Custom MIR: ICE on assigning to reference in enum variant

#![feature(custom_mir, core_intrinsics)]
extern crate core;
use core::intrinsics::mir::*;

pub enum E {
    V0 { fld0: &'static u64 },
}

#[custom_mir(dialect = "runtime", phase = "initial")]
pub fn fn0() {
    mir! {
    let e: E;
    let n: u64;
    {
        n = 0;
        place!(Field::<&u64>(Variant(e, 0), 0)) = &n;
        Return()
    }

    }
}
pub fn main() {
    fn0();
}

Only ICE with -Zmir-opt-level=0. Not sure if this is a real bug or not. Maybe certain MIR passes is required for correctness?

% rustc -Zmir-opt-level=0 repro.rs
...
    error: internal compiler error: broken MIR in Item(DefId(0:8 ~ repro[d2aa]::fn0)) (after phase change to runtime-optimized) at bb0[0]:
                                    Free regions in optimized runtime-optimized MIR
      --> /Users/andy/Code/rustlantis/repro.rs:15:9
       |
    15 |         n = 0;
       |         ^^^^^
       |
    note: delayed at compiler/rustc_const_eval/src/transform/validate.rs:91:33
             0: std::backtrace::Backtrace::create
             1: <rustc_errors::DiagCtxtInner>::emit_diagnostic
             2: <rustc_errors::DiagCtxt>::emit_diagnostic
             3: <rustc_errors::diagnostic_builder::DiagnosticBuilder>::emit_producing_error_guaranteed
             4: <rustc_const_eval::transform::validate::CfgChecker>::fail::<alloc::string::String>
             5: <rustc_const_eval::transform::validate::Validator as rustc_middle::mir::MirPass>::run_pass
             6: rustc_mir_transform::pass_manager::run_passes_inner
             7: rustc_mir_transform::optimized_mir
             8: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
             9: <rustc_query_impl::query_impl::optimized_mir::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::DefId)>>::call_once
            10: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            11: rustc_query_impl::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
            12: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 8]>>>
            13: rustc_mir_transform::cross_crate_inline::cross_crate_inlinable
            14: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::cross_crate_inlinable::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
            15: <rustc_query_impl::query_impl::cross_crate_inlinable::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::DefId)>>::call_once
            16: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            17: rustc_query_impl::query_impl::cross_crate_inlinable::get_query_non_incr::__rust_end_short_backtrace
            18: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 1]>>>
            19: rustc_passes::reachable::reachable_set
            20: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::reachable_set::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
            21: <rustc_query_impl::query_impl::reachable_set::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, ())>>::call_once
            22: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            23: rustc_query_impl::query_impl::reachable_set::get_query_non_incr::__rust_end_short_backtrace
            24: rustc_codegen_ssa::back::symbol_export::reachable_non_generics_provider
            25: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::reachable_non_generics::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 8]>>
            26: <rustc_query_impl::query_impl::reachable_non_generics::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::CrateNum)>>::call_once
            27: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::VecCache<rustc_span::def_id::CrateNum, rustc_middle::query::erase::Erased<[u8; 8]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            28: rustc_query_impl::query_impl::reachable_non_generics::get_query_non_incr::__rust_end_short_backtrace
            29: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::VecCache<rustc_span::def_id::CrateNum, rustc_middle::query::erase::Erased<[u8; 8]>>>
            30: rustc_codegen_ssa::back::symbol_export::is_reachable_non_generic_provider_local
            31: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::is_reachable_non_generic::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 1]>>
            32: <rustc_query_impl::query_impl::is_reachable_non_generic::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, rustc_span::def_id::DefId)>>::call_once
            33: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 1]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            34: rustc_query_impl::query_impl::is_reachable_non_generic::get_query_non_incr::__rust_end_short_backtrace
            35: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefaultCache<rustc_span::def_id::DefId, rustc_middle::query::erase::Erased<[u8; 1]>>>
            36: <rustc_monomorphize::collector::RootCollector>::push_if_root
            37: rustc_monomorphize::collector::collect_roots
            38: <rustc_session::session::Session>::time::<alloc::vec::Vec<rustc_middle::mir::mono::MonoItem>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#0}>
            39: rustc_monomorphize::collector::collect_crate_mono_items
            40: rustc_monomorphize::partitioning::collect_and_partition_mono_items
            41: rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle::query::erase::Erased<[u8; 24]>>
            42: <rustc_query_impl::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core::ops::function::FnOnce<(rustc_middle::ty::context::TyCtxt, ())>>::call_once
            43: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::DynamicConfig<rustc_query_system::query::caches::SingleCache<rustc_middle::query::erase::Erased<[u8; 24]>>, false, false, false>, rustc_query_impl::plumbing::QueryCtxt, false>
            44: rustc_query_impl::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace
            45: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend>
            46: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
            47: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
            48: rustc_interface::passes::start_codegen
            49: <rustc_middle::ty::context::GlobalCtxt>::enter::<<rustc_interface::queries::Queries>::codegen_and_build_linker::{closure#0}, core::result::Result<rustc_interface::queries::Linker, rustc_span::ErrorGuaranteed>>
            50: <rustc_interface::queries::Queries>::codegen_and_build_linker
            51: <rustc_interface::interface::Compiler>::enter::<rustc_driver_impl::run_compiler::{closure#0}::{closure#0}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
            52: rustc_span::set_source_map::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#0}::{closure#0}>
            53: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
            54: rustc_span::create_session_globals_then::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}>
            55: std::sys_common::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<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
            56: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
            57: std::sys::unix::thread::Thread::new::thread_start
            58: __pthread_joiner_wake
      --> /Users/andy/Code/rustlantis/repro.rs:15:9
       |
    15 |         n = 0;
       |         ^^^^^

Save the issue's reproducer as repro.rs and rerun rustc -Zmir-opt-level=0; the failure points to compiler/rustc_const_eval/src/transform/validate.rs:91:33 and the MIR validator. Trace how the custom MIR assignment to the enum-variant reference reaches that validator, and consider it resolved when the reproducer no longer ICEs at MIR opt level 0.