vapid tusk
I have been working on a library and have had the following code snippet:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=60c0f0196a14fd6a8b53e6d04065b46b
pub struct Never(());
pub struct Attributes<I, W> {
pub(crate) id: I,
pub(crate) weight: W,
}
impl<I, W> From<(I, W)> for Attributes<I, W>
{
fn from(value: (I, W)) -> Self {
Self {
id: value.0,
weight: value.1,
}
}
}
impl<W> From<(W,)> for Attributes<Never, W> {
fn from((weight,): (W,)) -> Self {
Self {
id: Never(()),
weight,
}
}
}
impl<W> From<W> for Attributes<Never, W> {
fn from(weight: W) -> Self {
Self {
id: Never(()),
weight,
}
}
}
You might immediately see it, but even though I provide a blanket implementation (via From<W> for Attributes<Never, W>) I am able to implement (and subsequently compile) with a more specialised implementation of From<(W, )> for Attributes<Never, W>, but this should violate the coherence rules. Take for example (u8, ), both the second and third From implementation would match both implementation, therefor violating the rule, and last time I checked tuples are no fundamental types either.
So I am baffled - why does this compile? What am I overlooking?
orchid hamlet