Some details here... every language other than assembly languages has some amount of runtime. This is Rust's: https://github.com/rust-lang/rust/blob/master/src/libstd/rt.... (as you allude to, many people refer to this amount of runtime as "no runtime" since it's very, very small.)
You could also consider some other things as part of a runtime; for example, by default on most platforms, Rust includes jemalloc. That can be removed, though not in stable Rust. Same with the standard library code, which can be removed for libraries, but not binaries just yet (due to a small technicality regarding the stability of some attributes.) Things that do this are likely to use some of Rust's other unstable features at the moment, so it's not generally a burden to do so, and those interfaces rarely change right now.