ICE: `Got a scalar pair where a scalar was expected` when using the `Self` on `fn`, a wrong place.

[TsiyukiRitsuki]

A really similar case I found is: #135128
But I think they are different, as when trying the code in that issue, it didn't panic in the latest rustc: 1.88.0-nightly, but mine does.

Code

fn f() -> &'static str
where
    Self: Sized,
{
    ""
}

Command: rustc -C opt-level=1 -Zvalidate-mir

Meta

rustc --version --verbose:

rustc 1.88.0-nightly (10fa3c449 2025-04-26)
binary: rustc
commit-hash: 10fa3c449f6b1613b352a6cbf78d3d91fd9a1d81
commit-date: 2025-04-26
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

Error output

error[E0411]: cannot find type `Self` in this scope
 --> test/tmp.rs:3:5
  |
1 | fn f() -> &'static str
  |    - `Self` not allowed in a function
2 | where
3 |     Self: Sized,
  |     ^^^^ `Self` is only available in impls, traits, and type definitions

error: internal compiler error: /rustc/10fa3c449f6b1613b352a6cbf78d3d91fd9a1d81/compiler/rustc_const_eval/src/interpret/operand.rs:84:42: Got a scalar pair where a scalar was expected


thread 'rustc' panicked at /rustc/10fa3c449f6b1613b352a6cbf78d3d91fd9a1d81/compiler/rustc_const_eval/src/interpret/operand.rs:84:42:
Box<dyn Any>

Backtrace

error[E0411]: cannot find type `Self` in this scope
 --> test/tmp.rs:3:5
  |
1 | fn f() -> &'static str
  |    - `Self` not allowed in a function
2 | where
3 |     Self: Sized,
  |     ^^^^ `Self` is only available in impls, traits, and type definitions

error: internal compiler error: /rustc/10fa3c449f6b1613b352a6cbf78d3d91fd9a1d81/compiler/rustc_const_eval/src/interpret/operand.rs:84:42: Got a scalar pair where a scalar was expected


thread 'rustc' panicked at /rustc/10fa3c449f6b1613b352a6cbf78d3d91fd9a1d81/compiler/rustc_const_eval/src/interpret/operand.rs:84:42:
Box<dyn Any>
stack backtrace:
   0:     0x7837b4ce90f3 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::he1cc6de51b8e3e46
   1:     0x7837b5405ac7 - core::fmt::write::h546351ef89cac2c3
   2:     0x7837b683e6d1 - std::io::Write::write_fmt::hdd0a9df47aae8e9f
   3:     0x7837b4ce8f52 - std::sys::backtrace::BacktraceLock::print::h575bf1241b79e77a
   4:     0x7837b4cecd9a - std::panicking::default_hook::{{closure}}::h9889804a123e7c5b
   5:     0x7837b4cec91f - std::panicking::default_hook::h0459a96a46f07fba
   6:     0x7837b3d710d3 - std[799ab5ff8c92313d]::panicking::update_hook::<alloc[8b075dae1d9a4a4]::boxed::Box<rustc_driver_impl[8ffa39a1f100e9bb]::install_ice_hook::{closure#1}>>::{closure#0}
   7:     0x7837b4ced613 - std::panicking::rust_panic_with_hook::hb01102643345a267
   8:     0x7837b3dacac1 - std[799ab5ff8c92313d]::panicking::begin_panic::<rustc_errors[3ee20515ee49c7f0]::ExplicitBug>::{closure#0}
   9:     0x7837b3da0af6 - std[799ab5ff8c92313d]::sys::backtrace::__rust_end_short_backtrace::<std[799ab5ff8c92313d]::panicking::begin_panic<rustc_errors[3ee20515ee49c7f0]::ExplicitBug>::{closure#0}, !>
  10:     0x7837b3da097b - std[799ab5ff8c92313d]::panicking::begin_panic::<rustc_errors[3ee20515ee49c7f0]::ExplicitBug>
  11:     0x7837b3db6fb1 - <rustc_errors[3ee20515ee49c7f0]::diagnostic::BugAbort as rustc_errors[3ee20515ee49c7f0]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  12:     0x7837b43f1f9a - rustc_middle[d1b324fb2b9fb44f]::util::bug::opt_span_bug_fmt::<rustc_span[9169b51a7177056a]::span_encoding::Span>::{closure#0}
  13:     0x7837b43ccdfa - rustc_middle[d1b324fb2b9fb44f]::ty::context::tls::with_opt::<rustc_middle[d1b324fb2b9fb44f]::util::bug::opt_span_bug_fmt<rustc_span[9169b51a7177056a]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7837b43ccc6b - rustc_middle[d1b324fb2b9fb44f]::ty::context::tls::with_context_opt::<rustc_middle[d1b324fb2b9fb44f]::ty::context::tls::with_opt<rustc_middle[d1b324fb2b9fb44f]::util::bug::opt_span_bug_fmt<rustc_span[9169b51a7177056a]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7837b168a6c0 - rustc_middle[d1b324fb2b9fb44f]::util::bug::bug_fmt
  16:     0x7837b2ad897c - <rustc_mir_transform[b9389225760552bd]::gvn::VnState as rustc_middle[d1b324fb2b9fb44f]::mir::visit::MutVisitor>::visit_statement
  17:     0x7837b2acea5b - <rustc_mir_transform[b9389225760552bd]::gvn::GVN as rustc_mir_transform[b9389225760552bd]::pass_manager::MirPass>::run_pass
  18:     0x7837b54039fb - rustc_mir_transform[b9389225760552bd]::pass_manager::run_passes_inner
  19:     0x7837b5d87c1f - rustc_mir_transform[b9389225760552bd]::optimized_mir
  20:     0x7837b5d8720d - rustc_query_impl[5054593ac04cc94b]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[5054593ac04cc94b]::query_impl::optimized_mir::dynamic_query::{closure#2}::{closure#0}, rustc_middle[d1b324fb2b9fb44f]::query::erase::Erased<[u8; 8usize]>>
  21:     0x7837b57b646f - rustc_query_system[868e915c35b94eeb]::query::plumbing::try_execute_query::<rustc_query_impl[5054593ac04cc94b]::DynamicConfig<rustc_query_system[868e915c35b94eeb]::query::caches::DefIdCache<rustc_middle[d1b324fb2b9fb44f]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[5054593ac04cc94b]::plumbing::QueryCtxt, false>
  22:     0x7837b57b589d - rustc_query_impl[5054593ac04cc94b]::query_impl::optimized_mir::get_query_non_incr::__rust_end_short_backtrace
  23:     0x7837b58d782f - <rustc_middle[d1b324fb2b9fb44f]::ty::context::TyCtxt>::instance_mir
  24:     0x7837b5ef588c - rustc_interface[4f0825a59a6ff88a]::passes::run_required_analyses
  25:     0x7837b62d7c5e - rustc_interface[4f0825a59a6ff88a]::passes::analysis
  26:     0x7837b62d7c2d - rustc_query_impl[5054593ac04cc94b]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[5054593ac04cc94b]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[d1b324fb2b9fb44f]::query::erase::Erased<[u8; 0usize]>>
  27:     0x7837b62dd73d - rustc_query_system[868e915c35b94eeb]::query::plumbing::try_execute_query::<rustc_query_impl[5054593ac04cc94b]::DynamicConfig<rustc_query_system[868e915c35b94eeb]::query::caches::SingleCache<rustc_middle[d1b324fb2b9fb44f]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[5054593ac04cc94b]::plumbing::QueryCtxt, false>
  28:     0x7837b62dd438 - rustc_query_impl[5054593ac04cc94b]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  29:     0x7837b66341c0 - rustc_interface[4f0825a59a6ff88a]::passes::create_and_enter_global_ctxt::<core[30c64d6a260d8eb8]::option::Option<rustc_interface[4f0825a59a6ff88a]::queries::Linker>, rustc_driver_impl[8ffa39a1f100e9bb]::run_compiler::{closure#0}::{closure#2}>::{closure#2}::{closure#0}
  30:     0x7837b64aeb84 - rustc_interface[4f0825a59a6ff88a]::interface::run_compiler::<(), rustc_driver_impl[8ffa39a1f100e9bb]::run_compiler::{closure#0}>::{closure#1}
  31:     0x7837b6353b28 - std[799ab5ff8c92313d]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[4f0825a59a6ff88a]::util::run_in_thread_with_globals<rustc_interface[4f0825a59a6ff88a]::util::run_in_thread_pool_with_globals<rustc_interface[4f0825a59a6ff88a]::interface::run_compiler<(), rustc_driver_impl[8ffa39a1f100e9bb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  32:     0x7837b6353f74 - <<std[799ab5ff8c92313d]::thread::Builder>::spawn_unchecked_<rustc_interface[4f0825a59a6ff88a]::util::run_in_thread_with_globals<rustc_interface[4f0825a59a6ff88a]::util::run_in_thread_pool_with_globals<rustc_interface[4f0825a59a6ff88a]::interface::run_compiler<(), rustc_driver_impl[8ffa39a1f100e9bb]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[30c64d6a260d8eb8]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  33:     0x7837b6355377 - std::sys::pal::unix::thread::Thread::new::thread_start::h6104b903ae36f332
  34:     0x7837b02a3708 - <unknown>
  35:     0x7837b0327aac - <unknown>
  36:                0x0 - <unknown>

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 1.88.0-nightly (10fa3c449 2025-04-26) running on x86_64-unknown-linux-gnu

note: compiler flags: -C opt-level=3 -Z no-codegen -Z validate-mir --crate-type lib

query stack during panic:
#0 [optimized_mir] optimizing MIR for `f`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0411`.

[theemathas]

@rustbot labels +-Zvalidate-mir

[moxian]

Reproduces with rustc -Zvalidate-mir -Zmir-enable-passes=+GVN
Bisects to #132527 ; cc @dianqk

@rustbot label: +A-mir-opt-GVN -needs-triage +S-has-bisection +S-has-mcve

[dianqk]

This seems to be a duplicate of #121363. When enabling debug assertions with -Zmir-opt-level=0, I can encounter:

thread 'rustc' panicked at /home/dianqk/rs/rust/compiler/rustc_const_eval/src/interpret/operand.rs:266:9:
immediate ScalarPair(alloc1<imm>, 0x0000000000000000) does not fit to layout TyAndLayout { ty: &str, layout: Layout { size: Size(8 bytes), align: AbiAndPrefAlign { abi: Align(8 bytes), pref: Align(8 bytes) }, backend_repr: Scalar(Initialized { value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), fields: Primitive, largest_niche: Some(Niche { offset: Size(0 bytes), value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), uninhabited: false, variants: Single { index: 0 }, max_repr_align: None, unadjusted_abi_align: Align(8 bytes), randomization_seed: 281492156579847 } }
...
query stack during panic:
#0 [mir_drops_elaborated_and_const_checked] elaborating drops for `f`
#1 [analysis] running analysis passes on this crate
end of query stack