ICE: type mismatch when copying

[DutchGhost]

Code

use core::mem::{transmute, ManuallyDrop};

// `&mut "literal"` has a type of `&mut &str`, not `&mut str`
const S: &'static mut str = &mut " hello ";

const fn trigger() -> [(); unsafe {
      let s = transmute::<(*const u8, usize), &ManuallyDrop<str>>((S.as_ptr(), 3));
      0
  }] {
    [(); 0]
}

Meta

rustc --version --verbose:

rustc 1.46.0 (04488afe3 2020-08-24)
rustc 1.47.0-beta.2 (84b047bf6 2020-08-28)
rustc 1.48.0-nightly (5099914a1 2020-09-08)

Error output

error: internal compiler error: src/librustc_mir/interpret/place.rs:860:13: type mismatch when copying!

Backtrace

   Compiling playground v0.0.1 (/playground)
error[E0764]: mutable references are not allowed in constants
 --> src/lib.rs:3:29
  |
3 | const S: &'static mut str = &mut " hello ";
  |                             ^^^^^^^^^^^^^^ `&mut` is only allowed in `const fn`

error[E0019]: constant contains unimplemented expression type
 --> src/lib.rs:3:29
  |
3 | const S: &'static mut str = &mut " hello ";
  |                             ^^^^^^^^^^^^^^
  |
  = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable

error[E0596]: cannot borrow data in a `&` reference as mutable
 --> src/lib.rs:3:29
  |
3 | const S: &'static mut str = &mut " hello ";
  |                             ^^^^^^^^^^^^^^ cannot borrow as mutable

error: internal compiler error: src/librustc_mir/interpret/place.rs:860:13: type mismatch when copying!
src: &str,
dest: &mut str
 --> src/lib.rs:3:29
  |
3 | const S: &'static mut str = &mut " hello ";
  |                             ^^^^^^^^^^^^^^

thread 'rustc' panicked at 'Box<Any>', /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/macros.rs:13:23
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::span_bug_fmt
  20: rustc_mir::interpret::place::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::copy_op_no_validate
  21: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  22: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  23: rustc_mir::const_eval::eval_queries::const_eval_raw_provider
  24: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_raw>::compute
  25: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  26: rustc_data_structures::stack::ensure_sufficient_stack
  27: rustc_query_system::query::plumbing::get_query_impl
  28: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  29: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  30: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  31: rustc_data_structures::stack::ensure_sufficient_stack
  32: rustc_query_system::query::plumbing::get_query_impl
  33: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  34: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  35: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  36: rustc_data_structures::stack::ensure_sufficient_stack
  37: rustc_query_system::query::plumbing::get_query_impl
  38: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  39: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  40: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  41: <rustc_infer::infer::at::At as rustc_trait_selection::traits::query::normalize::AtExt>::normalize
  42: rustc_infer::infer::InferCtxtBuilder::enter
  43: rustc_traits::normalize_erasing_regions::normalize_generic_arg_after_erasing_regions
  44: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::normalize_generic_arg_after_erasing_regions>::compute
  45: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  46: rustc_data_structures::stack::ensure_sufficient_stack
  47: rustc_query_system::query::plumbing::get_query_impl
  48: <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_middle::ty::fold::TypeFolder>::fold_const
  49: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::subst_and_normalize_erasing_regions
  50: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  51: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  52: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::run
  53: rustc_mir::const_eval::eval_queries::const_eval_raw_provider
  54: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_raw>::compute
  55: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  56: rustc_data_structures::stack::ensure_sufficient_stack
  57: rustc_query_system::query::plumbing::get_query_impl
  58: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  59: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  60: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  61: rustc_data_structures::stack::ensure_sufficient_stack
  62: rustc_query_system::query::plumbing::get_query_impl
  63: rustc_mir::const_eval::eval_queries::const_eval_validated_provider
  64: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::const_eval_validated>::compute
  65: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  66: rustc_data_structures::stack::ensure_sufficient_stack
  67: rustc_query_system::query::plumbing::get_query_impl
  68: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_global_id
  69: rustc_middle::mir::interpret::queries::<impl rustc_middle::ty::context::TyCtxt>::const_eval_resolve
  70: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_const
  71: <rustc_trait_selection::traits::project::AssocTypeNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
  72: rustc_middle::ty::fold::TypeFoldable::fold_with
  73: rustc_trait_selection::traits::project::AssocTypeNormalizer::fold
  74: rustc_trait_selection::traits::project::normalize
  75: <rustc_infer::infer::InferCtxt as rustc_trait_selection::infer::InferCtxtExt>::partially_normalize_associated_types_in
  76: rustc_typeck::check::FnCtxt::normalize_associated_types_in
  77: rustc_infer::infer::InferCtxtBuilder::enter
  78: rustc_typeck::check::wfcheck::check_item_well_formed
  79: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_item_well_formed>::compute
  80: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  81: rustc_data_structures::stack::ensure_sufficient_stack
  82: rustc_query_system::query::plumbing::get_query_impl
  83: rustc_query_system::query::plumbing::ensure_query_impl
  84: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::itemlikevisit::ParItemLikeVisitor>::visit_item
  85: rustc_data_structures::sync::par_for_each_in
  86: rustc_hir::hir::Crate::par_visit_all_item_likes
  87: rustc_session::session::Session::track_errors
  88: rustc_typeck::check_crate
  89: rustc_interface::passes::analysis
  90: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  91: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  92: rustc_query_system::query::plumbing::get_query_impl
  93: rustc_middle::ty::context::tls::enter_global
  94: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  95: rustc_span::with_source_map
  96: rustc_interface::interface::create_compiler_and_run
  97: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.46.0 (04488afe3 2020-08-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [const_eval_raw] const-evaluating `S`
#1 [const_eval_validated] const-evaluating + checking `S`
#2 [const_eval_validated] const-evaluating + checking `S`
#3 [normalize_generic_arg_after_erasing_regions] normalizing `S`
#4 [const_eval_raw] const-evaluating `trigger::{{constant}}#0`
#5 [const_eval_validated] const-evaluating + checking `trigger::{{constant}}#0`
#6 [const_eval_validated] const-evaluating + checking `trigger::{{constant}}#0`
#7 [check_item_well_formed] checking that `trigger` is well-formed
#8 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 4 previous errors

[DutchGhost]

A slightly more minimal example that does not use unsafe:

struct Dummy;

impl Dummy {
    const fn func(&mut self) -> usize {
        42
    }
}

const _: &[usize] = &[0; {
    const DUMMY: &Dummy = &Dummy;
    DUMMY.func()
}];

[DutchGhost]

This bug was introduced in 1.46. Rustc 1.45.2 rightfully rejects the code, https://godbolt.org/z/bYEhYc.

1.37 - 1.44 ICEs with error: internal compiler error: src/librustc_mir/interpret/intern.rs:182: const qualif failed to prevent mutable references

[Badel2]

Run cargo bisect on this:

regression in 25687ca

Given that the panic is in place.rs:860:13, the likely culprit is PR #72796 because of the changes to mir_assign_valid_types

[Alexendoo]

No longer ICEs since #78407