Integrate to the bootstrap process of rust-lang/rust

[dalance]

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?

[mgeisler]

Hi @dalance,

Thanks for the descriptive issue!

• 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.

So far, we have not paid much attention to the library interface of mdbook-i18n-helpers, but I'm not against making it usable as a library. @kdarkhan has been making similar changes to enable us to do fuzz testing against the binaries.