feat(AztecMacro): Let function convey mutability

[LHerskind]

Currently it is not possible for a function to convey to other contracts, that it will not be manipulating state, e.g., that it should be called using a static_call.

It is currently the burden of the caller, to do a static_call when needed, instead of a call.

This means that for contract such as a token, other contracts that need to fetch a balance on that token, must explicitly use a static_call instead of having the interface it uses know that it should be static, if the contract was defined as

#[aztec(static)]
#[aztec(public)]
fn balance_of(owner: AztecAddress) -> U128;

Allowing this data to be easily conveyed makes it much less error prone to build on top, as it is now just required to remember the static at the interface, instead at every time you will make a call.

If a call is made to some user provided "token", and it is not static pain ensures as it could really be calling anything, and the control flow is passed on.

[rahul-kothari]

Love the idea! I think it is needed but I wonder if there is a better name? I feel like the name should align with whatever is used on AztecjS side

[fcarreiro]

Nice. Would it be possible to limit static (contract) methods to only (external) call other static methods at compile time?

[Thunkar]

Nice. Would it be possible to limit static (contract) methods to only (external) call other static methods at compile time?

@fcarreiro it is very tricky to do at compile time since we don't have "storage modification opcodes", we just push stuff to the context inputs. However the way I've implemented it in #6338 only generates "Static" versions of the CallInterfaces for the functions that are marked as #[aztec(view)], so if users try to use the .call(&mut context) versions, they'll get a compile time error. Nothing prevents them from trying to do a low-level call, that will fail at runtime.