Cargo uses git-rs which is made to be built against the bundled libgit2
version that hasn't been part of a stable release yet. Using our libgit2
instead of the master version fails during runtime as they are not
compatible anymore.
After the next libgit2 update we can try again but it is likely that
there will also be yet another cargo release at that point in time…
This has several advantages:
1. It takes up less space on disk in-between builds in the nix store.
2. It uses less space in the binary cache for vendor derivation packages.
3. It uses less network traffic downloading from the binary cache.
4. It plays nicely with hashed mirrors like tarballs.nixos.org, which only
substitute --flat hashes on single files (not recursive directory hashes).
5. It's consistent with how simple `fetchurl` src derivations work.
6. It provides a stronger abstraction between input src-package and output
package, e.g., it's harder to accidentally depend on the src derivation at
runtime by referencing something like `${src}/etc/index.html`. Likewise, in
the store it's harder to get confused with something that is just there as a
build-time dependency vs. a runtime dependency, since the build-time
src dependencies are tarred up.
Disadvantages are:
1. It takes slightly longer to untar at the start of a build.
As currently implemented, this attaches the compacted vendor.tar.gz feature as a
rider on `verifyCargoDeps`, since both of them are relatively newly implemented
behavior that change the `cargoSha256`.
If this PR is accepted, I will push forward the remaining rust packages with a
series of treewide PRs to update the `cargoSha256`s.
Package is marked as broken for >2 years and used a fairly old
snapshot from the gcc7-branch, so I fairly doubt that this is
somewhere used (and is also pretty misleading as you don't expect a
random snapshot from gcc7 at `pkgs.gcc-snapshot`).
