Breakage in future Rust version

[lcnr]

Hello there, unfortunately frost-core uses an impl which should result in a coherence error, but due to an oversight in the existing compiler implementation, it has not been detected up until now. This impl must not be allowed as it may overlap.

frost/frost-core/src/identifier.rs

Lines 152 to 159 in 6e5aff8

	impl<C> std::ops::MulAssign<Identifier<C>> for Scalar<C>
	where
	C: Ciphersuite,
	{
	fn mul_assign(&mut self, identifier: Identifier<C>) {
	*self = *self * identifier.0
	}
	}

Scalar is defined as the associated type <<<C as Ciphersuite>::Group as Group>::Field as Field>::Scalar. Were a user to implement Field for their own type they could choose an associated type which already implements MulAssign<Identifier<Itself>>, causing an overlap. This is rust-lang/rust#99554.

We're currently working on a fix in rust-lang/rust#117164 and will likely land it as a future compat warning for now. This will then change to a hard error in the medium term. I can't tell how to best avoid this error? It may potentially be easiest to remove that impl entirely?

I apologize for the inconvenience and thank you for your understanding

[conradoplg]

Thank you for reporting this. Since the impl is only used internally it should be easy to remove it.