Crashes on macro input containing function signature

[dtolnay]

Minimal case:

m![fn f();];

I started hitting this in syn/tests/test_receiver.rs@189aea8 which hasn't been touched since October, so I guess this is a regression.

To reproduce against the current master branch of rustfmt (9124dd8):

$ echo 'm![fn f();];' | RUST_BACKTRACE=1 cargo +nightly-2020-03-22 run --bin rustfmt

thread 'main' panicked at 'internal error: entered unreachable code', rustfmt-core/rustfmt-lib/src/visitor.rs:367:18
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.45/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.45/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:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1427
   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:218
  10: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:511
  11: std::panicking::begin_panic
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libstd/panicking.rs:438
  12: rustfmt_lib::visitor::FmtVisitor::visit_fn
             at rustfmt-core/rustfmt-lib/src/visitor.rs:367
  13: rustfmt_lib::visitor::FmtVisitor::visit_item
             at rustfmt-core/rustfmt-lib/src/visitor.rs:511
  14: rustfmt_lib::macros::rewrite_macro_with_items
             at rustfmt-core/rustfmt-lib/src/macros.rs:1537
  15: rustfmt_lib::macros::rewrite_macro_inner
             at rustfmt-core/rustfmt-lib/src/macros.rs:346
  16: rustfmt_lib::macros::rewrite_macro::{{closure}}
             at rustfmt-core/rustfmt-lib/src/macros.rs:204
  17: core::ops::function::FnOnce::call_once
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libcore/ops/function.rs:232
  18: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libstd/panic.rs:318
  19: std::panicking::try::do_call
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libstd/panicking.rs:331
  20: __rust_try
  21: std::panicking::try
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libstd/panicking.rs:274
  22: std::panic::catch_unwind
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libstd/panic.rs:394
  23: rustfmt_lib::macros::rewrite_macro
             at rustfmt-core/rustfmt-lib/src/macros.rs:203
  24: rustfmt_lib::visitor::FmtVisitor::visit_mac::{{closure}}
             at rustfmt-core/rustfmt-lib/src/visitor.rs:682
  25: rustfmt_lib::visitor::FmtVisitor::with_context
             at rustfmt-core/rustfmt-lib/src/visitor.rs:955
  26: rustfmt_lib::visitor::FmtVisitor::visit_mac
             at rustfmt-core/rustfmt-lib/src/visitor.rs:682
  27: rustfmt_lib::visitor::FmtVisitor::visit_item
             at rustfmt-core/rustfmt-lib/src/visitor.rs:496
  28: rustfmt_lib::reorder::<impl rustfmt_lib::visitor::FmtVisitor>::visit_items_with_reordering
             at rustfmt-core/rustfmt-lib/src/reorder.rs:261
  29: rustfmt_lib::visitor::FmtVisitor::walk_mod_items
             at rustfmt-core/rustfmt-lib/src/visitor.rs:854
  30: rustfmt_lib::visitor::FmtVisitor::format_separate_mod
             at rustfmt-core/rustfmt-lib/src/visitor.rs:931
  31: rustfmt_lib::formatting::FormatContext<T>::format_file
             at ./rustfmt-core/rustfmt-lib/src/formatting.rs:182
  32: rustfmt_lib::formatting::format_project
             at ./rustfmt-core/rustfmt-lib/src/formatting.rs:109
  33: rustfmt_lib::formatting::<impl rustfmt_lib::Session<T>>::format_input_inner::{{closure}}
             at ./rustfmt-core/rustfmt-lib/src/formatting.rs:44
  34: scoped_tls::ScopedKey<T>::set
             at /home/david/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  35: rustc_ap_syntax::attr::with_globals::{{closure}}
             at /home/david/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-ap-syntax-642.0.0/attr/mod.rs:44
  36: scoped_tls::ScopedKey<T>::set
             at /home/david/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  37: rustc_ap_syntax::attr::with_globals
             at /home/david/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-ap-syntax-642.0.0/attr/mod.rs:44
  38: rustfmt_lib::formatting::<impl rustfmt_lib::Session<T>>::format_input_inner
             at ./rustfmt-core/rustfmt-lib/src/formatting.rs:32
  39: rustfmt_lib::Session<T>::format
             at ./rustfmt-core/rustfmt-lib/src/lib.rs:426
  40: rustfmt::format_and_emit_report
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:530
  41: rustfmt::format_string
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:415
  42: rustfmt::format
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:472
  43: rustfmt::execute
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:364
  44: rustfmt::main
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:25
  45: std::rt::lang_start::{{closure}}
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libstd/rt.rs:67
  46: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  47: std::panicking::try::do_call
             at src/libstd/panicking.rs:331
  48: std::panicking::try
             at src/libstd/panicking.rs:274
  49: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  50: std::rt::lang_start_internal
             at src/libstd/rt.rs:51
  51: std::rt::lang_start
             at /rustc/38114ff16e7856f98b2b4be7ab4cd29b38bed59a/src/libstd/rt.rs:67
  52: main
  53: __libc_start_main
  54: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[calebcartwright]

Different input, but same underlying issue as #4068 and #4077.

This has been fixed and will be included in the next release of rustfmt via #4100 and resolved on master once #4069 is merged