Add report formatter concept for customization of report output

[zanieb]

Adds a ReportFormatter trait and a Reporter::report_with_formatter method allowing customization of formatting the report without reimplementing reporter internals. Currently, we use format_external and format_terms but we can refine formatting in the future as needed e.g. all the explain methods in the reporter.

Adds a DefaultStringReportFormatter which implements the existing format for the DefaultStringReporter.

You can see how much easier this is to use in the example diff at fd30571

[zanieb]

I pass the &impl ReportFormatter type around instead of adding a &dyn ReportFormatter field to the reporter to avoid proliferation of lifetimes. Perhaps a more elegant solution around for someone who wants to fight with the compiler.

[Eh2406]

TLDR. I love it!

I had been working toward something like this in https://github.com/pubgrub-rs/pubgrub/tree/reporter_trait, if there are any of those changes that look like good ideas please take them. But your re-factor seems much more user focused and a better place to start.

I would eventually like our reporting infrastructure to support directly outputting to &mut Write instead of having to be buffered in strings, but having seen your branch it is a re-factor better done separately.

[zanieb]

Good to hear! Here's an upstream pull request pubgrub-rs#158

I had been working toward something like this in https://github.com/pubgrub-rs/pubgrub/tree/reporter_trait, if there are any of those changes that look like good ideas please take them. But your re-factor seems much more user focused and a better place to start.

Ah interesting. I agree it seems a bit more complicated than we need to get started. I think there are some ideas there we can build on though.

I would eventually like our reporting infrastructure to support directly outputting to &mut Write instead of having to be buffered in strings, but having seen your branch it is a re-factor better done separately.

Agreed!

[zanieb]

Closing in favor of pubgrub-rs#158