mdbook 0.4.44 will break `./x.py doc`

[pvdrz]

What the title says, if you update the Cargo.lock file so it updates mdbook from 0.4.43 to 0.4.44, running ./x.py doc will panic:

thread 'main' panicked at /home/christian/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/mdbook-0.4.44/src/renderer/html_handlebars/search.rs:47:81:
called `Option::unwrap()` on a `None` value
stack backtrace:
   0:     0x793b8ae44089 - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hfc1f21f8db201d29
   1:     0x793b8ae9fec3 - core::fmt::write::h016056faa7a6981e
   2:     0x793b8ae0669f - std::io::Write::write_fmt::h24b894747b35c3c0
   3:     0x793b8ae43ed3 - std::sys::backtrace::BacktraceLock::print::heebf86472dec8fe7
   4:     0x793b8ae241fc - std::panicking::default_hook::{{closure}}::h6d72acfbda71ec98
   5:     0x793b8ae24109 - std::panicking::default_hook::h8be7653a5e9957aa
   6:     0x793b8ae2473e - std::panicking::rust_panic_with_hook::h6aa9de01d2a3556f
   7:     0x793b8ae45066 - std::panicking::begin_panic_handler::{{closure}}::he4940efa6b78295e
   8:     0x793b8ae442b9 - std::sys::backtrace::__rust_end_short_backtrace::h6d8392e54739a970
   9:     0x793b8ae2429d - rust_begin_unwind
  10:     0x793b87896880 - core::panicking::panic_fmt::he144472e74486d6c
  11:     0x793b8789690c - core::panicking::panic::h9b72aee951677e47
  12:     0x793b878967f9 - core::option::unwrap_failed::h6e1b719b223197a6
  13:     0x5ab541711bef - mdbook[a979f49934072b30]::renderer::html_handlebars::search::create_files
  14:     0x5ab541782d36 - <mdbook[a979f49934072b30]::renderer::html_handlebars::hbs_renderer::HtmlHandlebars as mdbook[a979f49934072b30]::renderer::Renderer>::render
  15:     0x5ab54171f512 - <mdbook[a979f49934072b30]::book::MDBook>::execute_build_process
  16:     0x5ab54171ea06 - <mdbook[a979f49934072b30]::book::MDBook>::build
  17:     0x5ab5416f07d3 - error_index_generator[7ec2f52f1ffd143e]::main
  18:     0x5ab5416f1373 - std[778bddf7b4c93277]::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
  19:     0x5ab5416f49e9 - std[778bddf7b4c93277]::rt::lang_start::<()>::{closure#0}
  20:     0x793b8ae3f915 - std::rt::lang_start_internal::h808f256575e6d97e
  21:     0x5ab5416f11e8 - main
  22:     0x793b86e35488 - <unknown>
  23:     0x793b86e3554c - __libc_start_main
  24:     0x5ab5416ece35 - _start
  25:                0x0 - <unknown>
Command has failed. Rerun with -v to see more details.

[GrigorenkoPV]

Can reproduce on master (273465e).

1. Apply this patch:

   diff --git a/Cargo.lock b/Cargo.lock
   index 38a861727a9..9d736356bb5 100644
   --- a/Cargo.lock
   +++ b/Cargo.lock
   @@ -2230,9 +2230,9 @@ dependencies = [
   
   [[package]]
   name = "mdbook"
   -version = "0.4.43"
   +version = "0.4.44"
   source = "registry+https://github.com/rust-lang/crates.io-index"
   -checksum = "fe1f98b8d66e537d2f0ba06e7dec4f44001deec539a2d18bfc102d6a86189148"
   +checksum = "f9da1e54401fe5d45a664c57e112e70f18e8c5a73e268c179305b932ee864574"
   dependencies = [
   "ammonia",
   "anyhow",

2. Then ./x doc src/tools/error_index_generator

[ehuss]

Thanks for the report! I will take a look at this.