ICE on function definition in extern blocks

[lf-]

Hi! I encountered this ICE while putting bad code into my compiler while learning FFI for the first time. It also reproduced on stable 1.45.2, with the different location:

thread 'rustc' panicked at 'index out of bounds: the len is 13 but the index is 25', /rustc/d3fb005a39e62501b8b0b356166e515ae24e2e54/src/librustc_hir/definitions.rs:347:9

Code

extern "C" {
    fn lol() {
        println!("hello world!");
// whoops, forgot to close my function block
}

Meta

rustc --version --verbose:

dev/nix-doc - [main●] » rustc --version --verbose
rustc 1.47.0-nightly (6c8927b0c 2020-07-26)
binary: rustc
commit-hash: 6c8927b0cf80ceee19386026cf9d7fd4fd9d486f
commit-date: 2020-07-26
host: x86_64-unknown-linux-gnu
release: 1.47.0-nightly
LLVM version: 10.0

# STABLE
dev/nix-doc - [main●] » rustup run stable rustc --version --verbose
rustc 1.45.2 (d3fb005a3 2020-07-31)
binary: rustc
commit-hash: d3fb005a39e62501b8b0b356166e515ae24e2e54
commit-date: 2020-07-31
host: x86_64-unknown-linux-gnu
release: 1.45.2
LLVM version: 10.0

Error output

dev/nix-doc - [main●] » RUST_BACKTRACE=0 rustc -Zno-codegen ice-repro.rs
error: this file contains an unclosed delimiter
 --> ice-repro.rs:4:3
  |
1 | extern "C" {
  |            - unclosed delimiter
2 |     fn lol() {
  |              - this delimiter might not be properly closed...
3 |         println!("hello world!");
4 | }
  | - ^
  | |
  | ...as it matches this but it has different indentation

error: incorrect function inside `extern` block
 --> ice-repro.rs:2:8
  |
1 |   extern "C" {
  |   ---------- `extern` blocks define existing foreign functions and functions inside of them cannot have a body
2 |       fn lol() {
  |  ________^^^___-
  | |        |
  | |        cannot have a body
3 | |         println!("hello world!");
4 | | }
  | |_- help: remove the invalid body: `;`
  |
  = help: you might have meant to write a function accessible through FFI, which can be done by writing `extern fn` outside of the `extern` block
  = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html

thread 'rustc' panicked at 'index out of bounds: the len is 13 but the index is 25', src/librustc_ast_lowering/lib.rs:577:29
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-nightly (6c8927b0c 2020-07-26) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z no-codegen

error: aborting due to 2 previous errors

Backtrace

thread 'rustc' panicked at 'index out of bounds: the len is 13 but the index is 25', src/librustc_ast_lowering/lib.rs:577:29
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1117
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1508
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  12: rust_begin_unwind
             at src/libstd/panicking.rs:437
  13: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  14: core::panicking::panic_bounds_check
             at src/libcore/panicking.rs:62
  15: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  16: rustc_ast_lowering::lower_crate
  17: rustc_interface::passes::BoxedResolver::access::{{closure}}
  18: rustc_interface::passes::configure_and_expand::{{closure}}
  19: rustc_interface::passes::BoxedResolver::access
  20: rustc_interface::queries::Queries::lower_to_hir
  21: rustc_interface::queries::Queries::global_ctxt
  22: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  23: rustc_span::with_source_map
  24: rustc_interface::interface::create_compiler_and_run
  25: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[estebank]

The unclosed delimiters were a red herring, it reproduces even with correct delimiters. My guess is that we're trying to perform some evaluation on the body that should not have been done and instead treated as no body present for extern items.