Resource management in Rust

TODO: I think it is clearer if we step trough the example instead.

Apart from the memory that the application needs to store its CPU-instructions, it also needs the following resources when executing the function write_to_file described above:

• A piece of memory to store the the variable named file. Because this is a local variable, it is stored on the stack.
• The file name to be passed to the function File::create. Because this is a constant, it is stored in static memory.
• The file name needs to be passed to the function File::create. In fact, the function File::create does not expect the file name itself as a parameter, but only the memory address of the file name and the length. Because the file name is stored in static memory, its memory address is in fact a constant and this constant can be stored as part of the CPU-instructions of the application. Also the length of the file-name is a constant.
• When the function File::create is called, the function File::create also needs some resources. For instance, it stores its parameters (in this case the memory address and the length of the file name) on the stack while it executes.
• Of course, the open file itself is also a resource used by the application. A handle to this file (a file descriptor) is stored in the variable file.
• The text that will be written to the file. This is again a constant, so it is stored in static memory.
• The function File::write_all expects the memory address of a variable of type File. This memory address is notated as &mut file in the example. In fact, & means "take the memory address of". The mut indicates that the function File::write_all has exclusive access to this memory address while it is executed. This will be explained in further detail when we talk about "borrowing as mutable".
• The function File::write_all also expects the memory address and the length of the data to be written. This is similar to the file name parameter of the function File::create.
• The caller of write_to_file will store the return value of write_to_file.
• For completeness: when the function returns, it needs to know where it has to return to. This information is also stored on the stack.