Allow cheap conversions info/from ScalarValue for custom string types

[tyranron]

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