Non-terminal divergence between parser and macro matcher

[pnkfelix]

Summary

Spawned off of rust-lang/rust#86730 (which in turn was spawned off of rust-lang/rust#84364 and previous lang team meetings)

The meaning of non-terminals such as "expr" according to the rustc parser have changed (or would like to change) from its meaning according to the current rustc macro matcher. Each time this happens, we have to decide what to do about it (e.g. whether to add a new macro fragment specifier, such as expr202x).

The main topic to discuss is what to do about expr itself, in the short term. (There is a side-topic of how we should try to manage future changes on this front. Answers here might be a broad as changing the macro system itself, but that in turn means it may need its own design meeting...)

Background reading

pnkfelix plans to produce a document that folks will be able to consume at the meeting itself.

The main background material available now is linked from rust-lang/rust#86730

About this issue

This issue corresponds to a lang-team design meeting proposal. It corresponds
to a possible topic of discussion that may be scheduled for deeper discussion
during one of our design meetings.

[pnkfelix]

meeting scheduled for 2021-08-25

[pnkfelix]

meeting was cancelled, so removing "scheduled" tag, since we may still want to have it.

[nikomatsakis]

I don't think we're doing this meeting :)