Document Noir PR 3045

[github-actions]

Document PR #3045 on the Noir repository.

[guipublic]

PR #3045 allows users to filter functions for specific field.
This is conditional compilation because functions that do not match the filter are not compiled.
To use such filter, you need to add a field attribute. For instance:

#[field(bn254)]
fn foo() -> u32 {
    1
}

#[field(23)]
fn foo() -> u32 {
    2
}

#[field(bls12_381)]
fn foo() -> u32 {
    3
}

The field attributes that we currently support are: bn254, bls12_381, and any number (in decimal or hexa form)
When the field attribute is set, noir will compile the function only if it match the native field of noir. This native field is given by feature defined on acir_field cargo.toml. Default is bn254.
In the example given, we can define a function 3 times, but only the first one will be used, because it matches the default noir field. If you re-compile noir with bs12-381, then it will use the last one. The middle one will only match if the noir field is 23. If you use the bn254 modulus value instead, then noir will complain that there are duplicate function definitions. If you use a curve name which is not defined, e.g #[field(my_great_curve)], then the field attribute will not match and the function is discarded (i.e we do not produce an error).

[critesjosh]

thanks @guipublic! Is this useful because it allows Noir devs to write code that is more proving system agnostic? Is this being used anywhere in the Noir codebase?

[guipublic]

thanks @guipublic! Is this useful because it allows Noir devs to write code that is more proving system agnostic? Is this being used anywhere in the Noir codebase?

Yes, the proving system specifies the curve and hence the field noir has to use. So when we write code which depends on the field, it won't work for another proving system.
This is not used in the Noir codebase, but we already have code which depends on the field and we should use the field attribute for it. I will have a look at adding it in our std-lid.

[signorecello]

This is great! I think we don't have a dedicated page about other backends... So I'll move this to the docs monorepo and ask if someone wants to write a specific doc page about this :)

[guipublic]

I am closing this because the docs have already been updated by PR #3209