Implement Display for Duration.

[SimonSapin]

… or was there a reason not to do it?

r? @alexcrichton

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[SimonSapin]

Oh, I just found #25481. This may need to be more involved than I thought (rounding vs truncating, the meaning of width/padding, …) but I’m less convinced about using a different unit based on the range of value.

CC @kballard

[lilyball]

@SimonSapin If you don't use a different unit, then Display will be useless for most uses, because it will only be appropriate for durations of a scale that makes sense with the unit you chose.

[alexcrichton]

I do think this type should implement Display, but as #25481 has shown I think it's a pretty tricky situation about what it should actually be displaying. We may want to figure out what format should be displayed before pushing forward with an implementation.

For example, if I have a 3 nanosecond duration it seems like the "most readable" representation would be along the lines of 3ns rather than 0.000000003 as I believe this would print.

[alexcrichton]

The libs team discussed this during triage yesterday, and the conclusion was that we probably don't actually want a Display implementation on Duration. Following the guidelines for Display an interesting point about durations is that there seems to be no "one true way" to display them that is both not lossy and useful. As a result, it's likely that we may want to provide adapters which themselves implement Display to get different methods of formatting a duration, but we probably don't want a Display implementation itself on the type.

As a result I'm going to close this for now, but we also decided we'd be fine with an unstable method returning a display-able adapter, so feel free to open a PR with that!

[alexcrichton]

Ah and another point brought up by @brson is that once you start dealing with time + printing the question of locales can come into play, so we either need to have a story around that or ensure that we're stepping around the problem.