refine detail about what "old behavior" of as is.

[pnkfelix]

refine detail about what "old behavior" of as is.

[alexcrichton]

Looks good to me!

[bombless]

Seems that this doesn't cover the case when integer is casted to float.

[bombless]

cc rust-lang/rust#10185

[pnkfelix]

I think it was left implicit in the RFC text that when it spoke on the semantics of as, it was referring solely to the semantics of as with respect to int-type -> int-type conversions. It does not address using as to-or-from references, nor floats, nor unsafe pointers, etc.

I don't think this RFC text would be the right place to try to document the full semantics of as. The detail here was solely about int-type -> int-type casts, and that was just because the RFC as originally accepted had mandated that as would start dynamically checking the behavior of as, which was then rejected based on discussion here: http://internals.rust-lang.org/t/on-casts-and-checked-overflow/1710/