Here's[0] a breakdown from the foundation on that budget category. They also commit to breaking it down better in the future:

  Salaries, benefits, payroll taxes, payroll service provider fees: $1.17m  
  Travel, event sponsorship, & support: $192k  
  Legal and Professional Fees: $66k
  Fund transfer to the Grants Program from membership fees: $40k
  Fund transfer to the Specification work from membership fees: $20k
  Marketing: $36k
  General Admin: (software, bank fees, meeting rooms, etc.) $20k

It's mostly salaries. They employ four engineers (software, security, infrastructure) that work on Rust, who they pay "at or above the average in their local market."

[0]: https://rust-lang.zulipchat.com/#narrow/stream/335408-founda...