Add compile-time error when using brw on if attribute

[jam1garner]

No description provided.

[csnover]

The problem alluded to by this ticket exists for any rw directive that accesses fields because read fields are by-value whereas write fields are by-reference. It is actually fine to have #[brw(if)] when it doesn’t try to use fields (e.g. imports, globals).

[kitlith]

There are also ways to make it work by using auto(de)ref. Take this example:

#[binrw]
struct Test {
    #[brw(assert(field.ne(&0u8)))]
    field: u8,
}

by writing the condition as field.ne(&0u8) instead of field != 0u8 as usual, we can take advantage of autoref to write code that works in both cases. Alternatively, you could imagine an impl of PartialEq that allows you to compare a value and a ref.

[csnover]

Is there a way to handle this in the general case (ab)using auto-deref so users don’t have to, or is this just some thing that should be documented?

[kitlith]

Unsure. For operators we might be able to get something to work by parsing and replacing uses of operators with uses of traits. This strikes me as error-prone, though.

I'd probably just go ahead and document it.