Diesel's build is failing on beta and nightly

[sgrif]

There appears to be some regression in trait resolution between beta and nightly. Our builds are failing on nightly with this error: https://travis-ci.org/diesel-rs/diesel/jobs/324077372#L1081

I haven't yet looked into what the exact cause is, but the same code is compiling successfully on stable and beta.

[sgrif]

This is now a regression from stable to beta

[shssoichiro]

I have some time now, I can go through and do a bisect to find the cause.

[sgrif]

@shssoichiro Thank you! <3 I'm sorry I haven't provided a more specific reproduction case

[shssoichiro]

It looks like the regression originated in 8c59418. My first guess at what's happening is that the blanket impl<T> SliceContains for T where T: PartialEq is causing an infinite recursion in trying to resolve diesel's Insertable trait, although I haven't created a minimized reproduction.

[nikomatsakis]

Hmm. Any idea how those two traits are connected? That seems surprising.

[nikomatsakis]

I'd like to dig a bit more into this and understand better what's going on.

[sgrif]

I'm not sure how SliceContains could affect Diesel, but I agree that it's the only thing in that commit that looks like it could possibly cause any problems here.

[sgrif]

FWIW the impls it appears to be recursing through look like this:

impl<T, Tab> Insertable<Tab> for Option<T>
where
    T: Insertable<Tab>,
    T::Values: Default,
{
    // ...
}

impl<'a, T, Tab> Insertable<Tab> for &'a Option<T>
where
    Option<&'a T>: Insertable<Tab>,
{
    // ...
}