make proc not implicitly convert to pointer with a preview define

[metagn]

closes nim-lang/RFCs#229

Preview define for now because a package broke.

There is also cstring to pointer conversion which was not removed in #20761 or mentioned in #13790, however this is also documented so I didn't touch it for now.

[Araq]

-1 from me, we allow var p: pointer = addr(x) too, so we should allow var p: pointer = someProc.

[juancarlospaco]

Maybe it can be an optional Hint or Warning ?. 🙂

[Araq]

Why? Nobody reported any bugs due to this behavior.

[metagn]

The problem mentioned in nim-lang/RFCs#229 is that the conversion can happen implicitly in proc parameters.

# defined in system:
proc `==`(a, b: pointer)
proc `==`[T: proc](a, b: proc)

proc foo(a: int) = discard
proc bar(a: float) = discard

echo foo == bar # does pointer(foo) == pointer(bar)

This conversion is safe but maybe not always wanted. I don't know which mechanism makes it so this kind of "implicit" conversion doesn't work but var p: pointer does (don't know if this makes sense either). ptr is different because pointer is basically ptr any.

[github-actions]

Thanks for your hard work on this PR!
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