from rust docs:
Note: “Zero-cost” means zero runtime cost, not zero compile cost. The compiler does real work — monomorphization and inlining — to make abstractions disappear, and that work shows up as longer build times. See Reducing Compile Time for the trade-off.
most of the benefits come from a shared code base, one compiler (which is insanely fast with incremental rebuilds and can watch files for changes) means changes in the c code reflect in a single build step within seconds. the benefit is primarily for the developer as the compiler output should be relatively similar.