Correct. That has no bearing at all on whether it's smart to write the rest of the library in Rust.

Most non-trivial applications will at some point call out into code written in C, or compiled from some other language. So what?

Ah I thought you meant Rust generally, not this project. Makes sense!

(I still don't think that's enough to justify using C over Rust here, just that I understand you now.)

Datapoint: GNOME's Federico Mena Quintero is working on a file-by-file port of librsvg to Rust. Compiled Rust objects are linked with compiled C objects. Both Rust and C call functions in librsvg's C dependencies, such as Cairo. None of this is weird; it's precisely what Rust was designed for.