clarify what `static` means

[thestinger]

Are these compile-time constants? If they are, we need to drop support for foreign constants. If they aren't, this code is going to need a rewrite because it makes the assumption that any static is a known constant.

See #9866 and #9909 for some of the issues caused by this being unspecified and not handled correctly.

[alexcrichton]

Nominating.

[catamorphism]

1.0 backcompat

[brson]

Is there anything to do here beyond significant addresses (#8958)?

[brson]

Nominating for closing. This doesn't appear actionable.

[pnkfelix]

Closing as nonactionable beyond #8958.