Help stabilize a subset of Macros 2.0!

[nikomatsakis]

@durka

Moreover, can you elaborate on the concerns that make it more palatable to stabilize proc macros in module-level item position than in any other position?

Well, certainly expanding to expressions means that the lack of hygiene becomes much more visible as a concern.

@durka

For example, if let’s say something like lazy_static! became a proc macro then I couldn’t use it to declare a function-local static, which is a break from the way other kinds of items work, and it means a user has to care what “kind” of macro it is, which is weird.

That's a good point. We might be able to accommodate expanding to items within a fn (as opposed to expanding to an expression) without undue concern over hygiene, since they don't share name resolution scopes.