Prohibit the use of Self in non-init functions

[itegulov]

The use of Self in non-init context might be harmful as sometimes it indicates that the function was meant to be marked with #[init]. It might make sense to gradually prohibit this pattern, e.g. give a deprecation warning in 4.2 and give a compilation error in 5.0.

Originally pointed out in #972 (comment).

[uint]

Once the deprecation warning is there, should we create a 5.0 milestone and attach this issue to it so that we don't forget to switch to the compilation error when it's time?

[uint]

Also, should passing a Self argument still be fine? The original motivation was about using it as the return type from what I see.

[itegulov]

We already have a tracking issue for 5.0: #946

Also, should passing a Self argument still be fine? The original motivation was about using it as the return type from what I see.

I personally think that all Self usage is potentially harmful in this context, but this is very opinionated