Diagnostic for mismatched pointers type could be improved

[tesuji]

The following snippet is build with Rust 1.38.0: https://rust.godbolt.org/z/NGTGo-

snippet

mod unique {
    type HANDLE = *mut ::std::os::raw::c_void;
    type PHANDLE = *mut HANDLE;
    fn foo(token: PHANDLE) {}

    fn bar() {
        let token: HANDLE = ::std::ptr::null_mut();
        foo(token as *mut _);
        foo(&mut token);
        foo(&token);
        foo(token);
    }
}

mod shared {
    type HANDLE = *const ::std::os::raw::c_void;
    type PHANDLE = *const HANDLE;
    fn foo(token: PHANDLE) {}

    fn bar() {
        let token: HANDLE = ::std::ptr::null_mut();
        foo(token as *const _);
        foo(&mut token);
        foo(&token);
        foo(token);
    }
}

mod unique_mut {
    type HANDLE = *mut ::std::os::raw::c_void;
    type PHANDLE = *mut HANDLE;
    fn foo(token: PHANDLE) {}

    fn bar() {
        let mut token: HANDLE = ::std::ptr::null_mut();
        foo(token as *mut _);
        foo(&mut token);
        foo(&token);
        foo(token);
    }
}

The compiler outputs:

rustc's output

error[E0308]: mismatched types
  --> src/lib.rs:10:13
   |
10 |         foo(&token);
   |             ^^^^^^ types differ in mutability
   |
   = note: expected type `*mut *mut std::ffi::c_void`
              found type `&*mut std::ffi::c_void`

error[E0308]: mismatched types
  --> src/lib.rs:11:13
   |
11 |         foo(token);
   |             ^^^^^ expected *-ptr, found enum `std::ffi::c_void`
   |
   = note: expected type `*mut *mut std::ffi::c_void`
              found type `*mut std::ffi::c_void`

error[E0308]: mismatched types
  --> src/lib.rs:25:13
   |
25 |         foo(token);
   |             ^^^^^ expected *-ptr, found enum `std::ffi::c_void`
   |
   = note: expected type `*const *const std::ffi::c_void`
              found type `*const std::ffi::c_void`

error[E0308]: mismatched types
  --> src/lib.rs:38:13
   |
38 |         foo(&token);
   |             ^^^^^^ types differ in mutability
   |
   = note: expected type `*mut *mut std::ffi::c_void`
              found type `&*mut std::ffi::c_void`

error[E0308]: mismatched types
  --> src/lib.rs:39:13
   |
39 |         foo(token);
   |             ^^^^^ expected *-ptr, found enum `std::ffi::c_void`
   |
   = note: expected type `*mut *mut std::ffi::c_void`
              found type `*mut std::ffi::c_void`

error: aborting due to 5 previous errors

I'm a beginner in Rust's FFI. So my questions may seems silly.

1. In module unique, line 8: we mutually borrow token, but token is never defined
   with let mut syntax. Is this accepted?

2. Same question but for module shared, line 23.

3. Can we suggest borrowing with appropriate mutability in foo(token) lines?

   For example:

   error[E0308]: mismatched types
     --> src/lib.rs:10:13
      |
   10 |         foo(&token);
      |             ^^^^^^ types differ in mutability
      |
      = note: expected type `*mut *mut std::ffi::c_void`
                 found type `&*mut std::ffi::c_void`
        try: `&mut token`
   
   error[E0308]: mismatched types
     --> src/lib.rs:11:13
      |
   11 |         foo(token);
      |             ^^^^^ expected *-ptr, found enum `std::ffi::c_void`
      |
      = note: expected type `*mut *mut std::ffi::c_void`
                 found type `*mut std::ffi::c_void`
        try: `&mut token`