyn Generic<i64> + 'static)`

error[E0277]: the size for values of type `(dyn Generic<i64> + 'static)` cannot be known at compilation time
  --> src/lib.rs:16:5
   |
16 |     x as *mut TraitObject;
   |     ^ doesn't have a size known at compile-time
   |
   = help: within `Wrap`, the trait `Sized` is not implemented for `(dyn Generic<i64> + 'static)`, which is required by `Wrap: Sized`
note: required because it appears within the type `Wrap`
  --> src/lib.rs:13:8
   |
13 | struct Wrap(TraitObject);
   |        ^^^^
   = note: required for the cast from `*mut Wrap` to `*mut (dyn Generic<i64> + 'static)`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to 2 previous errors

[apiraino]

bisects to #120248 🤔 cc @WaffleLapkin

Just for context (unsure if relevant here) I see in that PR a question for T-lang about a breaking change at this comment and in the following FCP that was approved

[WaffleLapkin]

So while the breaking change was approved, I believe this code should compile (and definitely not ICE).

The issue seems to be that borrowck can't prove a bound (after typeck did). Seeing for you need a projection to trigger the issue (<i32 as Trait>::Associated>) I think there is normalization missing somewhere.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high