ICE: `Size::sub: 0 - 1 would result in negative size`

[matthiaskrgr]

auto-reduced (treereduce-rust):

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

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

#[repr(C)]
struct Zst;

#[repr(u8)]
enum V0 {
    V = 0,
}

enum V2 {
    V = 2,
}

enum Lopsided {
    Smol(Zst),
    Lorg(V0),
}

#[repr(C)]
#[repr(C)]
struct Dst(Lopsided, V2);

fn should_pad_variants() {
    assert::is_transmutable::<Src, Dst>();
}

original code

original: ````rust //! The variants of an enum must be padded with uninit bytes such that they have //! the same length (in bytes). #![crate_type = "lib"] #![feature(transmutability)] #![allow(dead_code)] mod assert { use std::mem::{Assume, BikeshedIntrinsicFrom}; pub fn is_transmutable() where Dst: BikeshedIntrinsicFrom {} } #[derive(Clone, Copy)] #[repr(C)] struct Zst; #[derive(Clone, Copy)] #[repr(u8)] enum V0 { V = 0 } #[derive(Clone, Copy)] #[repr(u8)] enum V2 { V = 2 } #[repr(transmutability)] enum Lopsided { Smol(Zst), Lorg(V0), } #[repr(C)] struct Src(V0, Zst V2); #[repr(C)] struct Dst(Lopsided, V2); fn should_pad_variants() { // If the implementation (incorrectly) fails to pad `Lopsided::Smol` with // an uninitialized byte, this transmutation might be (wrongly) accepted: assert::is_transmutable::(); //~ ERROR cannot be safely transmuted } ````

Version information

rustc 1.79.0-nightly (033becf83 2024-04-09)
binary: rustc
commit-hash: 033becf83c62814357f4810db149471db46ab816
commit-date: 2024-04-09
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.3

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

Program output

``` error[E0412]: cannot find type `Src` in this scope --> /tmp/icemaker_global_tempdir.7GMaangOEeD3/rustc_testrunner_tmpdir_reporting.sKS3rJHCTOdj/mvce.rs:33:31 | 33 | assert::is_transmutable::(); | ^^^ not found in this scope | help: you might be missing a type parameter | 32 | fn should_pad_variants() { | +++++ warning: unused import: `Assume` --> /tmp/icemaker_global_tempdir.7GMaangOEeD3/rustc_testrunner_tmpdir_reporting.sKS3rJHCTOdj/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.7GMaangOEeD3/rustc_testrunner_tmpdir_reporting.sKS3rJHCTOdj/mvce.rs:34:2 | 34 | } | ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.7GMaangOEeD3/rustc_testrunner_tmpdir_reporting.sKS3rJHCTOdj/mvce.rs` error[E0658]: use of unstable library feature 'transmutability' --> /tmp/icemaker_global_tempdir.7GMaangOEeD3/rustc_testrunner_tmpdir_reporting.sKS3rJHCTOdj/mvce.rs:2:20 | 2 | use std::mem::{Assume, BikeshedIntrinsicFrom}; | ^^^^^^ | = note: see issue #99571 for more information = help: add `#![feature(transmutability)]` to the crate attributes to enable = note: this compiler was built on 2024-04-09; consider upgrading it if it is out of date error[E0658]: use of unstable library feature 'transmutability' --> /tmp/icemaker_global_tempdir.7GMaangOEeD3/rustc_testrunner_tmpdir_reporting.sKS3rJHCTOdj/mvce.rs:2:28 | 2 | use std::mem::{Assume, BikeshedIntrinsicFrom}; | ^^^^^^^^^^^^^^^^^^^^^ | = note: see issue #99571 for more information = help: add `#![feature(transmutability)]` to the crate attributes to enable = note: this compiler was built on 2024-04-09; consider upgrading it if it is out of date error[E0658]: use of unstable library feature 'transmutability' --> /tmp/icemaker_global_tempdir.7GMaangOEeD3/rustc_testrunner_tmpdir_reporting.sKS3rJHCTOdj/mvce.rs:6:14 | 6 | Dst: BikeshedIntrinsicFrom, | ^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: see issue #99571 for more information = help: add `#![feature(transmutability)]` to the crate attributes to enable = note: this compiler was built on 2024-04-09; consider upgrading it if it is out of date thread 'rustc' panicked at /rustc/033becf83c62814357f4810db149471db46ab816/compiler/rustc_abi/src/lib.rs:581:13: Size::sub: 0 - 1 would result in negative size stack backtrace: 0: 0x79db92be6885 - std::backtrace_rs::backtrace::libunwind::trace::he2880c2699337794 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5 1: 0x79db92be6885 - std::backtrace_rs::backtrace::trace_unsynchronized::h460ebc9cd0eb6331 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5 2: 0x79db92be6885 - std::sys_common::backtrace::_print_fmt::h47b746c0604572c3 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/sys_common/backtrace.rs:68:5 3: 0x79db92be6885 - ::fmt::ha907b996746f56a0 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/sys_common/backtrace.rs:44:22 4: 0x79db92c35abb - core::fmt::rt::Argument::fmt::hfe22b646b1687193 at /rustc/033becf83c62814357f4810db149471db46ab816/library/core/src/fmt/rt.rs:142:9 5: 0x79db92c35abb - core::fmt::write::h6b4a267d08f3d00d at /rustc/033becf83c62814357f4810db149471db46ab816/library/core/src/fmt/mod.rs:1153:17 6: 0x79db92bdb3ff - std::io::Write::write_fmt::h0abdd14d11c1702a at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/io/mod.rs:1843:15 7: 0x79db92be665e - std::sys_common::backtrace::_print::hbeb8c9c7793a4a1e at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/sys_common/backtrace.rs:47:5 8: 0x79db92be665e - std::sys_common::backtrace::print::h6ca4866d684d9b00 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/sys_common/backtrace.rs:34:9 9: 0x79db92be9159 - std::panicking::default_hook::{{closure}}::h33647e3098e0b657 10: 0x79db92be8e75 - std::panicking::default_hook::ha6320bcd5d0e9cc8 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/panicking.rs:291:9 11: 0x79db8f46649f - std[5013708940097fa8]::panicking::update_hook::>::{closure#0} 12: 0x79db92be985c - as core::ops::function::Fn>::call::hf428354f3713bf5b at /rustc/033becf83c62814357f4810db149471db46ab816/library/alloc/src/boxed.rs:2032:9 13: 0x79db92be985c - std::panicking::rust_panic_with_hook::h0014da0b7030d171 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/panicking.rs:792:13 14: 0x79db92be9606 - std::panicking::begin_panic_handler::{{closure}}::h639e3e9de7a3e2e4 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/panicking.rs:657:13 15: 0x79db92be6d49 - std::sys_common::backtrace::__rust_end_short_backtrace::h66ed5715b81f24d6 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/sys_common/backtrace.rs:171:18 16: 0x79db92be9337 - rust_begin_unwind at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/panicking.rs:645:5 17: 0x79db92c31f56 - core::panicking::panic_fmt::h2fea69f54f6dc828 at /rustc/033becf83c62814357f4810db149471db46ab816/library/core/src/panicking.rs:72:14 18: 0x79db8fffc247 - ::sub::{closure#0} 19: 0x79db9000cb17 - >::from_variant 20: 0x79db9000bffa - >::from_enum 21: 0x79db9000c756 - >::from_variant 22: 0x79db9000b43f - >::from_struct 23: 0x79db9000f833 - ::is_transmutable 24: 0x79db910150a5 - ::confirm_candidate 25: 0x79db8d4ba166 - ::evaluate_candidate 26: 0x79db9115a88c - ::evaluate_trait_predicate_recursively 27: 0x79db90f0f668 - ::evaluation_probe::<::evaluate_root_obligation::{closure#0}> 28: 0x79db90f0dc9a - rustc_traits[ccae72c98d3fa03e]::evaluate_obligation::evaluate_obligation 29: 0x79db90f0d62b - rustc_query_impl[a13915abec574cc4]::plumbing::__rust_begin_short_backtrace::> 30: 0x79db90f0cb92 - rustc_query_system[37cb0ad7182a86b8]::query::plumbing::try_execute_query::>, rustc_middle[867c8a7f7f3067fc]::query::erase::Erased<[u8; 2usize]>>, false, false, false>, rustc_query_impl[a13915abec574cc4]::plumbing::QueryCtxt, false> 31: 0x79db90f0c7da - rustc_query_impl[a13915abec574cc4]::query_impl::evaluate_obligation::get_query_non_incr::__rust_end_short_backtrace 32: 0x79db8d404081 - ::process_obligation 33: 0x79db9080d663 - >::process_obligations:: 34: 0x79db90fe11bf - ::check_argument_types 35: 0x79db90fd71eb - ::check_call 36: 0x79db9128d159 - ::check_expr_with_expectation_and_args 37: 0x79db91287a26 - ::check_block_with_expected 38: 0x79db9128d63d - ::check_expr_with_expectation_and_args 39: 0x79db909ee20e - rustc_hir_typeck[90aadd5cc69e56e5]::check::check_fn 40: 0x79db909e4749 - rustc_hir_typeck[90aadd5cc69e56e5]::typeck 41: 0x79db909e4109 - rustc_query_impl[a13915abec574cc4]::plumbing::__rust_begin_short_backtrace::> 42: 0x79db909d0871 - rustc_query_system[37cb0ad7182a86b8]::query::plumbing::try_execute_query::>, false, false, false>, rustc_query_impl[a13915abec574cc4]::plumbing::QueryCtxt, false> 43: 0x79db909cf18c - rustc_query_impl[a13915abec574cc4]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace 44: 0x79db909ced54 - ::par_body_owners::::{closure#0} 45: 0x79db909cda1f - rustc_hir_analysis[341fcfcfa163e951]::check_crate 46: 0x79db910ee93a - rustc_interface[791da96d69a7cf4c]::passes::analysis 47: 0x79db910ee495 - rustc_query_impl[a13915abec574cc4]::plumbing::__rust_begin_short_backtrace::> 48: 0x79db915fa6a5 - rustc_query_system[37cb0ad7182a86b8]::query::plumbing::try_execute_query::>, false, false, false>, rustc_query_impl[a13915abec574cc4]::plumbing::QueryCtxt, false> 49: 0x79db915fa409 - rustc_query_impl[a13915abec574cc4]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace 50: 0x79db91480cd4 - rustc_interface[791da96d69a7cf4c]::interface::run_compiler::, rustc_driver_impl[83e8eb7d3795c9f9]::run_compiler::{closure#0}>::{closure#0} 51: 0x79db9154deff - std[5013708940097fa8]::sys_common::backtrace::__rust_begin_short_backtrace::, rustc_driver_impl[83e8eb7d3795c9f9]::run_compiler::{closure#0}>::{closure#0}, core[b815557cbe89e06]::result::Result<(), rustc_span[ef049dfdad08a076]::ErrorGuaranteed>>::{closure#0}, core[b815557cbe89e06]::result::Result<(), rustc_span[ef049dfdad08a076]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[b815557cbe89e06]::result::Result<(), rustc_span[ef049dfdad08a076]::ErrorGuaranteed>> 52: 0x79db9154dcee - <::spawn_unchecked_, rustc_driver_impl[83e8eb7d3795c9f9]::run_compiler::{closure#0}>::{closure#0}, core[b815557cbe89e06]::result::Result<(), rustc_span[ef049dfdad08a076]::ErrorGuaranteed>>::{closure#0}, core[b815557cbe89e06]::result::Result<(), rustc_span[ef049dfdad08a076]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[b815557cbe89e06]::result::Result<(), rustc_span[ef049dfdad08a076]::ErrorGuaranteed>>::{closure#1} as core[b815557cbe89e06]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} 53: 0x79db92bf328b - as core::ops::function::FnOnce>::call_once::h53814b1241345312 at /rustc/033becf83c62814357f4810db149471db46ab816/library/alloc/src/boxed.rs:2018:9 54: 0x79db92bf328b - as core::ops::function::FnOnce>::call_once::h68f33d2823fce7a0 at /rustc/033becf83c62814357f4810db149471db46ab816/library/alloc/src/boxed.rs:2018:9 55: 0x79db92bf328b - std::sys::pal::unix::thread::Thread::new::thread_start::h5c5183b404e56855 at /rustc/033becf83c62814357f4810db149471db46ab816/library/std/src/sys/pal/unix/thread.rs:108:17 56: 0x79db9299155a - 57: 0x79db92a0ea3c - 58: 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: rustc 1.79.0-nightly (033becf83 2024-04-09) running on x86_64-unknown-linux-gnu query stack during panic: #0 [evaluate_obligation] evaluating trait selection obligation `Dst: core::mem::transmutability::BikeshedIntrinsicFrom<{type error}, core::mem::transmutability::Assume { alignment: false, lifetimes: false, safety: false, validity: false }>` #1 [typeck] type-checking `should_pad_variants` #2 [analysis] running analysis passes on this crate end of query stack error: aborting due to 5 previous errors; 1 warning emitted Some errors have detailed explanations: E0412, E0601, E0658. For more information about an error, try `rustc --explain E0412`. ```

[compiler-errors]

cc @jswrenn

[jswrenn]

On it! Thanks for the report.

[jswrenn]

Further minimized:

#![feature(transmutability)]

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

    pub fn is_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src, { Assume::NOTHING }>,
    {
    }
}

enum Lopsided {
    Smol(()),
    Lorg(bool),
}

fn should_pad_variants() {
    assert::is_transmutable::<Lopsided, ()>();
}

[jswrenn]

The cause here is neat. Lopsided here is niche optimizable, with its second variant being subject to niche optimization. Typically, a niche-optimized enum has one tagless variant, and zero or more tagged variants laid out in the usual manner: the tag is placed, and then the rest of the fields follow. But #[rustc_layout(debug)] reveals that's not what happens here. Because () is a ZST, () is placed at offset 0, rather than 1 (following the tag). In other words: the tag is co-located with the field. This produces an ICE, because our assumption that fields follow tag is violated (a checked sub panics).

[jswrenn]

@rustbot claim