[Bug 2119693] [NEW] Erroneously-vendored C library causes build failures on higher C standards

[Max Gilmour]

Public bug reported:

Newer versions of GCC cause problems with the oniguruma C library, which
is bundled as a dependency of the vendored orig_sys crate. The higher C
standard causes the oniguruma build to fail.

This isn't a GCC problem — as I recently learned, it's advised to use
system C libraries when possible. This bug will be fixed by replacing
the vendored oniguruma C library with the libonig-dev package.

One example of a failure caused by this issue is in a `rust-comrak`
autopkgtest[1]:

```
878s [onig_sys 69.8.1] cargo:warning=oniguruma/src/regparse.c:588:5: error: initialization of 'int (*)(void)' from incompatible pointer type 'int (*)(st_str_end_key *, st_str_end_key *)' [-Wincompatible-pointer-types]
```

[1]:
https://objectstorage.prodstack5.canonical.com/swift/v1/AUTH_0f9aae918d5b4744bf7b827671c86842/autopkgtest-
questing/questing/armhf/r/rust-comrak/20250806_000912_1db42@/log.gz

** Affects: rustc-1.85 (Ubuntu)
     Importance: Undecided
     Assignee: Max Gilmour (maxgmr)
         Status: In Progress


** Tags: questing

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https://bugs.launchpad.net/bugs/2119693

Title:
  Erroneously-vendored C library causes build failures on higher C
  standards

Status in rustc-1.85 package in Ubuntu:
  In Progress

Bug description:
  Newer versions of GCC cause problems with the oniguruma C library,
  which is bundled as a dependency of the vendored orig_sys crate. The
  higher C standard causes the oniguruma build to fail.

  This isn't a GCC problem — as I recently learned, it's advised to use
  system C libraries when possible. This bug will be fixed by replacing
  the vendored oniguruma C library with the libonig-dev package.

  One example of a failure caused by this issue is in a `rust-comrak`
  autopkgtest[1]:

  ```
  878s [onig_sys 69.8.1] cargo:warning=oniguruma/src/regparse.c:588:5: error: initialization of 'int (*)(void)' from incompatible pointer type 'int (*)(st_str_end_key *, st_str_end_key *)' [-Wincompatible-pointer-types]
  ```

  [1]:
  https://objectstorage.prodstack5.canonical.com/swift/v1/AUTH_0f9aae918d5b4744bf7b827671c86842/autopkgtest-
  questing/questing/armhf/r/rust-comrak/20250806_000912_1db42@/log.gz

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