ICE in librustc_ast_pretty printing of multiline Mixed comments

[dtolnay]

main.rs:

fn main(/*
    ---
*/) {}

$ RUST_BACKTRACE=1 rustc main.rs -Zunstable-options --pretty=expanded

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `3`,
 right: `1`', src/librustc_ast_pretty/pprust.rs:441:17
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:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   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:477
  12: rust_begin_unwind
             at src/libstd/panicking.rs:384
  13: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:338
  14: rustc_ast_pretty::pprust::PrintState::print_comment
  15: rustc_ast_pretty::pprust::State::print_block_maybe_unclosed
  16: rustc_ast_pretty::pprust::State::print_item
  17: rustc_ast_pretty::pprust::print_crate
  18: rustc_driver::pretty::print_after_hir_lowering::{{closure}}
  19: rustc_driver::pretty::print_after_hir_lowering
  20: rustc_middle::ty::context::tls::enter_global
  21: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  22: rustc_span::with_source_map
  23: rustc_interface::interface::run_compiler_in_existing_thread_pool
  24: scoped_tls::ScopedKey<T>::set
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.46.0-nightly (a8cf39911 2020-06-21) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z unstable-options

query stack during panic:
end of query stack

rust/src/librustc_ast_pretty/pprust.rs

Lines 438 to 441 in 62878c2

	fn print_comment(&mut self, cmnt: &comments::Comment) {
	match cmnt.style {
	comments::Mixed => {
	assert_eq!(cmnt.lines.len(), 1);