'internal error: entered unreachable code' when formatting doc comment

[Aaron1011]

The following code:

/// ```
/// # #[rustversion::since(1.36)]
/// # fn dox() {
/// # use std::pin::Pin;
/// # type Projection<'a> = &'a ();
/// # type ProjectionRef<'a> = &'a ();
/// # trait Dox {
/// fn project_ex(self: Pin<&mut Self>) -> Projection<'_>;
/// fn project_ref(self: Pin<&Self>) -> ProjectionRef<'_>;
/// # }
/// # }
/// ```
struct Foo;

produces the following error when formatted with rustfmt:

thread 'main' panicked at 'internal error: entered unreachable code', rustfmt-core/rustfmt-lib/src/visitor.rs:368:18
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.44/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:1063
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   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:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:470
  11: std::panicking::begin_panic
             at /rustc/564758c4c329e89722454dd2fbb35f1ac0b8b47c/src/libstd/panicking.rs:397
  12: rustfmt_lib::visitor::FmtVisitor::visit_fn
             at rustfmt-core/rustfmt-lib/src/visitor.rs:368
  13: rustfmt_lib::visitor::FmtVisitor::visit_item
             at rustfmt-core/rustfmt-lib/src/visitor.rs:513
  14: rustfmt_lib::visitor::FmtVisitor::visit_stmt
             at rustfmt-core/rustfmt-lib/src/visitor.rs:132
  15: rustfmt_lib::visitor::FmtVisitor::walk_stmts
             at rustfmt-core/rustfmt-lib/src/visitor.rs:872
  16: rustfmt_lib::visitor::FmtVisitor::walk_block_stmts
             at rustfmt-core/rustfmt-lib/src/visitor.rs:881
  17: rustfmt_lib::visitor::FmtVisitor::visit_block
             at rustfmt-core/rustfmt-lib/src/visitor.rs:218
  18: rustfmt_lib::visitor::FmtVisitor::visit_fn
             at rustfmt-core/rustfmt-lib/src/visitor.rs:393
  19: rustfmt_lib::visitor::FmtVisitor::visit_item
             at rustfmt-core/rustfmt-lib/src/visitor.rs:513
  20: rustfmt_lib::reorder::<impl rustfmt_lib::visitor::FmtVisitor>::visit_items_with_reordering
             at rustfmt-core/rustfmt-lib/src/reorder.rs:261
  21: rustfmt_lib::visitor::FmtVisitor::walk_mod_items
             at rustfmt-core/rustfmt-lib/src/visitor.rs:856
  22: rustfmt_lib::visitor::FmtVisitor::format_separate_mod
             at rustfmt-core/rustfmt-lib/src/visitor.rs:933
  23: rustfmt_lib::formatting::FormatContext<T>::format_file
             at rustfmt-core/rustfmt-lib/src/formatting.rs:182
  24: rustfmt_lib::formatting::format_project
             at rustfmt-core/rustfmt-lib/src/formatting.rs:109
  25: rustfmt_lib::formatting::<impl rustfmt_lib::Session<T>>::format_input_inner::{{closure}}
             at rustfmt-core/rustfmt-lib/src/formatting.rs:44
  26: scoped_tls::ScopedKey<T>::set
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  27: rustc_ap_syntax::attr::with_globals::{{closure}}
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/rustc-ap-syntax-642.0.0/attr/mod.rs:44
  28: scoped_tls::ScopedKey<T>::set
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  29: rustc_ap_syntax::attr::with_globals
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/rustc-ap-syntax-642.0.0/attr/mod.rs:44
  30: rustfmt_lib::formatting::<impl rustfmt_lib::Session<T>>::format_input_inner
             at rustfmt-core/rustfmt-lib/src/formatting.rs:32
  31: rustfmt_lib::Session<T>::format
             at rustfmt-core/rustfmt-lib/src/lib.rs:426
  32: rustfmt_lib::format_snippet::{{closure}}
             at rustfmt-core/rustfmt-lib/src/lib.rs:288
  33: std::panicking::try::do_call
             at /rustc/564758c4c329e89722454dd2fbb35f1ac0b8b47c/src/libstd/panicking.rs:303
  34: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:86
  35: std::panicking::try
             at /rustc/564758c4c329e89722454dd2fbb35f1ac0b8b47c/src/libstd/panicking.rs:281
  36: std::panic::catch_unwind
             at /rustc/564758c4c329e89722454dd2fbb35f1ac0b8b47c/src/libstd/panic.rs:394
  37: rustfmt_lib::format_snippet
             at rustfmt-core/rustfmt-lib/src/lib.rs:279
  38: rustfmt_lib::format_code_block
             at rustfmt-core/rustfmt-lib/src/lib.rs:347
  39: rustfmt_lib::comment::CommentRewrite::handle_line
             at rustfmt-core/rustfmt-lib/src/comment.rs:662
  40: rustfmt_lib::comment::rewrite_comment_inner
             at rustfmt-core/rustfmt-lib/src/comment.rs:816
  41: rustfmt_lib::comment::identify_comment
             at rustfmt-core/rustfmt-lib/src/comment.rs:361
  42: rustfmt_lib::comment::rewrite_doc_comment
             at rustfmt-core/rustfmt-lib/src/comment.rs:244
  43: rustfmt_lib::attr::rewrite_initial_doc_comments
             at rustfmt-core/rustfmt-lib/src/attr.rs:179
  44: rustfmt_lib::attr::<impl rustfmt_lib::rewrite::Rewrite for [rustc_ap_syntax::ast::Attribute]>::rewrite
             at rustfmt-core/rustfmt-lib/src/attr.rs:385
  45: rustfmt_lib::visitor::FmtVisitor::visit_attrs
             at rustfmt-core/rustfmt-lib/src/visitor.rs:841
  46: rustfmt_lib::visitor::FmtVisitor::visit_item
             at rustfmt-core/rustfmt-lib/src/visitor.rs:420
  47: rustfmt_lib::reorder::<impl rustfmt_lib::visitor::FmtVisitor>::visit_items_with_reordering
             at rustfmt-core/rustfmt-lib/src/reorder.rs:261
  48: rustfmt_lib::visitor::FmtVisitor::walk_mod_items
             at rustfmt-core/rustfmt-lib/src/visitor.rs:856
  49: rustfmt_lib::visitor::FmtVisitor::format_separate_mod
             at rustfmt-core/rustfmt-lib/src/visitor.rs:933
  50: rustfmt_lib::formatting::FormatContext<T>::format_file
             at /home/aaron/repos/rustfmt/rustfmt-core/rustfmt-lib/src/formatting.rs:182
  51: rustfmt_lib::formatting::format_project
             at /home/aaron/repos/rustfmt/rustfmt-core/rustfmt-lib/src/formatting.rs:109
  52: rustfmt_lib::formatting::<impl rustfmt_lib::Session<T>>::format_input_inner::{{closure}}
             at /home/aaron/repos/rustfmt/rustfmt-core/rustfmt-lib/src/formatting.rs:44
  53: scoped_tls::ScopedKey<T>::set
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  54: rustc_ap_syntax::attr::with_globals::{{closure}}
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/rustc-ap-syntax-642.0.0/attr/mod.rs:44
  55: scoped_tls::ScopedKey<T>::set
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137
  56: rustc_ap_syntax::attr::with_globals
             at /home/aaron/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/rustc-ap-syntax-642.0.0/attr/mod.rs:44
  57: rustfmt_lib::formatting::<impl rustfmt_lib::Session<T>>::format_input_inner
             at /home/aaron/repos/rustfmt/rustfmt-core/rustfmt-lib/src/formatting.rs:32
  58: rustfmt_lib::Session<T>::format
             at /home/aaron/repos/rustfmt/rustfmt-core/rustfmt-lib/src/lib.rs:426
  59: rustfmt::format_and_emit_report
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:530
  60: rustfmt::format::{{closure}}
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:511
  61: rustfmt_lib::Session<T>::override_config
             at /home/aaron/repos/rustfmt/rustfmt-core/rustfmt-lib/src/lib.rs:434
  62: rustfmt::format
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:510
  63: rustfmt::execute
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:364
  64: rustfmt::main
             at rustfmt-core/rustfmt-bin/src/bin/main.rs:25
  65: std::rt::lang_start::{{closure}}
             at /rustc/564758c4c329e89722454dd2fbb35f1ac0b8b47c/src/libstd/rt.rs:67
  66: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  67: std::panicking::try::do_call
             at src/libstd/panicking.rs:303
  68: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:86
  69: std::panicking::try
             at src/libstd/panicking.rs:281
  70: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  71: std::rt::lang_start_internal
             at src/libstd/rt.rs:51
  72: std::rt::lang_start
             at /rustc/564758c4c329e89722454dd2fbb35f1ac0b8b47c/src/libstd/rt.rs:67
  73: main
  74: __libc_start_main
  75: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

The panic is caught, so rustfmt continues executing. However, it would be nice if rustfmt properly handled this kind of doc comment.

Found while working on taiki-e/pin-project#181

[calebcartwright]

This is a different input (and an only-with-option type of issue with format_code_in_doc_comments config option) but same underlying cause as #4068 and this would also be resolved with #4069

[topecongiro]

Closed via #4069.