[Macros] generate diagnostics from thrown error

[elizablock]

Macro expansion methods throw an error to indicate that the expansion failed. This patch allows the error thrown to add full-fledged diagnostics, rather than autogenerating a diagnostic based on the error's description.

It adds an error protocol:

public protocol DiagnosticsProviding: Error {
  var diagnostics: [Diagnostic] { get }
}

...and a utility method on MacroExpansionContext to add diagnostics from an error. The implementation of this method checks if the error is diagnostic providing; if not, it generates the same diagnostic as before. If so, it adds all the provided diagnostics.

[elizablock]

@swift-ci please test

[ahoppen]

This looks good to me. I’m not a huge fan of the name DiagnosticProviding but I don’t have any better names so it’s good with me.

[elizablock]

@swift-ci please test

[elizablock]

This looks good to me. I’m not a huge fan of the name DiagnosticProviding but I don’t have any better names so it’s good with me.

Happy to rename ... DiagnosticError? DiagnosticGenerating?

[elizablock]

@swift-ci please test

[ahoppen]

What about naming it DiagnosticsError? If we do that, we can also assert in the initializer of DiagnosticsError that at least one diagnostics with severity error is provided.