make use of impl T wherever possible

[bilelmoussaoui]

Except in ClosureExpression because we can't mix both impl & generic types due to
rust-lang/rust#83701

Tried to fix #838 but ended up cleaning up few things, there is more to be done but that is for another time.

[sdroege]

Note that both changes are breaking changes, the de-manual change more likely causes problems (people sometimes don't include the prelude but only the traits they use).

[bilelmoussaoui]

Note that both changes are breaking changes, the de-manual change more likely causes problems (people sometimes don't include the prelude but only the traits they use).

Yeah, well I don't know whether to merge those right now or wait till the 0.5 release

[sdroege]

master is what becomes 0.5, we're not backporting everything to the 0.4 branch :) So no need to wait either way.

[bilelmoussaoui]

I am still not familiar at all with what could be considered an API break or not, your help is very appreciated on that regard, thanks!

[sdroege]

FWIW, the other change is a breaking change because previously you could call the functions like foo::<A, B, C>() and now you can't anymore because impl trait does not give you a type parameter to the function. I would ignore that for our purposes here though because nobody calls it like that, there's no point in providing the type parameters like this.

The thing with the traits I wouldn't ignore because I know of people very selectively including specific traits because they don't like preludes.