Decide what we want about `macro_metavar_expr`

[traviscross]

The author of the stabilization PR for macro_metavar_expr has apparently lost interest in further pursuing it:

• Stabilize count, ignore, index, and len (macro_metavar_expr) #122808

We've reopened it for now, but probably it'll need a new PR regardless. To clear the way for that, probably we should decide what it is that we want to see here. Let's use this issue to track our answering of that.

@rustbot labels +T-lang +C-discussion

cc @rust-lang/lang @rust-lang/wg-macros

[tgross35]

Would it be possible to consider rust-lang/rfcs#3649 as an alternative here? It helps resolve the confusing numeric indexing, and it eliminates the need for having both of the similar count and len operators. (Also ignore is unneeded).

The reference-level explanation also spells out a more concrete set of rules for how metavariables need to behave in less straightforward cases. I haven't seen something similar for metavar expressions in their current state but it would probably help to draw that out. (I do address this to some extent in "Changes from current implementation" bullets).

Note that the RFC is still not completely polished yet, but I believe it is thorough enough to be discussed.