At beginning of learning rust, I mistakenly thought mut is a type decorator,

fn foo(v: mut i32) {
    // blah

This won’t compile since there’s no such type called mut i32. but in C,

void foo(int const bar, const int baz) {
    // blah

It seems like there’s a type called const int or int const.

However, I realize I had misunderstood the const decorator.

const decorates the variable v instead of type int, denoting the variable (the memory bound to this variable name) is immutable. The same thing applies to mut, whose usage is counterpart to const in C/C++. Everything in C/C++ is mutable by default and you need to use const to notify the compiler that you need special treatment. Rust does this in a reverse manner.

// C/C++ variable declaration
int const v;

// pseudo code for rust, and const is for demo
const v: int;

What rust does is to flip the order of the variable declaration from type decorator var into decorator var : type, with a : to visually separate type and variable declaration.

What about pointers and references?

RawPointerType : * ( mut | const ) TypeNoBounds ReferenceType : & Lifetime? mut? TypeNoBounds

In terms of the syntax definition, it’s the same as C/C++ that decorator mut and const is part of the pointer and reference type.

For C/C++, the reference and pointer type are like below.

// virtually separate & * with type and variable name
int * ptr;
int * const ptr;
const int * ptr;
int & ref; // ref variable is always `const`
const int & ref;

Let’s convert them to rust’s declaration.

  • add : to visually separate type declaration and variable declaration.
int * : ptr;
int * : const ptr;
const int * : ptr;
int & : ref; // ref variable is always `const`
const int & : ref;
  • swap variable and type declaration over : and reversely output tokens of the type declaration
ptr : * int;
ptr : * const int;
ref : & int;
ref : & const int;
  • remove const and add mut to mutable variable and pointer type
let mut ptr : * mut int;
let mut ptr : * int;
let ref : & mut int;
let ref : & int;

In rust, reference variable (memory that stores the address) can be mutable, whereas in C/C++, reference variable are immutable and value can be only set at initialization.

All in all, I’m in favor of C/C++ syntax and not into Rust’s variable declaration or definition syntax. However, Rust provides a visual separator, which is good for readability in a long run.