unexpected panic when trait not implemented

[XopheD]

Code

pub trait Callback { fn cb(); }

pub trait Processing {
    type Call:Callback;
}

fn f<P:Processing+?Sized>() {
    P::Call::cb();
}

fn main() {
    struct MyCall;
    f::<dyn Processing<Call=MyCall>>();
}

Meta

rustc --version --verbose:

rustc 1.45.2 (d3fb005a3 2020-07-31)
binary: rustc
commit-hash: d3fb005a39e62501b8b0b356166e515ae24e2e54
commit-date: 2020-07-31
host: x86_64-apple-darwin
release: 1.45.2
LLVM version: 10.0

Error output

rustc src/main.rs 
error: internal compiler error: src/librustc_trait_selection/traits/codegen/mod.rs:62: Encountered error `Unimplemented` selecting `Binder(<main::MyCall as Callback>)` during codegen

Backtrace

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: rustc_driver::report_ice
   6: std::panicking::rust_panic_with_hook
   7: std::panicking::begin_panic
   8: rustc_errors::HandlerInner::bug
   9: rustc_errors::Handler::bug
  10: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  11: rustc_middle::ty::context::tls::with_opt::{{closure}}
  12: rustc_middle::ty::context::tls::with_opt
  13: rustc_middle::util::bug::opt_span_bug_fmt
  14: rustc_middle::util::bug::bug_fmt
  15: rustc_middle::ty::context::GlobalCtxt::enter_local
  16: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
  17: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::codegen_fulfill_obligation>::compute
  18: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  19: rustc_data_structures::stack::ensure_sufficient_stack
  20: rustc_query_system::query::plumbing::get_query_impl
  21: rustc_ty::instance::resolve_instance
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::resolve_instance>::compute
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: rustc_middle::ty::instance::Instance::resolve
  27: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator_kind
  28: rustc_mir::monomorphize::collector::collect_neighbours
  29: rustc_mir::monomorphize::collector::collect_items_rec
  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::interface::run_compiler_in_existing_thread_pool
  43: 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 1.45.2 (d3fb005a3 2020-07-31) running on x86_64-apple-darwin

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `Callback` fulfills its obligations
#1 [resolve_instance] resolving instance `<main::MyCall as Callback>::cb`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

[spastorino]

Given that it doesn't reproduce on godbolt I guess this is apple specific?.

[tesuji]

I can reproduce it in playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=6cf3b529e30212cf30d3e1af0a106f51
godbolt builds Rust code as crate-type=lib by default.

[spastorino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

[tesuji]

There is no need for containing main. Renaming main to anything and make it pub is suffice.
The error traces back to 1.0 Rust release: https://rust.godbolt.org/z/bs7vWY

[csmoe]

here is the unwrap to blame.

rust/src/librustc_mir/monomorphize/collector.rs

Line 722 in 6cc268e

ty::Instance::resolve(tcx, ty::ParamEnv::reveal_all(), def_id, substs).unwrap().unwrap()

[JohnTitor]

Triage: This is no longer ICE with the latest nightly, marking as E-needs-test.