ICE given unclosed angle-bracket *only* in doctest

[plaidfinch]

Code

The following doctest causes an ICE when compiled in a doctest via cargo test. However, the same code causes the expected parse error when compiled outside the context of a doctest.

//! ```
//! fn test() -> Result< {}
//! ```

Meta

rustc --version --verbose:

rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-unknown-linux-gnu
release: 1.49.0

This error still exists in beta and nightly as well.

Error output (when backtrace enabled)

Standard 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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.50.0-beta.5 (ff5998292 2021-01-05) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no --crate-type lib

error: test failed, to rerun pass '--doc'

Standard output:

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s


running 1 test
test src/lib.rs - (line 1) ... FAILED

failures:

---- src/lib.rs - (line 1) stdout ----
thread 'src/lib.rs - (line 1)' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:974:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

query stack during panic:
end of query stack


failures:
    src/lib.rs - (line 1)

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Backtrace

thread 'src/lib.rs - (line 1)' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:974:13
stack backtrace:
   0: rust_begin_unwind
             at /rustc/ff59982926d98c8508008f0559f8a055260ac05e/library/std/src/panicking.rs:493:5
   1: std::panicking::begin_panic_fmt
             at /rustc/ff59982926d98c8508008f0559f8a055260ac05e/library/std/src/panicking.rs:435:5
   2: rustc_errors::HandlerInner::flush_delayed
   3: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   4: core::ptr::drop_in_place
   5: core::ptr::drop_in_place
   6: scoped_tls::ScopedKey<T>::set
   7: std::panic::catch_unwind
   8: rustc_driver::catch_fatal_errors
   9: rustdoc::doctest::make_test
  10: rustdoc::doctest::run_test
  11: core::ops::function::FnOnce::call_once{{vtable.shim}}
  12: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
             at /rustc/ff59982926d98c8508008f0559f8a055260ac05e/library/alloc/src/boxed.rs:1328:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.