cargo: `cargo update` hangs

I’ve tried to minimize this but I haven’t made any progress.

When I run cargo update, it seems to go forever (I let it go for 30 minutes with no output; it pegged a CPU but didn’t allocate a significant amount of memory). I’ve tried breaking into it in a debugger and it’s not always in the same place, but it’s been in jemalloc or HashMap internals. I am not low on RAM.

Here’s a gist of Cargo.toml and Cargo.lock. Cargo.toml looks weird because it was generated by cargo-outdated.

If I comment out the c_vec line in Cargo.toml, the update finishes in a couple of seconds. However, trimming down Cargo.toml and Cargo.lock to only mention c_vec also doesn’t hang. I also tried bisecting by commenting out the first half of the dependencies list, and the second half, and neither hang. So it’s some combinatorial interaction between c_vec and… some other crate(s) in the list!