document constant expressions in rust.md reference manual

[pnkfelix]

The section on "static items" (formerly entitled "constants") refers to some class of "constant expressions", but we have not defined what these are.

(I think catamorphism has noted this via comments in a few issues I have seen, but I have not seen an actual Issue for this topic; so I am promoting it to a full fledged issue.)

In particular, if we need a subgrammar, or some other sort of static analysis to classify the constant expressions correctly, then we should document it.

[pnkfelix]

For example, should constant expressions include the production
const-expr ::= (const-expr as type)
as desired by issue #4063

[catamorphism]

Visiting for triage. Still important.

[pnkfelix]

Another question, should constant expressions include the production
const-expr ::= const-expr . id
for field access on static constant struct values? (We currently explicitly fail to const-eval such expressions; but it seems like a potentially desirable feature; see e.g. #9762)

[nikomatsakis]

I just want to point to this comment of mine which describes some of the impl challenges for intertwining const expr evaluation and type checking

[brson]

Nominating for removal from milestone.

[pnkfelix]

@brson you think we can release 1.0 without a formal definition for the constant expression sublanguage?