That's not the same, and not what pornel is talking about. The x86 ADD instruction has a well-defined behavior on overflow, and i32 + i32 in Rust will usually be translated to an ADD instruction, same as int + int in C. But a C compiler is allowed to assume that a signed addition operation will never overflow (the dreaded Undefined Behavior), while a Rust compiler must not make that assumption. This means that i32 + i32 must not be translated to int + int.

For example, a C compiler is allowed to optimize the expression a+1<a to false (if a is signed), but a Rust compiler isn't allowed to do this.