ICE when combining "\r", unicode, and unused parameter in format string

[Reconcyl]

The following code causes an ICE:

fn main() {
    print!("\r¡{}");
}

As far as I can tell:

• Any number of characters can be inserted between \r and ¡ and the error will still happen.
• ¡ can be replaced with any unicode character and the error will still happen.
• No escape sequences other than \r cause the error.

The error is reproducible on the stable or nightly compiler at https://play.rust-lang.org.

rustc --version --verbose outputs the following on my machine:

rustc 1.40.0 (73528e339 2019-12-16)
binary: rustc
commit-hash: 73528e339aae0f17a15ffa49a8ac608f50c6cf14
commit-date: 2019-12-16
host: x86_64-apple-darwin
release: 1.40.0
LLVM version: 9.0

Error output

thread 'rustc' panicked at 'assertion failed: bpos.to_u32() >= mbc.pos.to_u32() + mbc.bytes as u32', src/libsyntax/source_map.rs:875:17
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.40.0 (73528e339 2019-12-16) running on x86_64-apple-darwin

Backtrace

stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: rustc_driver::report_ice
   6: std::panicking::rust_panic_with_hook
   7: std::panicking::begin_panic
   8: syntax::source_map::SourceMap::bytepos_to_file_charpos
   9: syntax::source_map::SourceMap::lookup_char_pos
  10: syntax::source_map::SourceMap::span_to_filename
  11: <syntax::source_map::SourceMap as rustc_errors::SourceMapper>::call_span_if_macro
  12: rustc_errors::emitter::Emitter::fix_multispan_in_std_macros
  13: rustc_errors::emitter::Emitter::fix_multispans_in_std_macros
  14: <rustc_errors::emitter::EmitterWriter as rustc_errors::emitter::Emitter>::emit_diagnostic
  15: rustc_errors::HandlerInner::emit_diagnostic
  16: rustc_errors::diagnostic_builder::DiagnosticBuilder::emit
  17: syntax_ext::format::expand_preparsed_format_args
  18: syntax_ext::format::expand_format_args_impl
  19: <F as syntax_expand::base::TTMacroExpander>::expand
  20: syntax_expand::expand::MacroExpander::fully_expand_fragment
  21: syntax_expand::expand::MacroExpander::expand_crate
  22: rustc_interface::passes::configure_and_expand_inner::{{closure}}
  23: rustc_interface::passes::configure_and_expand_inner
  24: rustc_interface::passes::configure_and_expand::{{closure}}
  25: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
  26: rustc_interface::queries::Query<T>::compute
  27: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::expansion
  28: rustc_interface::interface::run_compiler_in_existing_thread_pool
  29: std::thread::local::LocalKey<T>::with
  30: scoped_tls::ScopedKey<T>::set
  31: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[Centril]

Godbolt says this regressed in 1.29.0.

[spastorino]

pre-triage: this doesn't seem to be very important but would be of course nice to fix. Tagging it as P-low.

[moonheart08]

@rustbot claim
Time to try an easy issue