Disclaimer: I haven’t tested this yet, not slacking off, code is compiling.
Pre this update, comparing empty structs (mutable or not) is a NOP if types are inferred (it is literally free and the answer is, they are equal if and only if they have the same type). Post this update, comparing empty immutable structs is still free, and comparing empty mutable structs is exactly as expensive as comparing a pointer or an Int64.
You can maybe expect a speedup for comparison over e.g. 256 bit nonces if you are very paranoid about collisions of random numbers.
Pre this update, mutable empty structs made no sense at all (immutable empty structs were always better) and were a historical leftover. Post this update, mutable empty structs now serve a function different from immutable empty structs, and there was nothing before serving this new function (nonce). GC controlled and finalizable nonces are kinda niche, but this update looks “more coherent” in the sense that default equality for user-defined mutable objects never looks at the fields and only compares pointers; except for the former special case of mutable empty structs.
If your code on 0.6 or 0.7 used mutable empty structs before this update then your code was sub-optimal (could be improved by switching to the semantically equivalent but sometimes more performant immutable variant, because the old mutable empty struct infected composites with non-
bitstype-ness for no conceivable upside).
Note that the new mutable empty struct allocates.