keyword `become` yields ICE now.

[Neutron3529]

Code

I tried this code:

#![feature(explicit_tail_calls)]
fn foo() -> i32 {
    become 42 // ICE since Rust 1.81
}
pub fn main() {
    println!("{}", foo())
}

I expected to see this happen: Compile

Instead, this happened: ICE

rustc-ice-date.txt

thread 'rustc' panicked at compiler/rustc_mir_build/src/build/expr/stmt.rs:106:21:
Box<dyn Any>
stack backtrace:
   0:     0x7221523163e5 - std::backtrace::Backtrace::create::hd648a4a8e855af1e
   1:     0x722150ac7355 - std::backtrace::Backtrace::force_capture::he87459478c0a3ec6
   2:     0x72214fba33b4 - std[170610031590a965]::panicking::update_hook::<alloc[66d4c008c69e86cd]::boxed::Box<rustc_driver_impl[26aaba0b9f674e85]::install_ice_hook::{closure#0}>>::{closure#0}
   3:     0x722150adea08 - std::panicking::rust_panic_with_hook::hf9da426777cf33fb
   4:     0x72214fbdcf11 - std[170610031590a965]::panicking::begin_panic::<rustc_errors[2b9f4ebbc725b026]::ExplicitBug>::{closure#0}
   5:     0x72214fbcffb6 - std[170610031590a965]::sys::backtrace::__rust_end_short_backtrace::<std[170610031590a965]::panicking::begin_panic<rustc_errors[2b9f4ebbc725b026]::ExplicitBug>::{closure#0}, !>
   6:     0x72214fbcb499 - std[170610031590a965]::panicking::begin_panic::<rustc_errors[2b9f4ebbc725b026]::ExplicitBug>
   7:     0x72214fbe67a1 - <rustc_errors[2b9f4ebbc725b026]::diagnostic::BugAbort as rustc_errors[2b9f4ebbc725b026]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   8:     0x7221502332ed - <rustc_errors[2b9f4ebbc725b026]::DiagCtxtHandle>::span_bug::<rustc_span[d0163105bd3f350c]::span_encoding::Span, alloc[66d4c008c69e86cd]::string::String>
   9:     0x7221502463e8 - rustc_middle[1155dac776b01d50]::util::bug::opt_span_bug_fmt::<rustc_span[d0163105bd3f350c]::span_encoding::Span>::{closure#0}
  10:     0x72215024641a - rustc_middle[1155dac776b01d50]::ty::context::tls::with_opt::<rustc_middle[1155dac776b01d50]::util::bug::opt_span_bug_fmt<rustc_span[d0163105bd3f350c]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  11:     0x72215023be1b - rustc_middle[1155dac776b01d50]::ty::context::tls::with_context_opt::<rustc_middle[1155dac776b01d50]::ty::context::tls::with_opt<rustc_middle[1155dac776b01d50]::util::bug::opt_span_bug_fmt<rustc_span[d0163105bd3f350c]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  12:     0x72214f47a277 - rustc_middle[1155dac776b01d50]::util::bug::span_bug_fmt::<rustc_span[d0163105bd3f350c]::span_encoding::Span>
  13:     0x7221514c9738 - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::stmt_expr
  14:     0x722151527163 - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::expr_into_dest
  15:     0x72215151c585 - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::as_temp::{closure#0}
  16:     0x7221515263fb - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::expr_into_dest
  17:     0x72215152666f - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::expr_into_dest
  18:     0x722151daf14d - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::ast_block_stmts
  19:     0x722151dae147 - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::ast_block
  20:     0x7221515245a1 - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::expr_into_dest
  21:     0x72215152666f - <rustc_mir_build[bcefb118f41dfad5]::build::Builder>::expr_into_dest
  22:     0x722151b70fc1 - rustc_mir_build[bcefb118f41dfad5]::build::mir_build
  23:     0x722151204f06 - rustc_mir_transform[de8989103c0dc9d3]::mir_built
  24:     0x722151204ec9 - rustc_query_impl[180b75f2ddc39270]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[180b75f2ddc39270]::query_impl::mir_built::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1155dac776b01d50]::query::erase::Erased<[u8; 8usize]>>
  25:     0x7221515b16ba - rustc_query_system[9f857c887a67b714]::query::plumbing::try_execute_query::<rustc_query_impl[180b75f2ddc39270]::DynamicConfig<rustc_query_system[9f857c887a67b714]::query::caches::VecCache<rustc_span[d0163105bd3f350c]::def_id::LocalDefId, rustc_middle[1155dac776b01d50]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[180b75f2ddc39270]::plumbing::QueryCtxt, false>
  26:     0x7221515b11cd - rustc_query_impl[180b75f2ddc39270]::query_impl::mir_built::get_query_non_incr::__rust_end_short_backtrace
  27:     0x72214d8ee8a3 - rustc_mir_build[bcefb118f41dfad5]::check_unsafety::check_unsafety
  28:     0x722151a3b9e7 - rustc_query_impl[180b75f2ddc39270]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[180b75f2ddc39270]::query_impl::check_unsafety::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1155dac776b01d50]::query::erase::Erased<[u8; 0usize]>>
  29:     0x722151a3bc8b - rustc_query_system[9f857c887a67b714]::query::plumbing::try_execute_query::<rustc_query_impl[180b75f2ddc39270]::DynamicConfig<rustc_query_system[9f857c887a67b714]::query::caches::VecCache<rustc_span[d0163105bd3f350c]::def_id::LocalDefId, rustc_middle[1155dac776b01d50]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[180b75f2ddc39270]::plumbing::QueryCtxt, false>
  30:     0x722151a3b917 - rustc_query_impl[180b75f2ddc39270]::query_impl::check_unsafety::get_query_non_incr::__rust_end_short_backtrace
  31:     0x7221515aaf47 - rustc_interface[67c5fa7b03b13b84]::passes::run_required_analyses
  32:     0x722151d5ee9e - rustc_interface[67c5fa7b03b13b84]::passes::analysis
  33:     0x722151d5ee71 - rustc_query_impl[180b75f2ddc39270]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[180b75f2ddc39270]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1155dac776b01d50]::query::erase::Erased<[u8; 1usize]>>
  34:     0x72215207cbee - rustc_query_system[9f857c887a67b714]::query::plumbing::try_execute_query::<rustc_query_impl[180b75f2ddc39270]::DynamicConfig<rustc_query_system[9f857c887a67b714]::query::caches::SingleCache<rustc_middle[1155dac776b01d50]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[180b75f2ddc39270]::plumbing::QueryCtxt, false>
  35:     0x72215207c8cf - rustc_query_impl[180b75f2ddc39270]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  36:     0x722151f0939e - rustc_interface[67c5fa7b03b13b84]::interface::run_compiler::<core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>, rustc_driver_impl[26aaba0b9f674e85]::run_compiler::{closure#0}>::{closure#1}
  37:     0x722152029650 - std[170610031590a965]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[67c5fa7b03b13b84]::util::run_in_thread_with_globals<rustc_interface[67c5fa7b03b13b84]::util::run_in_thread_pool_with_globals<rustc_interface[67c5fa7b03b13b84]::interface::run_compiler<core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>, rustc_driver_impl[26aaba0b9f674e85]::run_compiler::{closure#0}>::{closure#1}, core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>>::{closure#0}, core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>>
  38:     0x722152029cba - <<std[170610031590a965]::thread::Builder>::spawn_unchecked_<rustc_interface[67c5fa7b03b13b84]::util::run_in_thread_with_globals<rustc_interface[67c5fa7b03b13b84]::util::run_in_thread_pool_with_globals<rustc_interface[67c5fa7b03b13b84]::interface::run_compiler<core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>, rustc_driver_impl[26aaba0b9f674e85]::run_compiler::{closure#0}>::{closure#1}, core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>>::{closure#0}, core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[d68a251df926a28b]::result::Result<(), rustc_span[d0163105bd3f350c]::ErrorGuaranteed>>::{closure#1} as core[d68a251df926a28b]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  39:     0x72215202a0ab - std::sys::pal::unix::thread::Thread::new::thread_start::h5cf1ac2878833c32
  40:     0x7221535b739d - <unknown>
  41:     0x72215363c49c - <unknown>
  42:                0x0 - <unknown>


rustc version: 1.83.0-nightly (fb4aebddd 2024-09-30)
platform: x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_built] building MIR for `foo`
#1 [check_unsafety] unsafety-checking `foo`
#2 [analysis] running analysis passes on this crate
end of query stack

Version it worked on

It most recently worked on: Rust 1.80 (with RUSTC_BOOTSTRAP=1 set, which could be verified in godbolt)

Version with regression

rustc --version --verbose:

rustc 1.83.0-nightly (fb4aebddd 2024-09-30)
binary: rustc
commit-hash: fb4aebddd18d258046ddb51fd41589295259a0fa
commit-date: 2024-09-30
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

Backtrace

Backtrace

thread 'rustc' panicked at compiler/rustc_mir_build/src/build/expr/stmt.rs:106:21:
Box<dyn Any>
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   2: <rustc_errors::DiagCtxtHandle>::span_bug::<rustc_span::span_encoding::Span, alloc::string::String>
   3: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   5: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   6: rustc_middle::util::bug::span_bug_fmt::<rustc_span::span_encoding::Span>
   7: <rustc_mir_build::build::Builder>::stmt_expr
   8: <rustc_mir_build::build::Builder>::expr_into_dest
   9: <rustc_mir_build::build::Builder>::as_temp::{closure#0}
  10: <rustc_mir_build::build::Builder>::expr_into_dest
  11: <rustc_mir_build::build::Builder>::expr_into_dest
  12: <rustc_mir_build::build::Builder>::ast_block_stmts
  13: <rustc_mir_build::build::Builder>::ast_block
  14: <rustc_mir_build::build::Builder>::expr_into_dest
  15: <rustc_mir_build::build::Builder>::expr_into_dest
  16: rustc_mir_build::build::mir_build
  17: rustc_mir_transform::mir_built
      [... omitted 1 frame ...]
  18: rustc_mir_build::check_unsafety::check_unsafety
      [... omitted 1 frame ...]
  19: rustc_interface::passes::run_required_analyses
  20: rustc_interface::passes::analysis
      [... omitted 1 frame ...]
  21: rustc_interface::interface::run_compiler::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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: please attach the file at `/me/rustc-ice-2024-10-14T04_38_18-24201.txt` to your bug report

note: compiler flags: -Z unstable-options -C link-arg=-fuse-ld=mold

query stack during panic:
#0 [mir_built] building MIR for `foo`
#1 [check_unsafety] unsafety-checking `foo`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 1 previous error; 1 warning emitted

[WaffleLapkin]

explicit tail calls is an incomplete feature, it is not properly implemented yet (and never was, the fact that it compiled before did not mean it compiled to something sensible)