`syn2` and `syn3` are mutually exclusive

[datdenkikniet]

Hi,

It seems that the syn2 and syn3 features are mutually exclusive, as they both a copy of the following impl:

impl<T: quote::ToTokens> ManyhowToTokens<T> for &WhatType<T> {}

additionally, the following two blanket impls collide when both features are activated:

#[cfg(all(feature = "syn2", not(doc)))]
impl<T: syn2::parse::Parse> ManyhowParse<T> for &WhatType<T> {}

#[cfg(feature = "syn3")]
impl<T: syn3::parse::Parse> ManyhowParse<T> for &WhatType<T> {}

Is this intended? This suggests that using this crate in a setup that has both syn2 and syn3 based proc-macros that use the same version manyhow will never compile since (AFAIK) the features for manyhow will be unified.

Thanks,

Jona

[datdenkikniet]

To illustrate the problem, imagine two crates in my tree with the following Cargo.tomls:

[package]
name = "testy3"
version = "0.1.0"
edition = "2024"

[dependencies]
syn = "3"
manyhow = { version = "0.13", default-features = false,  features = [ "syn3" ]}

and

[package]
name = "testy2"
version = "0.1.0"
edition = "2024"

[dependencies]
syn = "2"
manyhow = { version = "0.13", default-features = false,  features = [ "syn2" ]}

manyhow (and by extension, my crate) will fail to compile and there is nothing I can do about it.

[ModProg]

true... sadly this is due to the fake specialisation I'm using. But maybe there is a way to achieve this in a different way, though it's difficult.

I wonder if there is a way to achieve this via a different approach than this trait.