Revisiting `expr` follow-set restrictions after type ascription removal

Currently, macro_rules! enforces follow-set ambiguity restrictions that prevent patterns like $i:expr : ... (an expr fragment directly followed by a colon). This appears to stem from historical ambiguity with type ascription syntax (expr: Ty), which allowed type annotations in expressions.
https://doc.rust-lang.org/reference/macros-by-example.html#r-macro.decl.follow-set.token-expr-stmt

However, since type ascription was officially removed via De-RFC #2522, the grammar no longer permits a colon directly after an expression.

Given that the original ambiguity source (type ascription) no longer exists:

  • Could we relax the follow-set restrictions for expr fragments followed by : in macros?
  • Are there remaining ambiguities I might be overlooking?

This change would unblock macro patterns like:

macro_rules! my_macro {
    ($i:expr : $t:ty) => { /* ... */ };
}

Without reintroducing parsing conflicts. I'm curious if there's either:
a) A technical reason this restriction must remain
b) Consensus that this could be safely changed

Would appreciate insights from compiler/parser experts!