[coldtea]

That doesn't answer the parent's point at all.

If anything it re-enforces it. From the repo introduction:

The current state of the code is quite rough. It's not known to compile with stable Rust 1.0 (the original version was written well before 1.0 stabilized), it doesn't handle composite glyphs, the code isn't organized with Cargo, and it's basically not ready for prime time.

[cm3]

It should build with 1.0 since a recent update, but your other points are valid. That said, it's a PoC and kinda like a MVP. Granted this is from a domain expert, so the time it took Ralph Levien to code this is substantially shorter than it would have taken a beginner in the field of font rendering. However, it's easy to see that something like font-rs can be incorporated into Firefox in the future even before Servo is complete. It's already happening with image decoders.

To answer the parent's questions, which I unfortunately forgot to: if we translate Harfbuzz and FreeType to Rust, it won't take that long. See Dropbox's recent blogpost about their translation of Brotli's C code to Rust.