关键点：

• Integer overflow is defined via the overflow-checks compile-time flag. If enabled, the program will panic (a controlled crash of the program), otherwise you get wrap-around semantics. By default, you get panics in debug mode (cargo build) and wrap-around in release mode (cargo build --release).

• 边界检查不能使用编译标志禁用，也不能直接通过 unsafe 关键字禁用。然而， unsafe 允许你调用 slice::get_unchecked 等不做边界检查的函数。