Derivation adds unnecessary bounds that can't be sat... | paritytech/parity-scale-codec

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Error encountered

error[E0277]: the trait bound `BrakesOthers<u32>: Decode` is not satisfied
   --> tests/mod.rs:898:14
    |
898 |     pub broken: runtime::BrakesOthers<u32>,
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Decode` is not implemented for `BrakesOthers<u32>`
    |
    = help: the trait `Decode` is implemented for `BrakesOthers<_1>`

error[E0277]: the trait bound `BrakesOthers<u32>: Encode` is not satisfied
   --> tests/mod.rs:896:24
    |
896 | #[derive(DeriveDecode, DeriveEncode, Debug)]
    |                        ^^^^^^^^^^^^ the trait `Encode` is not implemented for `BrakesOthers<u32>`, which is required by `&BrakesOthers<u32>: Encode`
    |
    = help: the trait `Encode` is implemented for `BrakesOthers<_1>`
    = note: required for `&BrakesOthers<u32>` to implement `Encode`
    = note: this error originates in the derive macro `DeriveEncode` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.

Minimal Repro:

#[derive(DeriveDecode, DeriveEncode, Debug)]
pub struct Foo2 {
	pub broken: runtime::BrakesOthers<u32>,
}

pub mod runtime {
	use std::marker::PhantomData;

	use parity_scale_codec_derive::{Decode as DeriveDecode, Encode as DeriveEncode};

	#[derive(DeriveEncode, DeriveDecode, Debug)]
	pub struct BrakesOthers<_1> {
		twtwt: Box<super::runtime::BrakesOthers<_1>>,
		marker: PhantomData<_1>,
	}
}

Produces trait with extra bounds(expansion below, note extra Containter<Self> bounds introduced):

Click me(`Encode` as the example, although same happens for `Decode` too)

// Recursive expansion of DeriveEncode macro
// ==========================================

#[allow(deprecated)]
const _: () = {
	#[automatically_derived]
	impl<_1> ::parity_scale_codec::Encode for BrakesOthers<_1>
	where
		Box<super::runtime::BrakesOthers<_1>>: ::parity_scale_codec::Encode,
		Box<super::runtime::BrakesOthers<_1>>: ::parity_scale_codec::Encode,
		PhantomData<_1>: ::parity_scale_codec::Encode,
		PhantomData<_1>: ::parity_scale_codec::Encode,
	{
		fn size_hint(&self) -> usize {
			0_usize
				.saturating_add(::parity_scale_codec::Encode::size_hint(&self.twtwt))
				.saturating_add(::parity_scale_codec::Encode::size_hint(&self.marker))
		}
		fn encode_to<__CodecOutputEdqy: ::parity_scale_codec::Output + ?::core::marker::Sized>(
			&self,
			__codec_dest_edqy: &mut __CodecOutputEdqy,
		) {
			::parity_scale_codec::Encode::encode_to(&self.twtwt, __codec_dest_edqy);
			::parity_scale_codec::Encode::encode_to(&self.marker, __codec_dest_edqy);
		}
	}
	#[automatically_derived]
	impl<_1> ::parity_scale_codec::EncodeLike for BrakesOthers<_1>
	where
		Box<super::runtime::BrakesOthers<_1>>: ::parity_scale_codec::Encode,
		Box<super::runtime::BrakesOthers<_1>>: ::parity_scale_codec::Encode,
		PhantomData<_1>: ::parity_scale_codec::Encode,
		PhantomData<_1>: ::parity_scale_codec::Encode,
	{
	}
};

changing this line to

  for segment in i.path.segments.iter() {

seems to solve the issue of correctly detecting the recursive ty_ident.

Tests pass after the change, however it might break things not tested in the ecosystem(?)
also see: paritytech/subxt#1603

Inspect derive/src/trait_bounds.rs around line 55 and the recursive type-identifier detection logic, using the provided BrakesOthers reproduction as the starting case. Run the existing test suite and verify that the generated Encode and Decode implementations no longer contain unsatisfiable recursive bounds. Check that other trait-bound cases remain unaffected, including the related paritytech/subxt#1603 scenario.

Written by the indexing model from the issue text.

Derivation adds unnecessary bounds that can't be satisfied for certain recursive types

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