Bogus trivial cast warning when comparing references cast to *const

[sfackler]

fn main() {
    let a = &true;
    let b = &false;

    assert!(a as *const _ == b as *const _);
}

test.rs:5:30: 5:43 warning: trivial cast: `&bool` as `*const bool`, #[warn(trivial_casts)] on by default
test.rs:5     assert!(a as *const _ == b as *const _);
                                       ^~~~~~~~~~~~~
<std macros>:1:1: 5:46 note: in expansion of assert!
test.rs:5:5: 5:45 note: expansion site

But removing the second cast doesn't compile:

fn main() {
    let a = &true;
    let b = &false;

    assert!(a as *const _ == b);
}

test.rs:5:30: 5:31 error: mismatched types:
 expected `*const _`,
    found `&bool`
(expected *-ptr,
    found &-ptr) [E0308]
test.rs:5     assert!(a as *const _ == b);
                                       ^
<std macros>:1:1: 5:46 note: in expansion of assert!
test.rs:5:5: 5:33 note: expansion site
error: aborting due to previous error

cc @nrc

[nrc]

The lint runs when a cast can be replaced by a coercion, not when a cast can be removed. So, this is not bogus, although it is kind of annoying to fix without type ascription.

[sfackler]

Uh, so what's the resolution here? Should I just turn the lint off?

[nrc]

You could add a type annotation to the declaration of b so that it has the expected type, or introduce a temporary variable (b_as_raw or something). You could also turn the lint off until type ascription lands (hopefully soon) which will give you a nice way to do this.