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My personal nightmare is Fn, FnMut, and FnOnce. I always got them in the opposite order as I intended.

Fn: a function that can be called many times without side effects. Can be shared. Almost exactly like regular functions besides the capturing.

FnMut: a function that can be called many times but can have side effects, because of this it must be exclusively held (&mut F)

FnOnce: a function that can be called only once, making use of by-ownership function calls

So I remember it like Fn: just a function FnMut: a function that needs &mut access FnOnce: only used once, moving ownership

If when trying to use them the compiler fails to provide adequate feedback to get you on track, please file diagnostics tickets! We take them seriously.

I think they're saying that it's counter-intuitive that the specialization relationship between Fn, FnMut, and FnOnce is the reverse of foo, &mut foo, and &foo.

I know it took me a long time for that to stick.

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