Could you elaborate on what potential mismatches there currently are between the semantics of Rust and C? Where particularly do you think there might be issues/dragons hiding in the semantics of a translation?
Float integer conversions (overflows are defined to saturate in rust, are undefined behavior in C).
Maybe things like how pointer casts are treated in correct (but unsafe) rust vs C. Or similarly in transmute. Generally it wouldn't surprise me if rust was standardizing a memory model that was subtly different from C.
I'm sure the list goes on, but "that sort of thing".
Rust also has different aliasing rules. Stricter than C for references, but more relaxed than C for raw pointers.
Edit: Now that I am coming back and reading this though I am going to make an edit, and like normal for substantive edits I'm going to mark it clearly...
Edit2: Or I can't edit, so let's put it here:
A better example than any of the above might be infinite loops, `while (true) {}` (or any similar loop without side effects) is undefined behavior in C, it's defined behavior in rust.