ICE in rustc_infer error reporting on elided lifetime in extern signature

[dtolnay]

Repro:

pub trait Trait {}

#[repr(transparent)]
pub struct Wrapper<T: Trait>(T);

#[repr(transparent)]
pub struct Ref<'a>(&'a u8);

impl Trait for Ref {}

extern "C" {
    pub fn repro(_: Wrapper<Ref>);
}

As of current nightly:

$ RUST_BACKTRACE=1 rustc +nightly-2020-12-29 src/lib.rs --crate-type lib
thread 'rustc' panicked at 'internal error: entered unreachable code', compiler/rustc_infer/src/infer/error_reporting/mod.rs:156:14
stack backtrace:
   0: rust_begin_unwind
             at /rustc/2987785df3d46d5ff144a5c67fbb8f5cca798d78/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/2987785df3d46d5ff144a5c67fbb8f5cca798d78/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /rustc/2987785df3d46d5ff144a5c67fbb8f5cca798d78/library/core/src/panicking.rs:50:5
   3: rustc_infer::infer::error_reporting::msg_span_from_free_region
   4: rustc_infer::infer::error_reporting::note_and_explain_region
   5: rustc_infer::infer::error_reporting::note::<impl rustc_infer::infer::InferCtxt>::report_concrete_failure
   6: rustc_infer::infer::error_reporting::<impl rustc_infer::infer::InferCtxt>::report_region_errors
   7: rustc_infer::infer::InferCtxt::resolve_regions_and_report_errors
   8: rustc_typeck::check::regionck::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::regionck_item
   9: rustc_infer::infer::InferCtxtBuilder::enter
  10: rustc_typeck::check::wfcheck::check_item_well_formed
  11: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_item_well_formed>::compute
  12: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  13: rustc_data_structures::stack::ensure_sufficient_stack
  14: rustc_query_system::query::plumbing::get_query_impl
  15: rustc_query_system::query::plumbing::ensure_query_impl
  16: <rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor as rustc_hir::itemlikevisit::ParItemLikeVisitor>::visit_item
  17: rustc_data_structures::sync::par_for_each_in
  18: rustc_hir::hir::Crate::par_visit_all_item_likes
  19: rustc_session::session::Session::track_errors
  20: rustc_typeck::check_crate
  21: rustc_interface::passes::analysis
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::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_interface::passes::QueryContext::enter
  27: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  28: rustc_span::with_source_map
  29: rustc_interface::interface::create_compiler_and_run

note: rustc 1.51.0-nightly (2987785df 2020-12-28) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [check_item_well_formed] checking that `` is well-formed
#1 [analysis] running analysis passes on this crate
end of query stack

This is the unreachable line:

rust/compiler/rustc_infer/src/infer/error_reporting/mod.rs

Line 156 in 2987785

_ => unreachable!(),

The same crash reproduces back to 1.47.0. Older stable compilers do not crash:

$ RUST_BACKTRACE=1 rustc +1.46.0 src/lib.rs --crate-type lib
error[E0726]: implicit elided lifetime not allowed here
 --> src/lib.rs:9:16
  |
9 | impl Trait for Ref {}
  |                ^^^- help: indicate the anonymous lifetime: `<'_>`

[dtolnay]

Bisect:

#!/bin/bash
rustc src/lib.rs --crate-type lib 2>&1 | not grep unreachable

#cargo-bisect-rustc --script=./bisect.sh --without-cargo --start=2020-07-10 --end=2020-08-27

searched nightlies: from nightly-2020-07-10 to nightly-2020-08-27
regressed nightly: nightly-2020-07-22
searched commits: from f9a3086 to 8ad7bc3
regressed commit: 734233d (#74569)

Mentioning @davidtwco @lcnr because #73323 appears the most relevant.

[rylev]

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