[Draft] Don't include PhantomData fields in IdlBuild impl

[ssadler]

Hi, I made this small change which addresses #3241 in order to be able to use a PhantomData field in an account.

It gets the IDL to build by filtering out PhantomData fields, however i have not tested it the other way, i.e., what happens if you want to encode a struct that includes a PhantomData field from a JS input.

Would like to know if this PR is acceptable as is, or what other changes need to be made also? For example:

• Test that IDL definition does not include PhantomData fields
• Signed Git commit
• Some comments would be nice

Best

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[acheroncrypto]

Thanks for taking the time to work on this!

I think we also need to remove the generics that are only used by the PhantomData to make the definition valid. Otherwise, using declare_program! with this IDL would result in a compile error because Rust doesn't allow unused generic parameters.

Would like to know if this PR is acceptable as is, or what other changes need to be made also?

It's helpful and has minimal changes, so yes, it's certainly acceptable. However, it would be great to have some tests for this in here. Tests should cover compilation of a definition with:

• Existing generic parameters (other than the one used by the PhantomData field)
• No existing generic parameters

The generated IDL should also be checked to make sure:

• The PhantomData fields don't exist
• Generic parameter definitions are correctly omitted.

[acheroncrypto]

Nit: match arms more or less follow the order of IdlType, so it would be great if you could move this logic either inside the Defined impl:

anchor/lang/syn/src/idl/defined.rs

Lines 484 to 485 in 6df05aa

	// Defined
	syn::Type::Path(path) => {

or just above it.

[ssadler]

I think we also need to remove the generics that are only used by the PhantomData to make the definition valid. Otherwise, using declare_program! with this IDL would result in a compile error because Rust doesn't allow unused generic parameters.

How would I produce this error? The change is Works On My Machine(TM), because though the account struct has a generic param, it's only the impl that omits encoding the PhantomData field and rust allows that. Updated title.

[acheroncrypto]

I was going to push a minimal reproduction to this branch, but I was unable to do so because of this branch belonging to an organization fork. However, it's quite easy to reproduce; you can simply define a type that uses PhantomData:

#[derive(AnchorSerialize, AnchorDeserialize, Clone)]
pub struct PhantomStruct<T> {
    pub field: u32,
    _phantom: PhantomData<T>,
}

and use that type as a parameter to an instruction:

// Compilation test for whether `PhantomData` can be used with IDL
pub fn test_compilation_phantom_data(
    _ctx: Context<TestCompilation>,
    _phantom_struct: PhantomStruct<u8>,
) -> Result<()> {
    Ok(())
}

in this program.

Trying to build declare-program after updating external's IDL results in:

error[E0392]: type parameter `T` is never used
 --> programs/declare-program/src/lib.rs:5:1
  |
5 | declare_program!(external);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^ unused type parameter
  |
  = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData`
  = help: if you intended `T` to be a const parameter, use `const T: /* Type */` instead
  = note: this error originates in the macro `declare_program` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0282]: type annotations needed
 --> programs/declare-program/src/lib.rs:5:1
  |
5 | declare_program!(external);
  | ^ cannot infer type
  |
  = note: this error originates in the derive macro `Clone` which comes from the expansion of the macro `declare_program` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0282]: type annotations needed
 --> programs/declare-program/src/lib.rs:5:1
  |
5 | declare_program!(external);
  | ^ cannot infer type
  |
  = note: this error originates in the derive macro `Default` which comes from the expansion of the macro `declare_program` (in Nightly builds, run with -Z macro-backtrace for more info)

Some errors have detailed explanations: E0282, E0392.
For more information about an error, try `rustc --explain E0282`.
error: could not compile `declare-program` (lib) due to 3 previous errors

[ssadler]

Ah, yes, I agree. I mentioned that possibility in my original comment. That still won't work. Let me know if it's acceptable to fix only the impls?