Add help when trying to use literal 0 as raw pointer

[Gaelan]

Code

extern "C" {
    // hypothetical C function which takes a nullable pointer
    fn foo(ptr: *const u8);
}

fn main() {
    unsafe { foo(0); }
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/main.rs:6:18
  |
6 |     unsafe { foo(0); }
  |              --- ^ expected `*const u8`, found `usize`
  |              |
  |              arguments to this function are incorrect
  |
  = note: expected raw pointer `*const u8`
                    found type `usize`
note: function defined here
 --> src/main.rs:2:8
  |
2 |     fn foo(ptr: *const u8);
  |        ^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

Desired output

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
 --> src/main.rs:6:18
  |
6 |     unsafe { foo(0); }
  |              --- ^ expected `*const u8`, found `usize`
  |              |
  |              arguments to this function are incorrect
  |
  = note: expected raw pointer `*const u8`
                    found type `usize`
  = help: if you're trying to create a null pointer, use
    std::ptr::null()
note: function defined here
 --> src/main.rs:2:8
  |
2 |     fn foo(ptr: *const u8);
  |        ^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

Rationale and extra context

(Note the added help line.)

C programers could reasonably expect 0 in a raw-pointer position to mean null. Seems like this could be a fairly simple check to save them a google.

(For reference: I'm a fairly experienced Rust programmer, and a novice C programmer, but I haven't done much FFI. I couldn't remember what std::ptr::null was called, so I tried 0; I was pretty sure it wasn't going to work, but I hoped I might at least get a helpful error.)

Other cases

No response

Anything else?

No response

[edward-shen]

Seems like an easy thing to fix and a reasonable line of logic?

@rustbot claim