Thanks! Was this originally a "just because" project, or do you expect it to offer some improvements or advantages over existing embeddable Schemes like Chibi (as you mentioned)?
Originally started it as a school project, which then during the pandemic morphed into something to work on while cooped up. It is really a passion project, working on it is fun! There are some interesting design things I wanted to explore, like how to get good performance out of safe Rust, using unrolled linked lists or vlists instead of naive linked lists, using contracts, etc.
Chibi is an impressive scheme implementation, and it will take a long time before Steel can hit the same level of compliance as Chibi. There is not a Chibi equivalent in native Rust that I am aware of. There are other embedded scripting languages for Rust that are pleasant - but no schemes of the maturity of Chibi. So in that regard, I'm hoping to offer a compelling scheme in native Rust to make integration with Rust applications relatively easy and painless.
I also don't have a particularly strong need to be 100% completely compliant with the scheme specs. The plan is to have compatibility layers so that portable scheme code can be used, however there are things about scheme that I think Racket (for example) improved on, and I'd like to explore that as well as much as I can.
I borrowed the R5RS test suite from chibi here - https://github.com/mattwparas/steel/blob/master/cogs/r5rs.sc... - only a few of these tests don't yet pass. Something like 135 pass, 4 fail, 20 skipped since I haven't implemented the primitives yet.
I've only tested against a few SRFIs so far, but am also attempting to run the R7RS benchmark suite https://ecraven.github.io/r7rs-benchmarks/ - So far you can view the progress here https://github.com/mattwparas/steel/tree/master/r7rs-benchma...
There are some more that aren't yet checked in. I plan to get a document up with the exact state of compliance soon.
The biggest difference right now is that, like Racket, Steel lists are immutable, so there I need a compatibility layer when running portable scheme.