Cargo build crash in project with git Split Index

[de-husk]

Problem

Cargo build is breaking when a build.rs file is present in a project with a git repo using split indexes.

$ cargo build

error: failed to determine package fingerprint for build script for split_index_bug v0.1.0 (/.../split_index_bug)

Caused by:
  failed to determine the most recently modified file in /.../split_index_bug

Caused by:
  failed to determine list of files in /.../split_index_bug

Caused by:
  failed to open git index at /.../split_index_bug/.git/

Caused by:
  unsupported mandatory extension: 'link'; class=Index (10)

Steps to Reproduce

I have made a small repo where you can reproduce the issue https://github.com/Samangan/cargo-split-index-bug

Version

cargo 1.56.0 (4ed5d137b 2021-10-04)
release: 1.56.0
commit-hash: 4ed5d137baff5eccf1bae5a7b2ae4b57efad4a7d
commit-date: 2021-10-04

[weihanglo]

Confirmed that libgit2 v1.3.0, the C library wrapped by Cargo's git2 dependency, doesn't yet support split-index extension at this moment.

https://github.com/libgit2/libgit2/blob/v1.3.0/src/index.c#L2604-L2616

[arlosi]

Bringing the discussion from #13498 back into the issue.

This issue is preventing me from vendoring crates into a repo that uses unsupported git extensions. When attempting to build any crate that has a build.rs file, the error is encountered.

Potential solutions:

1. Ignore the error (log it to tracing and fall back to the non-git heuristic approach): proposed in fix: reduce failing to open a git repo to a tracing warning #13498
   • As @weihanglo pointed out, this goes against Cargo documentation.
2. Ignore the error, but only for building, not for publishing (where it would still be an error)
3. Do the .gitignore processing in Cargo, rather than using libgit2 (either always, or only if libgit2 fails)
4. Have a config or CLI option that disables using git when calculating includes/excludes for a package.
5. Require build.rs scripts to use rerun-if-changed as needed in the 2024 edition so that enumerating the files in a path source is no longer required to build.

[epage]

iirc jj is looking to switch to ignore.