ICE on "index out of bounds: the len is 0 but the index is 1"

[dwrensha]

When I try to run rustc on this program (found by fuzz-rustc)

fn f() -> isize { fn f() -> isize {} pub f<

it crashes with this error:

error: this file contains an un-closed delimiter
 --> main.rs:1:45
  |
1 | fn f() -> isize { fn f() -> isize {} pub f<
  |                 - un-closed delimiter       ^

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 1', /rustc/273f42b5964c29dda2c5a349dd4655529767b07f/src/libcore/slice/mod.rs:2687:14
stack backtrace:
   0: std::panicking::default_hook::{{closure}}
   1: std::panicking::default_hook
   2: rustc::util::common::panic_hook
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::continue_panic_fmt
   5: rust_begin_unwind
   6: core::panicking::panic_fmt
   7: core::panicking::panic_bounds_check
   8: syntax::parse::parser::Parser::parse_item_
   9: syntax::parse::parser::Parser::parse_stmt_without_recovery
  10: syntax::parse::parser::Parser::parse_full_stmt
  11: syntax::parse::parser::Parser::parse_block_tail
  12: syntax::parse::parser::Parser::parse_inner_attrs_and_block
  13: syntax::parse::parser::Parser::parse_item_fn
  14: syntax::parse::parser::Parser::parse_item_implementation
  15: syntax::parse::parser::Parser::parse_item_
  16: syntax::parse::parser::Parser::parse_item
  17: syntax::parse::parser::Parser::parse_mod_items
  18: syntax::parse::parser::Parser::parse_crate_mod
  19: syntax::parse::parse_crate_from_file
  20: rustc_interface::passes::parse::{{closure}}
  21: rustc::util::common::time
  22: rustc_interface::passes::parse
  23: rustc_interface::queries::Query<T>::compute
  24: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::parse
  25: rustc_interface::interface::run_compiler_in_existing_thread_pool
  26: std::thread::local::LocalKey<T>::with
  27: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
end of query stack
error: aborting due to previous error


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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.38.0-nightly (273f42b59 2019-07-21) running on x86_64-apple-darwin

The error happens on stable, beta, and nightly.

[estebank]

Introduced in 1.33.

[dwrensha]

I'm seeing the error on nightly-2018-12-14 (f4a421e) but not on nightly-2018-12-08 (4a45578).

[dwrensha]

#56248 looks suspicious.