Wrong code generation when using the zero variant of an enum in if clauses

[skade]

The following code:

typedef enum
{
    false,
    true
} boolean;

boolean mayFail() {
    return false;
}

void avoidErasure() {

}

void checksForFail() {
    if (mayFail()) {
        avoidErasure();
    }
}

is converted to:

#[derive(Clone, Copy)]
#[repr(i32)]
pub enum Enum1 {
    false_,
    true_,
}

#[no_mangle]
pub unsafe extern fn mayFail() -> Enum1 { Enum1::false_ }

#[no_mangle]
pub unsafe extern fn avoidErasure() { }

#[no_mangle]
pub unsafe extern fn checksForFail() {
    if mayFail() != 0 {
        avoidErasure();
    }
}

which doesn't compile due to this:

$ rustc lets-play-bool.rs
error[E0369]: binary operation `!=` cannot be applied to type `Enum1`
  --> lets-play-bool.rs:16:8
   |
16 |     if mayFail() != 0 {
   |        ^^^^^^^^^
   |
   = note: an implementation of `std::cmp::PartialEq` might be missing for `Enum1`

error: aborting due to previous error

This implementation would work:

#[no_mangle]
pub unsafe extern fn checksForFail() {
    if (mayFail() as u32) != 0 {
        avoidErasure();
    }
}