The latest Rust by Example has a Japanese translation resource based on mdbook-i18n-helpers.
But it is not built because rust-lang/rust has the original mdbook build process which doesn't contain mdbook-i18n-helpers.

In my investigation, I found that the following changes are required to build it.

• Export Preprocessor struct for gettext
  The bootstrap process of rust-lang/rust uses the original rustbook binary to build docs.
  It uses mdbook crate as a library and the normal mdbook plugin system is not used.
  So exporting Preprocessor struct which can be integrated to rustbook at compile time is required.

• Fix "hidden lifetime parameters in types are deprecated"
  In the rustbook build, the following error is occured, it should be fixed.

error: hidden lifetime parameters in types are deprecated
  --> /home/hatta/work/repos/mdbook-i18n-helpers/i18n-helpers/src/lib.rs:95:34
   |
95 |     fn expand_shortcut_link(tag: Tag) -> Tag {
   |                                  ^^^ expected lifetime parameter
   |
   = note: `-D elided-lifetimes-in-paths` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(elided_lifetimes_in_paths)]`
help: indicate the anonymous lifetime
   |
95 |     fn expand_shortcut_link(tag: Tag<'_>) -> Tag {
   |                                     ++++

• Downgrade the minimum required version of regex to 1.9.4
regex 1.9.5 requires memchr 2.6.0, but it is blocked by windows-gnu thin-lto test linking error regression rust-lang/rust#109797.

I opened rust-lang/rust#124731.
It refers my forked mdbook-i18n-helpers https://github.com/dalance/mdbook-i18n-helpers/tree/export_preprocessor now.

How about the above changes?