How can I use a structure reference for isize?

Question

I want to get bin the following code.

unsafe {
    struct Test {
        num: i32,
    }
    let a = Test { num: 0 };
    let b = &mut a as isize;
}

But this leads to the following error message.

error: casting `&on_create::Test` as `isize` is invalid
   --> main.rs:181:15
    |
181 |       let b = &a as isize;
    |    

I thought he would be forced to *const _, and then apply ptr-addr-cast . What did I miss? Should i use mem::transmute?

Answer 1

I thought he would be forced to *const _, and then ptr-addr-cast is applied ....

Firstly, implicit coercion does not occur everywhere and, of course, not inside a part of an expression of the type e as T, as you have noticed. The places where coercion occurs are called coercion sites , usually you do an expression assessment, for example.

• RHS let/const/static:

  let x = foo
  //      ^~~
  

• :

  foo(bar)
  //  ^~~
  

• :

  fn f() -> u32 {
      foo
  //  ^~~
  }
  

• struct/array/tuple:

     [foo, bar, baz]
  //  ^~~  ^~~  ^~~
     (foo, bar, baz)
  //  ^~~  ^~~  ^~~
     Foo { field: foo }
  //              ^~~
  

• :

  { ...; foo }
  //     ^~~
  

foo as isize .

-, , , Rust , . &mut Test &Test *mut Test *const Test &mut SomeTrait .., ! , , :

#![feature(type_ascription)]

let b = {&a}: *const _ as isize;
//       ^~ ^~~~~~~~~~
//        |   we explicitly make the type a const raw pointer
//        |                             using type-ascription
//        a coercion site

, &a as *const _ as isize.

Answer 2

; "" / (-?):

struct Test {
    num: i32,
}
let a = Test { num: 0 };
let b = &a as *const _ as isize;

, . , unsafe .