I think this one is a bit unfortunate... But why not shortening all of them? That is, something like:

impl<LengthOrPercentageOrAuto> ToCss for BackgroundSize<LengthOrPercentageOrAuto>
where
    LengthOrPercentageOrAuto: ToCss + PartialEq,
{
    pub fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
    where
        W: Write,
    {
        match self {
            BackgroundSize::Explicit { width, height } => {
                width.to_css(dest)?;
                if width != height {
                    dest.write_str(" ")?;
                    height.to_css(dest)
                }
            }
            BackgroundSize::Cover => dest.write_str("cover"),
            BackgroundSize::Contain => dest.write_str("contain"),
        }
    }
}

In D10446#251192, @emilio wrote:

But why not shortening all of them?

Just because all other browsers are consistent about shortening only "auto auto" and not for any other value.

What's more unfortunate (IMO) is that it's impossible (or difficult enough that I didn't work out how) to add a trait bound on generic::BackgroundSize's LengthPercentageOrAuto type parameter for, say, an IsAuto trait, and then just add a #[css(skip_if = "is_auto")] on the height field or something.

In D10446#251207, @heycam wrote:

In D10446#251192, @emilio wrote:

But why not shortening all of them?

Just because all other browsers are consistent about shortening only "auto auto" and not for any other value.

That's too bad :(

In D10446#251210, @heycam wrote:

What's more unfortunate (IMO) is that it's impossible (or difficult enough that I didn't work out how) to add a trait bound on generic::BackgroundSize's LengthPercentageOrAuto type parameter for, say, an IsAuto trait, and then just add a #[css(skip_if = "is_auto")] on the height field or something.

That'd be wrong though, since you could have stretch auto or something like that, right? You'd need an skip_if_self or something of the sort... It shouldn't be hard to add a way to add extra bounds to the impl either, something like this should work:

https://gist.github.com/emilio/421418d12864929a7d0d9c4a98ff7ccb

Now there's the question of how useful it would generally be...

I think the extra_bounds bit should not be needed, right? enum BackgroundSize<LengthOrPercentageOrAuto: IsAuto> should work without that, right? The #[derive] code I think parses and understands that.

In D10446#251249, @emilio wrote:

In D10446#251210, @heycam wrote:

What's more unfortunate (IMO) is that it's impossible (or difficult enough that I didn't work out how) to add a trait bound on generic::BackgroundSize's LengthPercentageOrAuto type parameter for, say, an IsAuto trait, and then just add a #[css(skip_if = "is_auto")] on the height field or something.

That'd be wrong though, since you could have stretch auto or something like that, right? You'd need an skip_if_self or something of the sort...

Yeah, true.

It shouldn't be hard to add a way to add extra bounds to the impl either, something like this should work:

https://gist.github.com/emilio/421418d12864929a7d0d9c4a98ff7ccb

Cool.

I think the extra_bounds bit should not be needed, right? enum BackgroundSize<LengthOrPercentageOrAuto: IsAuto> should work without that, right? The #[derive] code I think parses and understands that.

I couldn't get it to work.

In D10446#253524, @emilio wrote:

0001-fixup-Bug-1501261-Part-2-Serialize-background-size-a.patch10 KBDownload

Seems to work, wdyt about that? It's neutral in amount of lines, but I think it's a bit nicer.

Huh, I thought I tried pretty much exactly the same thing and ran into troubles!

I bet the problem was that I was also trying to add the IsAuto trait bound to the BackgroundSize enum itself, and that was tripping up the derived traits.