In macros, expr followed by { is buggy

[Stebalien]

The following compiles:

macro_rules! test {
    ($e:expr {}) => {$e};
}

However, (1) this technically isn't allowed by the macro future proofing rules and (2) it doesn't work when $e is an ident because rust tries to parse ident {} as an invalid struct literal instantiation.

The following works:

test! {
    1 + 1 { } // 1 + 1 can be anything other than an ident.
};

And the following doesn't:

test! {
        ident { }
};

Error:

tmp.rs:10:15: 10:16 error: structure literal must either have at least one field or use functional structure update syntax
tmp.rs:10         ident { }
                        ^
tmp.rs:10:17: 10:18 error: unexpected end of macro invocation
tmp.rs:10         ident { }
                          ^

[nikomatsakis]

triage: P-high (decision needed)

For this and other macro rules, it's unclear how much we can change here, but I think we should make an effort to fix these bugs sooner rather than later if we are going to do so!

[pnkfelix]

This was fixed by #30694

[pnkfelix]

(though i will note yet again that there may be motivation for adding a new syntactic class to macro rules, which I have been referring to tongue-in-cheek as $e:moelarry, but obviously we'd want to bikeshed a better name before we'd actually put it in for real...)