[SR-16111] [SymbolKit] Provide a simplified method for reading a symbol's or relationship's mixins

[QuietMisdreavus]

Previous ID	SR-16111
Radar	None
Original Reporter	@QuietMisdreavus
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Swift-DocC
Labels	Improvement
Assignee	None
Priority	Medium

md5: 9c3463d48a54fbd06e0e7b6bba7d89d2

Issue Description:

SymbolKit currently defines optional data fields for symbols and relationships as a set of "mixins", which is represented as a map from a "mixin key" to the parsed value of that mixin. Swift-DocC uses these mixins for various uses, but checking for a value and reading it out is a bit cumbersome. For example, this is how Swift-DocC loads availability information from a symbol:

if var availability = symbol.mixins[SymbolGraph.Symbol.Availability.mixinKey] as? SymbolGraph.Symbol.Availability

This could be greatly simplified into some kind of getMixin method on symbols and relationships in SymbolKit. It could look something like this:

func getMixin<T>() -> T? where T: Mixin {
    self.mixins[T.mixinKey] as? T
}

In fact, Swift-DocC already defines something similar, as an extension on mixin dictionaries themselves:

extension Dictionary where Key == String, Value == Mixin {
    func getValueIfPresent<T>(for mixinType: T.Type) -> T? where T: Mixin {
        return self[mixinType.mixinKey] as? T
    }
}

Adding one of these methods to SymbolKit and encouraging its use throughout Swift-DocC would make its codebase much more readable.