ICE from public re-export of private enum variant in proc macro

[jam1garner]

Code

proc macro:

use proc_macro::TokenStream;

#[proc_macro]
pub fn cause_ice(_: TokenStream) -> TokenStream {
    quote::quote!(
        enum IceCause {
            Variant,
        }

        pub use IceCause::Variant;
    ).into()
}

usage:

ice_macro::cause_ice!();

ICE is caused by pub re-exporting a single variant of a non-pub enum generated within a proc macro. Cannot be recreated with a declarative macro. marking the enum as pub/pub(in super), or pub(super) fixes the issue, while pub(crate) and pub(self) do not.

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (ffa2e7ae8 2020-10-24)
binary: rustc
commit-hash: ffa2e7ae8fbf9badc035740db949b9dae271c29f
commit-date: 2020-10-24
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly
LLVM version: 11.0

Also recreated on stable:

rustc 1.47.0 (18bf6b4f0 2020-10-07)
binary: rustc
commit-hash: 18bf6b4f01a6feaf7259ba7cdae58031af1b7b39
commit-date: 2020-10-07
host: x86_64-unknown-linux-gnu
release: 1.47.0
LLVM version: 11.0

Error output

thread 'rustc' panicked at '`enum` keyword should exist in snippet', src/librustc_resolve/imports.rs:1453:26
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

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 (18bf6b4f0 2020-10-07) running on x86_64-unknown-linux-gnu

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

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

error: could not compile `ice-reproduction`

Backtrace

stack backtrace:
   0: rust_begin_unwind
             at /rustc/ffa2e7ae8fbf9badc035740db949b9dae271c29f/library/std/src/panicking.rs:483:5
   1: core::panicking::panic_fmt
             at /rustc/ffa2e7ae8fbf9badc035740db949b9dae271c29f/library/core/src/panicking.rs:85:14
   2: core::option::expect_failed
             at /rustc/ffa2e7ae8fbf9badc035740db949b9dae271c29f/library/core/src/option.rs:1260:5
   3: rustc_resolve::ModuleData::for_each_child
   4: rustc_resolve::imports::ImportResolver::finalize_imports
   5: rustc_resolve::Resolver::resolve_crate
   6: rustc_interface::passes::configure_and_expand_inner
   7: rustc_interface::passes::configure_and_expand::{{closure}}
   8: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
   9: rustc_interface::passes::configure_and_expand
  10: rustc_interface::queries::Queries::expansion
  11: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  12: rustc_span::with_source_map
  13: rustc_interface::interface::create_compiler_and_run
  14: scoped_tls::ScopedKey<T>::set

*backtrace is from nightly commit ffa2e7a

I'd love to implement a fix, but figured I should make an issue to ensure it's not dupe work first and would appreciate any guidance as far as where to look and possible concerns when fixing.

[jyn514]

Note that this requires calling the proc-macro, not just defining it.

use ice_reproduction::cause_ice;

cause_ice!();

Here's a backtrace from 1.49:

thread 'rustc' panicked at '`enum` keyword should exist in snippet', compiler/rustc_resolve/src/imports.rs:1443:26
stack backtrace:
   0: rust_begin_unwind
             at /rustc/cf9cf7c923eb01146971429044f216a3ca905e06/library/std/src/panicking.rs:495:5
   1: core::panicking::panic_fmt
             at /rustc/cf9cf7c923eb01146971429044f216a3ca905e06/library/core/src/panicking.rs:92:14
   2: core::option::expect_failed
             at /rustc/cf9cf7c923eb01146971429044f216a3ca905e06/library/core/src/option.rs:1260:5
   3: rustc_resolve::ModuleData::for_each_child
   4: rustc_resolve::imports::ImportResolver::finalize_imports
   5: rustc_resolve::Resolver::resolve_crate
   6: rustc_interface::passes::configure_and_expand_inner
   7: rustc_interface::passes::configure_and_expand::{{closure}}
   8: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
   9: rustc_interface::passes::configure_and_expand
  10: rustc_interface::queries::Queries::expansion
  11: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  12: rustc_span::with_source_map
  13: rustc_interface::interface::create_compiler_and_run
  14: rustc_span::with_session_globals

[cjgillot]

This is fixed on recent nightly.