Add a constant for ANCHOR_DISCRIMINATOR_SIZE?

[mikemaccana]

Heya! @nickfrosty and I were talking and wondering what you think here.

Right now when teaching Anchor we avoid magic numbers by getting people to add pub const ANCHOR_DISCRIMINATOR: usize = 8; to almost every new project, and get people to write...

space = ANCHOR_DISCRIMINATOR + SomeStruct::INIT_SPACE,

...when they need to initialize new accounts. I'm wondering what a better solution would be:

• Include an ANCHOR_DISCRIMINATOR const out of the box?
• Have something like INIT_SPACE that included the discriminator size, eg TOTAL_SPACE or similar?

I'm happy to send a PR. Just LMK what you think.

[acheroncrypto]

• Include an ANCHOR_DISCRIMINATOR const out of the box?

There are plans to support dynamic discriminator length which would make this redundant. This is also one of the main reasons why the new IDL spec have required discriminator field.

• Have something like INIT_SPACE that included the discriminator size, eg TOTAL_SPACE or similar?

Yeah, tbh I think InitSpace should have included the discriminator by default since it's pretty much only being used in init contexts anyway. That being said, making that change now would be a breaking change, and it's likely many people will keep adding the discriminator size if we were to make changes to InitSpace. Instead, we can deprecate InitSpace, and have a new one TotalSpace (or Space for simplicity) like you've suggested.

I also think it's possible to completely hide the discriminator logic from the user's POV.

[Aursen]

The macro needs to be rewrite totally. The thing with the approach is to differentiate account struct and included struct in the derive macro. Also derive automatically basic types to be able to call for example u64::INIT_SPACE. Also allow to bypass the space with an attribute raw_space. The macro INIT_SPACE is a giant mess

[acheroncrypto]

FYI, you can use

MyAccount::DISCRIMINATOR.len()

rather than hardcoding or declaring a new constant, which also works with custom discriminators:

anchor/tests/custom-discriminator/programs/custom-discriminator/src/lib.rs

Line 66 in 04b2153

space = MyAccount::DISCRIMINATOR.len() + core::mem::size_of::<MyAccount>(),

Closing as this solves the "magic number" problem, and #3184 is more fitting for future discussions.

[mikemaccana]

Thanks @acheroncrypto ! I gather DISCRIMINATOR appears in Anchor next release (I can't find it in 0.30.1)?

[acheroncrypto]

It also exists in v0.30.1, but it's not exported from prelude. You need to bring the trait to scope to get access to the DISCRIMINATOR constant:

use anchor_lang::Discriminator;

The import won't be needed in the next version (#3075).