fix: handle callbacks from functions without a return type in a special way

[itegulov]

Fixes #729

I fixed this in a very straightforward way by extracting the first generic type for all #[callback_result] function parameters and checking if it is unit or not. This also adds a nice benefit of a compile time check that all #[callback_result] parameters have the right type.

But I am not super happy with how it turned out in the end. Since we do not have access to semantic information in macros there is no certain way to check whether the given type is actually the kind of Result we want. If a user was to type a parameter as core::result::Result or via a Result type alias, the macro would panic even though it shouldn't. If the user had something like

use anyhow::Result;

fn foo(#[callback_result] a: Result<String, PromiseError>)

it won't panic even though it should.

The other alternative would be to change our serialization logic to make all functions that don't have an explicit return type (which is a much easier check) to return "null" as their response, but I am not sure if it is a good idea to change how all already existing contracts behave and I will be introducing maybe a miniscule, but still overhead. So what do you guys think?

[austinabell]

Suggested change

	near_sdk::PromiseResult::Successful(data) if data.is_empty() || data == vec!(110, 117, 108, 108) =>
	near_sdk::PromiseResult::Successful(data) if data.is_empty() || &data == b"null" =>

vec! will allocate, and we don't need this to check equality.

Also, null will be deserialized into (), so this code path isn't really necessary. Also, I'm pretty sure this will fail when using another serialization protocol, try it with borsh

[itegulov]

All good points. I made so that the flow goes through the regular deserialization process if the array is not empty, this should resolve all the weirdness you have pointed out.

[austinabell]

note that this doesn't create a compiler error with span info, so the error line will be on #[near_bindgen] like:

24 | #[near_bindgen]
   | ^^^^^^^^^^^^^^^
   |
   = help: message: Function parameters marked with #[callback_result] should have type Result<T, PromiseError>

but you can have this error be placed on the violating line with something like https://docs.rs/syn/latest/syn/struct.Error.html and converting it into a compile error.

Can be done later, not a big deal

[itegulov]

nice, didn't know there were proper diagnostics in Rust macros