Automatically deriving implementations about approx HOT 9 OPEN

I've created a repository at https://github.com/tylerreisinger/approx_derive to implement it. When it is production ready, I can transfer ownership or merge it into the approx repo with a cargo workspace.

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@joshburkart It's not ready for a PR yet, but I can work on it a bit this weekend and see if I can get something working. TryFrom being stabilized helps with an issue implementing it.

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Is the last blanket impl necessary? You could always just create a macro to create the implementation for foreign types if needed

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Here's a pretty quick and dirty example of the sort of thing needed: https://gist.github.com/tylerreisinger/886e56379beffabddaff7bad6a232f06. It can only handles structs and won't insert type bounds on generics, but otherwise works.

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Also, there is no clean way to deal with both non-primitive Epsilon types and a mix of f32/f64 types without TryFrom, which is currently unstable. I used as in my example, as I'm guessing non-primitive Epsilons aren't that common.

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A nice thing to have would be a derive-proc-macro, so you could do

#[derive(AbsDiffEq)]
struct MyStruct { ... }

thus avoiding all the repetitive boilerplate on a per-struct basis.

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This would be really great to have! Approximately comparing nested structs/enums is a pretty common use case when writing tests for numerical computing applications. @tylerreisinger Do you think the implementation you made is ready for a PR in some form? Or too hacky right now?

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Yeah, having this would be really nice! I know it'll be a bit annoying with the epsilon though, and haven't had much time at the moment to look into it.

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I'd love to have this as well!

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