That's very insightful. So my understanding now it that Rust provides low level "blocks" that, as a whole, is an extensible language. Thus language vs standard library doesn't really matter, since Rust = Language spec + standard library. Am I getting close to its definition?

BTW, I remember in the alpha days when the decision was made to make Rust support concurrency in the standard library rather in the language itself. So in that sense (Rust = language spec + stdlib) Rust does have concurrency built-in.