Is it possible to tell if a field is a certain type or implements a certain method in a procedural macro?

Question

I created a procedural macro that implements a trait, but in order for this to work I need to get the raw bytes for every field. The problem is how to get the bytes of a field differs depending on the type of field.

Is there some way of testing if a function exists on a field and if it does not tries another function?

E.g. something like this:

if item.field::function_exist {
    //do code
} else {
    //do other code
}

Currently I am looking at creating another trait/member function that I just have to create for all primitives and create a procedural macro for larger fields such as structs. For example:

if item.field::as_bytes().exists {
    (&self.#index).as_bytes()
} else {
    let bytes = (&self.#index).to_bytes();
    &bytes
}

With a string, it has a as_bytes member function, while i32 does not. This means I need extra code, when the member field of the struct is not a string. I might need a match rather than an if, but the if will suffice for the example.

I really don't understand what you want to achieve or what's your problem. This sounds very much as an XY problem to me. Please describe what your problem is, what you want to do and what you have done so far. Please have a look at [how to ask](https://stackoverflow.com/help/how-to-ask) and the rust specific information at the [rust info page](https://stackoverflow.com/tags/rust/info). — hellow, Apr 01 '19 at 11:10
I added in an extra piece of what I want to do. The trait implementation I have I require the raw bytes in memory. I dont really want to delve into unsafe if I dont have too. Yes I know those functions do. — blackwolfsa, Apr 01 '19 at 12:21
I thought I might add this as well. What I in essence want is the raw bytes in memory of the struct. My current macro steps through each field of the struct and asks the as_bytes(), but not every field has as_bytes(). — blackwolfsa, Apr 01 '19 at 12:54

score 3 · Accepted Answer · edited Oct 13 '20 at 13:01

Is it possible to tell if a field is a certain type or implements a certain method in a procedural macro?

No, it is not.

Macros operate on the abstract syntax tree (AST) of the Rust code. This means that you basically just get the characters that the user typed in.

If user code has something like type Foo = Option<Result<i32, MyError>>, and you process some code that uses Foo, the macro will not know that it's "really" an Option.

Even if it did know the type, knowing what methods are available would be even harder. Future crates can create traits which add methods to existing types. At the point in time that the procedural macro is running, these crates may not have even been compiled yet.

I am looking at creating another trait/member function that I just have to create for all primitives and create a procedural macro for larger fields such as structs.

This is the correct solution. If you look at any existing well-used procedural macro, that's exactly what it does. This allows the compiler to do what the compiler is intended to do.

This is also way better for maintainability — now these primitive implementations live in a standard Rust file, as opposed to embedded inside of a macro. Much easier to read and debug.

Your crate will have something like this:

// No real design put into this trait
trait ToBytes {
    fn encode(&self, buf: &mut Vec<u8>);
}

impl ToBytes for str {
    fn encode(&self, buf: &mut Vec<u8>) {
        buf.extend(self.as_bytes())
    }
}

// Other base implementations

And your procedural macro will implement this in the straightforward way:

#[derive(ToBytes)]
struct Foo {
    a: A,
    b: B,
}

becomes

impl ToBytes for Foo {
    fn encode(&self, buf: &mut Vec<u8>) {
        ToBytes::encode(&self.a, buf);
        ToBytes::encode(&self.b, buf);
    }
}

As a concrete example, Serde does the same thing, with multiple ways of serializing to and from binary data:

Is it possible to tell if a field is a certain type or implements a certain method in a procedural macro?

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