ICE with main lang items

[benbrittain]

Code

in library:

#[lang = "start"]                                                                                                                             
extern "C" fn start(main: fn(), _argc: isize, _argv: *const *const u8) -> isize {                                                             
    main();                                                                                                                                   
    0                                                                                                                                         
}                                                                                                                                             

in no_std binary:

fn main() {
...
}

This goes away if I turn on #![no_main] in the binary and annotate main with #[no_mangle].

Meta

rustc --version --verbose:

binary: rustc
commit-hash: f455e46eae1a227d735091091144601b467e1565
commit-date: 2020-06-20
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0

Error output

   Compiling rust-example v0.1.0 (/home/ben/workspace/nrf-test/rust-example)
error: internal compiler error: src/librustc_metadata/rmeta/decoder.rs:1106:17: get_optimized_mir: missing MIR for `DefId(4:22 ~ nrf52_sys[e357]::start[0])`

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:916:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_metadata::rmeta::decoder::<impl rustc_metadata::creader::CrateMetadataRef>::get_optimized_mir::{{closure}}
  21: rustc_metadata::rmeta::decoder::<impl rustc_metadata::creader::CrateMetadataRef>::get_optimized_mir
  22: rustc_metadata::rmeta::decoder::cstore_impl::provide_extern::optimized_mir
  23: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::optimized_mir>::compute
  24: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  25: rustc_data_structures::stack::ensure_sufficient_stack
  26: rustc_query_system::query::plumbing::get_query_impl
  27: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::instance_mir
  28: rustc_mir::monomorphize::collector::collect_neighbours
  29: rustc_data_structures::stack::ensure_sufficient_stack
  30: rustc_mir::monomorphize::collector::collect_items_rec
  31: rustc_mir::monomorphize::collector::collect_crate_mono_items
  32: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  33: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::collect_and_partition_mono_items>::compute
  34: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  35: rustc_data_structures::stack::ensure_sufficient_stack
  36: rustc_query_system::query::plumbing::get_query_impl
  37: rustc_codegen_ssa::base::codegen_crate
  38: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  39: rustc_interface::passes::start_codegen
  40: rustc_middle::ty::context::tls::enter_global
  41: rustc_interface::queries::Queries::ongoing_codegen
  42: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  43: rustc_span::with_source_map
  44: rustc_interface::interface::run_compiler_in_existing_thread_pool
  45: scoped_tls::ScopedKey<T>::set
  46: rustc_ast::attr::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.46.0-nightly (f455e46ea 2020-06-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -C panic=abort -C embed-bitcode=no -C debuginfo=2 -C linker=arm-none-eabi-gcc -C link-arg=-mthumb -C link-arg=-Tnrf52.ld -C link-arg=-mcpu=cortex-m4 -C link-arg=--specs=nano.specs -C link-arg=-Wl,--gc-sections -C link-arg=-Wl,--start-group -C link-arg=-Wl,--end-group -C link-arg=-Wl,--build-id=none -C link-arg=-mabi=aapcs -C link-arg=-mfloat-abi=hard -C link-arg=-mfpu=fpv4-sp-d16 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [optimized_mir] optimizing MIR for `nrf52_sys::start`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

[RalfJung]

FWIW, when a lang item is used incorrectly, ICEs are expected. The standard library is the only code that is ever expected to define lang items, and if those are wrong then an ICE is appropriate.

Better errors are nice, but no attempt is made to actually be exhaustive about that.