ICE: transmute: called Result::unwrap() on an Err value: ReferencesError(ErrorGuaranteed(()))

[matthiaskrgr]

auto-reduced (treereduce-rust):

mod assert {
    use std::mem::{Assume, BikeshedIntrinsicFrom};

    pub fn is_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src>,
    {
    }
}

fn test() {
    #[repr(C)]
    struct Src {
        a: &'a str,
        g: G,
    }

    assert::is_transmutable::<&mut Src, &mut Dst>();
}

original:

//@ check-pass

//! When safety is assumed, a transmutation over exclusive references should be
//! accepted if the source type potentially carries safety invariants.

#![crate_type = "lib"]
#![feature(transmutability)]
#![allow(dead_code)]

mod assert {
    use std::mem::{Assume, BikeshedIntrinsicFrom};

    pub fn is_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src, { Assume::SAFETY }>
    {}
}

fn test() {
    #[repr(C)]
    struct Src {
    a: &'a str,
    g: G,
}
    type Dst = u8;
    assert::is_transmutable::<&mut Src, &mut Dst>();
}

Version information

rustc 1.82.0-nightly (4d5b3b196 2024-08-20)
binary: rustc
commit-hash: 4d5b3b196284aded6ae99d12bcf149ffdc8ef379
commit-date: 2024-08-20
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

error[E0261]: use of undeclared lifetime name `'a`
  --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:14:13
   |
13 |     struct Src {
   |               - help: consider introducing lifetime `'a` here: `<'a>`
14 |         a: &'a str,
   |             ^^ undeclared lifetime

error[E0412]: cannot find type `G` in this scope
  --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:15:12
   |
15 |         g: G,
   |            ^ not found in this scope
   |
help: you might be missing a type parameter
   |
13 |     struct Src<G> {
   |               +++

error[E0412]: cannot find type `Dst` in this scope
  --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:18:46
   |
18 |     assert::is_transmutable::<&mut Src, &mut Dst>();
   |                                              ^^^ not found in this scope
   |
help: you might be missing a type parameter
   |
11 | fn test<Dst>() {
   |        +++++

warning: unused import: `Assume`
 --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:2:20
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                    ^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:19:2
   |
19 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs`

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:2:20
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                    ^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable
  = note: this compiler was built on 2024-08-20; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:2:28
  |
2 |     use std::mem::{Assume, BikeshedIntrinsicFrom};
  |                            ^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable
  = note: this compiler was built on 2024-08-20; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'transmutability'
 --> /tmp/icemaker_global_tempdir.11e6Po1ssj9b/rustc_testrunner_tmpdir_reporting.TfSZCUR9yS0l/mvce.rs:6:14
  |
6 |         Dst: BikeshedIntrinsicFrom<Src>,
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
  = help: add `#![feature(transmutability)]` to the crate attributes to enable
  = note: this compiler was built on 2024-08-20; consider upgrading it if it is out of date

thread 'rustc' panicked at compiler/rustc_transmute/src/layout/tree.rs:488:49:
called `Result::unwrap()` on an `Err` value: ReferencesError(ErrorGuaranteed(()))
stack backtrace:
   0:     0x75f78cfbc8bd - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hf9983800a10aa2c6
   1:     0x75f78d805457 - core::fmt::write::h878015f659953925
   2:     0x75f78e799351 - std::io::Write::write_fmt::h3c30dbfe90a8ec80
   3:     0x75f78cfbef9b - std::panicking::default_hook::{{closure}}::h4de9d0974edce2a2
   4:     0x75f78cfbec0e - std::panicking::default_hook::h37c112ea75e0ed52
   5:     0x75f78c140db9 - std[ad0fe81d36b825dd]::panicking::update_hook::<alloc[80838699efa87670]::boxed::Box<rustc_driver_impl[e626c935ad353586]::install_ice_hook::{closure#0}>>::{closure#0}
   6:     0x75f78cfbf8b7 - std::panicking::rust_panic_with_hook::he404808eae2fa6a3
   7:     0x75f78cfbf577 - std::panicking::begin_panic_handler::{{closure}}::h2e0f2172b8396a90
   8:     0x75f78cfbcd79 - std::sys::backtrace::__rust_end_short_backtrace::h8e04dc1a1de87bcf
   9:     0x75f78cfbf244 - rust_begin_unwind
  10:     0x75f789e9bd73 - core::panicking::panic_fmt::h7c850fd3e66ca6f3
  11:     0x75f789f5aad6 - core::result::unwrap_failed::h8a305cb559451044
  12:     0x75f78cefad67 - <rustc_transmute[a3340d9efd954e09]::layout::tree::Tree<rustc_transmute[a3340d9efd954e09]::layout::rustc::Def, rustc_transmute[a3340d9efd954e09]::layout::rustc::Ref>>::from_ty
  13:     0x75f78ceff8b7 - <rustc_transmute[a3340d9efd954e09]::rustc::TransmuteTypeEnv>::is_transmutable
  14:     0x75f78e58180e - <rustc_trait_selection[c090ce818561ca7c]::traits::select::SelectionContext>::poly_select::{closure#0}
  15:     0x75f789e60970 - <rustc_trait_selection[c090ce818561ca7c]::traits::fulfill::FulfillProcessor as rustc_data_structures[c066231c7fa32b84]::obligation_forest::ObligationProcessor>::process_obligation
  16:     0x75f78d8087f7 - <rustc_data_structures[c066231c7fa32b84]::obligation_forest::ObligationForest<rustc_trait_selection[c090ce818561ca7c]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[c090ce818561ca7c]::traits::fulfill::FulfillProcessor>
  17:     0x75f78a4aa3f3 - <rustc_hir_typeck[35f980454f904813]::fn_ctxt::FnCtxt>::confirm_builtin_call
  18:     0x75f78e2d8e65 - <rustc_hir_typeck[35f980454f904813]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  19:     0x75f78e2d21fd - <rustc_hir_typeck[35f980454f904813]::fn_ctxt::FnCtxt>::check_block_with_expected
  20:     0x75f78e2d96c8 - <rustc_hir_typeck[35f980454f904813]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args
  21:     0x75f78d8c01b7 - rustc_hir_typeck[35f980454f904813]::check::check_fn
  22:     0x75f78deda8f2 - rustc_hir_typeck[35f980454f904813]::typeck
  23:     0x75f78deda335 - rustc_query_impl[c25b9b67542a1b7e]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c25b9b67542a1b7e]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b9d4f7314f7035ba]::query::erase::Erased<[u8; 8usize]>>
  24:     0x75f78db84e7d - rustc_query_system[33396c7944b94add]::query::plumbing::try_execute_query::<rustc_query_impl[c25b9b67542a1b7e]::DynamicConfig<rustc_query_system[33396c7944b94add]::query::caches::VecCache<rustc_span[bd867921c9292620]::def_id::LocalDefId, rustc_middle[b9d4f7314f7035ba]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[c25b9b67542a1b7e]::plumbing::QueryCtxt, false>
  25:     0x75f78db84095 - rustc_query_impl[c25b9b67542a1b7e]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace
  26:     0x75f78db83d1b - <rustc_middle[b9d4f7314f7035ba]::hir::map::Map>::par_body_owners::<rustc_hir_analysis[9db2b2b48bb4004b]::check_crate::{closure#4}>::{closure#0}
  27:     0x75f78db81a64 - rustc_hir_analysis[9db2b2b48bb4004b]::check_crate
  28:     0x75f78db7a27f - rustc_interface[91cc5f479e6877b9]::passes::run_required_analyses
  29:     0x75f78e351bde - rustc_interface[91cc5f479e6877b9]::passes::analysis
  30:     0x75f78e351bb1 - rustc_query_impl[c25b9b67542a1b7e]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[c25b9b67542a1b7e]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b9d4f7314f7035ba]::query::erase::Erased<[u8; 1usize]>>
  31:     0x75f78e76b46e - rustc_query_system[33396c7944b94add]::query::plumbing::try_execute_query::<rustc_query_impl[c25b9b67542a1b7e]::DynamicConfig<rustc_query_system[33396c7944b94add]::query::caches::SingleCache<rustc_middle[b9d4f7314f7035ba]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[c25b9b67542a1b7e]::plumbing::QueryCtxt, false>
  32:     0x75f78e76b1cf - rustc_query_impl[c25b9b67542a1b7e]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  33:     0x75f78e5e57aa - rustc_interface[91cc5f479e6877b9]::interface::run_compiler::<core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>, rustc_driver_impl[e626c935ad353586]::run_compiler::{closure#0}>::{closure#1}
  34:     0x75f78e672a90 - std[ad0fe81d36b825dd]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[91cc5f479e6877b9]::util::run_in_thread_with_globals<rustc_interface[91cc5f479e6877b9]::util::run_in_thread_pool_with_globals<rustc_interface[91cc5f479e6877b9]::interface::run_compiler<core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>, rustc_driver_impl[e626c935ad353586]::run_compiler::{closure#0}>::{closure#1}, core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>>::{closure#0}, core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>>
  35:     0x75f78e6730fa - <<std[ad0fe81d36b825dd]::thread::Builder>::spawn_unchecked_<rustc_interface[91cc5f479e6877b9]::util::run_in_thread_with_globals<rustc_interface[91cc5f479e6877b9]::util::run_in_thread_pool_with_globals<rustc_interface[91cc5f479e6877b9]::interface::run_compiler<core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>, rustc_driver_impl[e626c935ad353586]::run_compiler::{closure#0}>::{closure#1}, core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>>::{closure#0}, core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[4a74943f2cbe9718]::result::Result<(), rustc_span[bd867921c9292620]::ErrorGuaranteed>>::{closure#1} as core[4a74943f2cbe9718]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  36:     0x75f78e67346b - std::sys::pal::unix::thread::Thread::new::thread_start::hf4aaee2b23b0d10a
  37:     0x75f788aa339d - <unknown>
  38:     0x75f788b2849c - <unknown>
  39:                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 (4d5b3b196 2024-08-20) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [typeck] type-checking `test`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 7 previous errors; 1 warning emitted

Some errors have detailed explanations: E0261, E0412, E0601, E0658.
For more information about an error, try `rustc --explain E0261`.

@rustbot label +F-transmutability

[matthiaskrgr]

#129217 cc @jswrenn ^^

[matthiaskrgr]

reduced a bit

use std::mem::BikeshedIntrinsicFrom;

pub fn is_transmutable<Src, Dst>()
where
    Dst: BikeshedIntrinsicFrom<Src>,
{
}

struct Src {
    g: G,
}

fn main() {
    is_transmutable::<&mut Src, &mut Dst>();
}

[jswrenn]

Hm, the problem seems to stem from G not being defined. I think there's been a few instances where we ICE on malformed types/parameters — hopefully there's some simple thing we can do at a top-level to catch these cases and fail gracefully.

[compiler-errors]

hopefully there's some simple thing we can do at a top-level

@jswrenn: I don't think so, since you're only encountering these error types on field projection. I recommend reworking the layout computation code to stop unwrapping and handle the layout errors gracefully.

[jswrenn]

The downside of doing so is that any misconceptions I have about layout_of will no longer be helpfully reported to me as ICEs.

Is there a way to query whether a Ty is well-formed?

[compiler-errors]

No, and also I think the particular unwrap in this case is very weird. I have no idea why it's unwrapping when it's only being called in functions that also only return Results.

[jswrenn]

Because it's important to get this right. For this scenario, the ideal thing to do might be to accept the transmute, because doing so will avoid emitting a distracting 'trait not implemented' error. But, in scenarios that don't involved malformed types where layout_of fails, this approach might mask a bug.

My ideal solution would be to detect at a top-level whether the involved types are well-formed, unconditionally accept the transmute if they are not, and retain the unwraps in tree.rs. Since that's not possible, the conservative fix will be to bubble up NotYetSupported errors. Emitting diagnostic might be distracting, but at least it won't ever be unsound.

[compiler-errors]

I don't believe the correct fix is to bubble up NotYetSupported errors. There literally is already a conversion from LayoutError to rustc_transmute::tree::rustc::Error: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_transmute/layout/tree.rs.html#195-204

That error struct already has variants to represent the layout errors this API should be handling. I don't see why you'd be turning them into NotYetSupported errors instead, and I'm skeptical of the claim that this will mask any bugs. I would consider any other code in rustc that's unwrapping a layout call (except for in post-monomorphization codegen) to be a bug, on the contrary.

[jswrenn]

Yeah, that sounds right — forgot we had that impl. For some reason, NotYetSupported is how we currently handle layout errors: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_transmute/maybe_transmutable/mod.rs.html#47-51

There's probably a UI test or issue somewhere that prompted that, but I agree that it smells.

[compiler-errors]

I recommend you change that one to handle layout errors gracefully, too.

[jswrenn]

Of course; will do!