derive(Serialize) regression with lifetimes

[arcnmx]

This used to work around version 0.7.10 of serde_codegen...

#[derive(Serialize)]
struct S<'a> {
    a: Option<&'static str>,
    b: Option<&'a str>,
}

error: mismatched types [E0308]
#[derive(Serialize)]
help: run `rustc --explain E0308` to see a detailed explanation
note: expected type `serde::Serialize`
note:    found type `serde::Serialize`
note: the lifetime 'a as defined on the impl at 31:9...
#[derive(Serialize)]
note: ...does not necessarily outlive the static lifetime

[dtolnay]

I started looking into this. We generate code like the following:

impl<'a> Serialize for S<'a>
    where Option<&'static str>: Serialize,
          Option<&'a str>: Serialize,
{
    fn serialize<SER>(&self, _ser: &mut SER) -> Result<(), SER::Error>
        where SER: Serializer,
    {
        unimplemented!()
    }
}

The error message is coming from Option<&'static str>: Serialize. I need to figure out why, because that seems like a reasonable bound.

[dtolnay]

The option impl looks like this:

impl<T> Serialize for Option<T> where T: Serialize { /* ... */ }

The ref impl looks like this:

impl<'a, T: ?Sized> Serialize for &'a T where T: Serialize { /* ... */ }

And the str impl looks like this:

impl Serialize for str { /* ... */ }

So it seems like Option<&'static str>: Serialize should be fine. Possibly a compiler issue? I will continue debugging after work.