'error constructed but not emitted' when parsing const keyword

[ZhangZhuoSJTU]

Code

trait t { const
impl

md5 of poc.rs: 7593012a00629a632a7b06982cbccb2f

The bug can be reproduced via rustc poc.rs

Meta

The bug is reproduced with nightly and beta versions, but not with stable version.

rustc --version --verbose:

rustc 1.51.0-nightly (4f4656d46 2021-02-04)
binary: rustc
commit-hash: 4f4656d46d84a488ae3df34b08f362d7071036a0
commit-date: 2021-02-04
host: x86_64-unknown-linux-gnu
release: 1.51.0-nightly
LLVM version: 11.0.1

rustc 1.50.0-beta.8 (1cd030396 2021-01-20)
binary: rustc
commit-hash: 1cd0303963629f317a08e7e52162ccca7232ae7f
commit-date: 2021-01-20
host: x86_64-unknown-linux-gnu
release: 1.50.0-beta.8

Error output

The output is from nightly version.

➜  playground rustc poc.rs
error: this file contains an unclosed delimiter
 --> out.rs:2:6
  |
1 | trait t { const
  |         - unclosed delimiter
2 | impl
  |      ^

error: internal compiler error: the following error was constructed but not emitted

error: expected identifier, found keyword `impl`
 --> out.rs:2:1
  |
2 | impl
  | ^^^^ expected identifier, found keyword
  |
help: you can escape reserved keywords to use them as identifiers
  |
2 | r#impl
  |

thread 'rustc' panicked at 'explicit panic', compiler/rustc_errors/src/diagnostic_builder.rs:460:13
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 1.51.0-nightly (4f4656d46 2021-02-04) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack
error: aborting due to 3 previous errors

Backtrace

➜  playground RURUST_BACKTRACE=1 rustc poc.rs
error: this file contains an unclosed delimiter
 --> out.rs:2:6
  |
1 | trait t { const
  |         - unclosed delimiter
2 | impl
  |      ^

error: internal compiler error: the following error was constructed but not emitted

error: expected identifier, found keyword `impl`
 --> out.rs:2:1
  |
2 | impl
  | ^^^^ expected identifier, found keyword
  |
help: you can escape reserved keywords to use them as identifiers
  |
2 | r#impl
  |

thread 'rustc' panicked at 'explicit panic', compiler/rustc_errors/src/diagnostic_builder.rs:460:13
stack backtrace:
   0: std::panicking::begin_panic
   1: <rustc_errors::diagnostic_builder::DiagnosticBuilder as core::ops::drop::Drop>::drop
   2: rustc_parse::parser::item::<impl rustc_parse::parser::Parser>::recover_const_impl
   3: rustc_parse::parser::item::<impl rustc_parse::parser::Parser>::parse_item_common_
   4: rustc_parse::parser::item::<impl rustc_parse::parser::Parser>::parse_item_common
   5: rustc_parse::parser::item::<impl rustc_parse::parser::Parser>::parse_assoc_item
   6: rustc_parse::parser::item::<impl rustc_parse::parser::Parser>::parse_item_common_
   7: rustc_parse::parser::item::<impl rustc_parse::parser::Parser>::parse_item_common
   8: rustc_parse::parser::item::<impl rustc_parse::parser::Parser>::parse_mod
   9: rustc_parse::parse_crate_from_file
  10: rustc_session::utils::<impl rustc_session::session::Session>::time
  11: rustc_interface::passes::parse
  12: rustc_interface::queries::Queries::parse
  13: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  14: rustc_span::with_source_map
  15: rustc_interface::interface::create_compiler_and_run
  16: rustc_span::with_session_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.51.0-nightly (4f4656d46 2021-02-04) running on x86_64-unknown-linux-gnu

query stack during panic:
end of query stack
error: aborting due to 3 previous errors

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