new DIP38: Safe references and rvalue references without runtime checks.

[Timothee Cour]

> BAM! Interprocedural analysis. Doesn't matter what name you invent for it.
> It's weird - you think you're in good shape, walking down the street, and
> suddenly you're in interprocedural analysis zone.

ok, call it interprocedural analysis, but what would be your arguments
against it, assuming:

A) I can convince (with code) that the propagation algorithm is simple
to implement and has negligible compile time overhead

B) it is safer than proposed approach in release builds (because
unittests in debug builds might not catch all such bugs, for example
things like min(ref int x, ref int y) which might be correct in some
code paths but not all)

C) it is faster in debug builds because no runtime checks