IDL doesn't support PhantomData

[cryptopapi997]

If we define an account like so

use std::marker::PhantomData;

use anchor_lang::prelude::*;

declare_id!("B8v9C4JA5CqMWcoKYfA2hQSEytFxPVk1iWeEjXQxCvS3");

#[program]
pub mod sample {
    use super::*;

    pub fn initialize(ctx: Context<Initialize>) -> Result<()> {
        msg!("Greetings from: {:?}", ctx.program_id);
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Initialize<'info> {
    pub data_obj_acc: Account<'info, MyAcc<MyStruct>>,
}

#[account]
pub struct MyAcc<T> {
    _p: PhantomData<T>,
}

#[derive(AnchorDeserialize, AnchorSerialize, Clone)]
pub struct MyStruct {}

it compiles fine if we compile with anchor build --no-idl. If we try to build the idl however, we get a bunch of errors (I'm guessing because the idl tries to include PhantomData in the idl itself but it isn't serializable). I'm not sure if this is intentional or not, but if it isn't it would be nice to support PhantomData.

[acheroncrypto]

IDL only supports a small subset of types:

anchor/idl/spec/src/lib.rs

Lines 282 to 310 in 340e9c1

	pub enum IdlType {
	Bool,
	U8,
	I8,
	U16,
	I16,
	U32,
	I32,
	F32,
	U64,
	I64,
	F64,
	U128,
	I128,
	U256,
	I256,
	Bytes,
	String,
	Pubkey,
	Option(Box<IdlType>),
	Vec(Box<IdlType>),
	Array(Box<IdlType>, IdlArrayLen),
	Defined {
	name: String,
	#[serde(default, skip_serializing_if = "is_default")]
	generics: Vec<IdlGenericArg>,
	},
	Generic(String),
	}

All these types are more or less language-agnostic, but PhantomData is not. Additionally, there is pretty much nothing useful a client can do with this information.

The best we can do is to skip PhantomData fields during the IDL generation.

[cryptopapi997]

If skipping PhantomData during idl generation is a valid option for you, happy to open a PR for this. Although I believe this will cause issues with the generic argument, as you mentioned in #3148 that the anchor idl never officially supported generics.

What do you suggest is the best approach here?

[acheroncrypto]

I think skipping is the better option at least for now.