They want to use rust but can't, because they need C compatibility, e.g. because they're providing a library or wants to support platforms that rust don't.

It's at least a less bad idea than I expected originally – I've mainly seen people try to use transpilers to sneakily start using the language they prefer as part of a larger code base, with all the obvious problems.

It's still not great as you now have an additional translation step to deal with. You will at some point need to look for bugs in the generated C layer, which will really suck. Users of the C library can't read your source to understand what it does internally without an extra step to figure out what rust code the thing they use corresponds to, etc.

If you want to provide a C library, write C. If rust isn't an option because it doesn't work for your customers who use C, don't use rust.