How does Rust allocate a zeroed Vec and what does it cost?

How does Rust return values, and does it make any difference to us programmers?

In Rust, what is the performance cost of using sync::Once after the initial setup?

In Rust, how does using a different primitive type (u8, i32, u64, …) change the generated assembly?

Assembly: In all the world of the programmer, there is no more important output.