Allow cheap conversion intoScalarValue for custom string types

[tyranron]

Part of #819
Revealed from #1247 (comment)

Synopsis

an idea: Could we somehow make From<ArcStr> a supertrait of ScalarValue? Then one could make a custom scalar value that doesn't need to clone string buffers that are already ArcStr. Think introspection queries, etc.

(Ideally From<&ArcStr> (which doesn't require cloning up front), but that's not possible without making ScalarValue lifetime generic. Another idea is to add a method fn from_arcstr(&ArcStr) -> Self to trait ScalarValue itself.)

edit: fn from_arcstr(&ArcStr) could be a method with a default implementation that just delegates to From<String>

Indeed, if an ArcStr is used (like in SchemaType types), and some custom ScalarValue is involved, which uses ArcStr internally, we want to operate on it without intermediate conversions into/from String.

Solution

To allow other custom string types as well, instead of just fn from_arcstr(&ArcStr) -> Self method, we introduce a fn from_displayable<Str: fmt::Display + Any + ?Sized>(s: &Str) -> Self method defaulting to s.to_string().into().

However, an author of a custom ScalarValue could redefine it to convert efficiently:

fn from_displayable<Str: fmt::Display + Any + ?Sized>(s: &Str) -> Self {
    use juniper::AnyExt as _;

    if let Some(arc_str) = s.downcast_ref::<ArcStr>() {
        Self::ArcStr(arc_str.clone())
    } else {
        s.to_string().into()
    }
}

Checklist

• Documentation is updated
• Examples are provided
• Tests are added
• CHANGELOG is updated

[tyranron]

ack @audunhalland

I went a little bit further with your idea, to not restrict only to ArcStr type, but allow arbitrary ones.

The second counterpart of this, allowing cheap conversion from InputValue into custom string type, would be done in a subsequent PR.

[audunhalland]

@tyranron I like it, it's a nice improvement.