Just one or two days ago I asked here on HN how memory allocations in C++ are considered to be more hidden than in Rust and got some good replies: Especially constructors, copy constructors, assignment operators etc. can introduce non-obvious allocations.

For example:

  T t;
  a = b;
  T t2 = t;

can all allocate in C++. The equivalent in Rust:

  let t: T; // won't allocate
  let t = T::new(); // might allocate
  a = b; // won't allocate
  let t2 = t; // won't allocate
  let t2 = t.clone(); // might allocate

So in Rust you can tell that as long as there is no function call, there won't be an allocation.