Strictly speaking, Isn't there still a way to express at least one Illegal string in ArchivedString? I'm not sure how to hint to the Rust compiler which values are illegal, but if the inline length (at most 15 characers) is aliased to the pointer string length (assume little-endian), wouldnt {ptr: null, len: 16} and {inline_data: {0...}, len: 16} both technically be an illegal value?

I'm not saying this is better than your solution, just curious :^)