Regression in nightly-2020-07-22: collection encountered polymorphic constant

[dtolnay]

The widely used reqwest crate began failing to compile in nightly-2020-07-22. To reproduce, clone https://github.com/seanmonstar/reqwest/tree/69288606bcf648967ddd09000f06e88161f80a2f and run cargo build.

error: internal compiler error: src/librustc_mir/monomorphize/collector.rs:629:54: collection encountered polymorphic constant
  --> /home/david/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.22/src/runtime/task/raw.rs:30:1
   |
30 | / pub(super) fn vtable<T: Future, S: Schedule>() -> &'static Vtable {
31 | |     &Vtable {
32 | |         poll: poll::<T, S>,
33 | |         dealloc: dealloc::<T, S>,
...  |
37 | |     }
38 | | }
   | |_^

Note the error is presented on tokio-0.2.22 but that crate compiles fine by itself. Running cargo build again indicates it's only reqwest being compiled when the ICE occurs.

Complete error and backtrace

$ RUST_BACKTRACE=1 cargo +nightly-2020-07-22 build
   Compiling reqwest v0.10.6 (/git/tmp/reqwest)
error: internal compiler error: src/librustc_mir/monomorphize/collector.rs:629:54: collection encountered polymorphic constant
  --> /home/david/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.22/src/runtime/task/raw.rs:30:1
   |
30 | / pub(super) fn vtable<T: Future, S: Schedule>() -> &'static Vtable {
31 | |     &Vtable {
32 | |         poll: poll::<T, S>,
33 | |         dealloc: dealloc::<T, S>,
...  |
37 | |     }
38 | | }
   | |_^

thread 'rustc' panicked at 'Box<Any>', /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/macros.rs:13:23
stack backtrace:
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/../backtrace/src/backtrace/libunwind.rs:96
   1: std::backtrace_rs::backtrace::trace_unsynchronized
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/../backtrace/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/sys_common/backtrace.rs:58
   4: core::fmt::write
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libcore/fmt/mod.rs:1117
   5: std::io::Write::write_fmt
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/io/mod.rs:1508
   6: std::sys_common::backtrace::_print
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/sys_common/backtrace.rs:61
   7: std::sys_common::backtrace::print
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/sys_common/backtrace.rs:48
   8: std::panicking::default_hook::{{closure}}
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at /rustc/8ad7bc3f428300aee6764f6e23527e19eb235e81/src/libstd/panicking.rs:530
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_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::span_bug_fmt
  20: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_const
  21: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_rvalue
  22: rustc_mir::monomorphize::collector::collect_neighbours
  23: rustc_mir::monomorphize::collector::collect_items_rec
  24: rustc_mir::monomorphize::collector::collect_items_rec
  25: rustc_mir::monomorphize::collector::collect_items_rec
  26: rustc_mir::monomorphize::collector::collect_items_rec
  27: rustc_mir::monomorphize::collector::collect_items_rec
  28: rustc_mir::monomorphize::collector::collect_items_rec
  29: rustc_mir::monomorphize::collector::collect_items_rec
  30: rustc_mir::monomorphize::collector::collect_items_rec
  31: rustc_mir::monomorphize::collector::collect_items_rec
  32: rustc_mir::monomorphize::collector::collect_items_rec
  33: rustc_mir::monomorphize::collector::collect_items_rec
  34: rustc_mir::monomorphize::collector::collect_items_rec
  35: rustc_mir::monomorphize::collector::collect_items_rec
  36: rustc_mir::monomorphize::collector::collect_items_rec
  37: rustc_mir::monomorphize::collector::collect_items_rec
  38: rustc_mir::monomorphize::collector::collect_items_rec
  39: rustc_mir::monomorphize::collector::collect_items_rec
  40: rustc_mir::monomorphize::collector::collect_items_rec
  41: rustc_mir::monomorphize::collector::collect_items_rec
  42: rustc_mir::monomorphize::collector::collect_items_rec
  43: rustc_mir::monomorphize::collector::collect_items_rec
  44: rustc_mir::monomorphize::collector::collect_items_rec
  45: rustc_mir::monomorphize::collector::collect_items_rec
  46: rustc_mir::monomorphize::collector::collect_items_rec
  47: rustc_mir::monomorphize::collector::collect_items_rec
  48: rustc_mir::monomorphize::collector::collect_items_rec
  49: rustc_mir::monomorphize::collector::collect_items_rec
  50: rustc_session::utils::<impl rustc_session::session::Session>::time
  51: rustc_mir::monomorphize::collector::collect_crate_mono_items
  52: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  53: 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
  54: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  55: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  56: rustc_data_structures::stack::ensure_sufficient_stack
  57: rustc_query_system::query::plumbing::get_query_impl
  58: rustc_codegen_ssa::back::symbol_export::exported_symbols_provider_local
  59: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::exported_symbols>::compute
  60: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  61: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  62: rustc_data_structures::stack::ensure_sufficient_stack
  63: rustc_query_system::query::plumbing::get_query_impl
  64: rustc_metadata::rmeta::encoder::encode_metadata_impl
  65: rustc_data_structures::sync::join
  66: rustc_metadata::rmeta::decoder::cstore_impl::<impl rustc_middle::middle::cstore::CrateStore for rustc_metadata::creader::CStore>::encode_metadata
  67: rustc_middle::ty::context::TyCtxt::encode_metadata
  68: rustc_interface::passes::start_codegen
  69: rustc_middle::ty::context::tls::enter_global
  70: rustc_interface::queries::Queries::ongoing_codegen
  71: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  72: rustc_span::with_source_map
  73: rustc_interface::interface::create_compiler_and_run
  74: scoped_tls::ScopedKey<T>::set
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.47.0-nightly (8ad7bc3f4 2020-07-21) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [collect_and_partition_mono_items] collect_and_partition_mono_items
#1 [exported_symbols] exported_symbols
end of query stack
error: aborting due to previous error

error: could not compile `reqwest`.

To learn more, run the command again with --verbose.

[JohnTitor]

I think the culprit PR is #69749 (and this is the first failure of "Polymorphization"), cc @davidtwco

[link2xt]

Same bug encountered here: chatmail/core#1743