ICE: `slices are not scalars` II

[matthiaskrgr]

File: /tmp/icemaker/033b7887410cef9d40e915f3b66f44d747d3e0d2.rs

#116513 season two

auto-reduced (treereduce-rust):

fn hey<T>(it: &[T])
where
    [T]: Clone,
{
}

fn main() {}

original:

// compile-flags: -Z mir-opt-level=0
// EMIT_MIR issue_91633.hey.mir_map.0.mir
fn hey<T> (it: &[T])
 where
     [T] : Clone,
 {
     let _ = {0;0};
 }

// EMIT_MIR issue_91633.bar.mir_map.0.mir
fn bar<T> (it: Box<[T]>)
 where
     [T] : std::ops::Index<usize>,
 {
     let _ = it[0];
 }

// EMIT_MIR issue_91633.fun.mir_map.0.mir
fn fun<T> (it: &[T]) -> &T
 {
     let f = &it[0];
     f
 }

// EMIT_MIR issue_91633.foo.mir_map.0.mir
fn foo<T: Clone> (it: Box<[T]>) -> T
 {
     let f = it[0].clone();
     f
 }
 fn main(){}

Version information

rustc 1.75.0-nightly (57ef88985 2023-10-13)
binary: rustc
commit-hash: 57ef8898520f7ff2c60341636bb86d94cb550e79
commit-date: 2023-10-13
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.2

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zmir-opt-level=3 --emit=mir

Program output

warning: unused variable: `it`
 --> /tmp/icemaker/033b7887410cef9d40e915f3b66f44d747d3e0d2.rs:3:12
  |
3 | fn hey<T> (it: &[T])
  |            ^^ help: if this is intentional, prefix it with an underscore: `_it`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: function `hey` is never used
 --> /tmp/icemaker/033b7887410cef9d40e915f3b66f44d747d3e0d2.rs:3:4
  |
3 | fn hey<T> (it: &[T])
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function `bar` is never used
  --> /tmp/icemaker/033b7887410cef9d40e915f3b66f44d747d3e0d2.rs:11:4
   |
11 | fn bar<T> (it: Box<[T]>)
   |    ^^^

warning: function `fun` is never used
  --> /tmp/icemaker/033b7887410cef9d40e915f3b66f44d747d3e0d2.rs:19:4
   |
19 | fn fun<T> (it: &[T]) -> &T
   |    ^^^

warning: function `foo` is never used
  --> /tmp/icemaker/033b7887410cef9d40e915f3b66f44d747d3e0d2.rs:26:4
   |
26 | fn foo<T: Clone> (it: Box<[T]>) -> T
   |    ^^^

thread 'rustc' panicked at compiler/rustc_mir_dataflow/src/value_analysis.rs:764:13:
slices are not scalars
stack backtrace:
   0:     0x7f2ba6b628fc - std::backtrace_rs::backtrace::libunwind::trace::h03c0bf785ff8ac95
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f2ba6b628fc - std::backtrace_rs::backtrace::trace_unsynchronized::heb554a194a8b8d76
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f2ba6b628fc - std::sys_common::backtrace::_print_fmt::h5104548cf2f1d04d
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f2ba6b628fc - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h60a89bd281db8532
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f2ba6bca350 - core::fmt::rt::Argument::fmt::h3f967ff7ed1eb864
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/core/src/fmt/rt.rs:142:9
   5:     0x7f2ba6bca350 - core::fmt::write::h6c7eacf90d438ccc
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/core/src/fmt/mod.rs:1117:17
   6:     0x7f2ba6b55c4f - std::io::Write::write_fmt::heed557ad0e7e227d
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/io/mod.rs:1762:15
   7:     0x7f2ba6b626e4 - std::sys_common::backtrace::_print::hc14194a28bba6478
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f2ba6b626e4 - std::sys_common::backtrace::print::h49644fe6309a893a
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f2ba6b654f7 - std::panicking::default_hook::{{closure}}::h2aa5ccff7297f9c8
  10:     0x7f2ba6b6523a - std::panicking::default_hook::h9e4bff1aa2bdf161
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/panicking.rs:292:9
  11:     0x7f2ba9718daa - std[5f80c380de201c9c]::panicking::update_hook::<alloc[9cbc8cef37f5f9a9]::boxed::Box<rustc_driver_impl[3ff10296a9c85239]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f2ba6b65c98 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h372bad5385f8c0c5
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/alloc/src/boxed.rs:2021:9
  13:     0x7f2ba6b65c98 - std::panicking::rust_panic_with_hook::h6c6741452089fc1b
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/panicking.rs:735:13
  14:     0x7f2ba6b659a6 - std::panicking::begin_panic_handler::{{closure}}::h281b553b64b3c920
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/panicking.rs:601:13
  15:     0x7f2ba6b62e16 - std::sys_common::backtrace::__rust_end_short_backtrace::h3fa2532f3895ceba
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/sys_common/backtrace.rs:170:18
  16:     0x7f2ba6b65722 - rust_begin_unwind
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/panicking.rs:597:5
  17:     0x7f2ba6bc6975 - core::panicking::panic_fmt::h7601551d8f1de7d6
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/core/src/panicking.rs:72:14
  18:     0x7f2ba9c2a8b7 - <rustc_mir_dataflow[68eaf0bb38c8fec3]::value_analysis::Map>::register_children
  19:     0x7f2ba9c299b8 - <rustc_mir_dataflow[68eaf0bb38c8fec3]::value_analysis::Map>::new
  20:     0x7f2ba9d0fd45 - <rustc_mir_transform[892ade550f628091]::dataflow_const_prop::DataflowConstProp as rustc_middle[48fc6e82f368dfd0]::mir::MirPass>::run_pass
  21:     0x7f2ba8560a5f - rustc_mir_transform[892ade550f628091]::pass_manager::run_passes_inner
  22:     0x7f2ba8d69a6c - rustc_mir_transform[892ade550f628091]::optimized_mir
  23:     0x7f2ba7a196dd - rustc_query_impl[d93ff4b57be0a3f9]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[d93ff4b57be0a3f9]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[48fc6e82f368dfd0]::query::erase::Erased<[u8; 8usize]>>
  24:     0x7f2ba7dcf2d1 - rustc_query_system[bf593801cb8c9f4a]::query::plumbing::try_execute_query::<rustc_query_impl[d93ff4b57be0a3f9]::DynamicConfig<rustc_query_system[bf593801cb8c9f4a]::query::caches::DefaultCache<rustc_span[3385b0a88a85fcb6]::def_id::DefId, rustc_middle[48fc6e82f368dfd0]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[d93ff4b57be0a3f9]::plumbing::QueryCtxt, false>
  25:     0x7f2ba91d8a22 - rustc_query_impl[d93ff4b57be0a3f9]::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
  26:     0x7f2ba7c8bba0 - <rustc_middle[48fc6e82f368dfd0]::ty::context::TyCtxt>::instance_mir
  27:     0x7f2ba9af9e5e - rustc_middle[48fc6e82f368dfd0]::mir::pretty::write_mir_pretty
  28:     0x7f2ba9cde8e3 - rustc_mir_transform[892ade550f628091]::dump_mir::emit_mir
  29:     0x7f2ba8ef940f - rustc_interface[d1c057f52199548c]::passes::start_codegen
  30:     0x7f2ba8ef8916 - <rustc_interface[d1c057f52199548c]::queries::Queries>::ongoing_codegen
  31:     0x7f2ba8ee7552 - std[5f80c380de201c9c]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[d1c057f52199548c]::util::run_in_thread_with_globals<rustc_interface[d1c057f52199548c]::interface::run_compiler<core[5ecddd5f6ab0df47]::result::Result<(), rustc_span[3385b0a88a85fcb6]::ErrorGuaranteed>, rustc_driver_impl[3ff10296a9c85239]::run_compiler::{closure#1}>::{closure#0}, core[5ecddd5f6ab0df47]::result::Result<(), rustc_span[3385b0a88a85fcb6]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5ecddd5f6ab0df47]::result::Result<(), rustc_span[3385b0a88a85fcb6]::ErrorGuaranteed>>
  32:     0x7f2ba8ee67d3 - <<std[5f80c380de201c9c]::thread::Builder>::spawn_unchecked_<rustc_interface[d1c057f52199548c]::util::run_in_thread_with_globals<rustc_interface[d1c057f52199548c]::interface::run_compiler<core[5ecddd5f6ab0df47]::result::Result<(), rustc_span[3385b0a88a85fcb6]::ErrorGuaranteed>, rustc_driver_impl[3ff10296a9c85239]::run_compiler::{closure#1}>::{closure#0}, core[5ecddd5f6ab0df47]::result::Result<(), rustc_span[3385b0a88a85fcb6]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5ecddd5f6ab0df47]::result::Result<(), rustc_span[3385b0a88a85fcb6]::ErrorGuaranteed>>::{closure#1} as core[5ecddd5f6ab0df47]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:     0x7f2ba6b70c05 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h0018b354bb46a254
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/alloc/src/boxed.rs:2007:9
  34:     0x7f2ba6b70c05 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hb166705f6d4a8b53
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/alloc/src/boxed.rs:2007:9
  35:     0x7f2ba6b70c05 - std::sys::unix::thread::Thread::new::thread_start::hdac26740529f6e03
                               at /rustc/57ef8898520f7ff2c60341636bb86d94cb550e79/library/std/src/sys/unix/thread.rs:108:17
  36:     0x7f2ba69139eb - <unknown>
  37:     0x7f2ba69977cc - <unknown>
  38:                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 1.75.0-nightly (57ef88985 2023-10-13) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z mir-opt-level=3 -Z dump-mir-dir=dir

query stack during panic:
#0 [optimized_mir] optimizing MIR for `hey`
end of query stack
warning: 5 warnings emitted

[ouz-a]

cc @cjgillot

[Noratrieb]

I cannot reproduce the original anymore, but #121357 does reproduce.
The body is impossible because of the where clauses, so no MIR opts should ever run on it...
I'll take a look.

[matthiaskrgr]

I can still reproduce this with the original mvce when I use the flags that I listed.

[Sky9x]

bisected #121357 with -Zmir-opt-level=4

searched nightlies: from nightly-2023-09-01 to nightly-2023-10-01
regressed nightly: nightly-2023-09-09
searched commit range: 1e746d7...62ebe3a
regressed commit: 3cd97ed

bisected with cargo-bisect-rustc v0.6.8

cc #115612
oli was right: #116513 (comment)

[Noratrieb]

The bisection here isn't useful sadly, but thank you anyways.
I took a look and yeah, the issue is what I suspected. impossible_predicates (

rust/compiler/rustc_mir_transform/src/lib.rs

Line 451 in bb59453

.filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });

) checks whether there are any impossible predicates (like Self: Sized here). It sadly has to filter out "non-global" predicates, which Self: Sized belongs to. It has to do this because of overflows as mentioned above.
@compiler-errors told me that the new trait solvers non-fatal handling of overflow will allow this to be fixed. But until then, there's nothing we can do.

As for getting the code working, the solution here so to avoid these impossible bounds wherever possible. In the example here, that's easy to do, just delete it. In #121357 that's a bit more annoying, I don't really see a solution.

We could try playing whack-a-mole by trying to make the MIR opts sorta-handle these cases that slip through, but I don't think that's very sustainable. Might work at least for the trivial cases like #121357 though, but I don't open PRs for that.

[matthiaskrgr]

another one

fn cast_same_meta<T: ?Sized, U: ?Sized>(ptr: *const T) -> *const U {}

fn sized_slice<T>(ptr: *const [T]) -> *const str
where
    [T]: Sized,
{
    cast_same_meta(ptr)
}