Interesting. I assumed it was merely never implemented. But from looking at that, it appears that this was just done as a workaround for an issue encoding/decoding metadata. That could presumably be fixed at some point. I would expect that if it's decided that it should never get fixed, then the attribute AST representation should change to only allow a string instead of any literal.

FWIW, I could theoretically write a syntax extension that expands to the equivalent of `#[default=3]`, because that's representable at the AST level.