Add allow omit rule =

[A4-Tacks]

This PR makes = ... optional for easy completion

rule x() -> cannot trigger completion, rule x() -> = can

[kevinmehall]

By "completion" do you mean allowing the macro expansion to generate enough output so that rust-analyzer can provide suggestions for the return type?

This would allow mistakes to be successfully compiled. Instead of fully allowing this, would it work to expand it to something like rule x() -> ty = { compile_error!("missing rule body") } such that rustc won't compile it, but rust-analyzer can still see the ty you've started typing?

[A4-Tacks]

Good idea!