Should structs be freezeable?

[takikawa]

Something that came up when I was working on a spec draft is what the interaction between freezing and structs should be. Normally sealed objects can still be frozen afterwards.

However, perhaps struct instances should not be freezable? I believe JS engines may need to modify flags, etc. in the hidden class on Object.freeze (e.g., https://searchfox.org/mozilla-central/source/js/src/vm/Shape.cpp#776), and struct hidden classes are intended to be fixed.

Additionally, in the shared struct case, there is a possibility of one thread freezing an object concurrent with mutations occurring in another thread. Would this be a problem?

[rbuckton]

Supporting Object.freeze could be problematic since it could be racy, but then again shared structs might be represented by an exotic object with a [[Set]] that could handle the race, similar to an SAB-backed TypedArray.