I always enjoy this one: https://crates.io/crates/inline-python

Apparently, Python's syntax is almost entirely a subset of Rust's syntax + a few keywords, so even though you have to pass valid Rust tokens into a macro, that's good enough to write Python with just a few quirks.

That's inside the macro though. Whatever you parse inside the macro is defined by the creator of the macro. The macro invocation itself is defined by the rust language.

Rust allows you to use any bracket because it doesn't know what the macro is supposed to do.

Some macros work like functions:

println!("Hello world");

Some macros work like array declarations:

let v = vec![1,2,3];

And some macros work like a code scope:

select!{
 a => myfunc(),
 b => my_other_func()
}

Since rust doesn't know what your macro does, it just lets the user choose whatever the user sees more fitting for that specific macro invocation.

Yeah, I always wonder, if it was an intentional decision or just some first draft that stayed in.

Because I do think, it's kind of smart. Some macros make sense to call with either () or {}, depending on if you pass a code snippet that's multi-line or not.
So, you could come up with a complicated syntax to define which bracket types are acceptable, or you could just not bother. 🙃