The spec does not allow immutable arrays atm. When optimizing things for j2wasm we noticed that it seems like we could support them, if we have array.init which takes the values to initialize with, which is also being experimented with.

It seems like the rules for an immutable array could be like those of a struct, that is, subtypes would need to match it precisely on everything? Structs can at least add more fields, but not arrays, so this makes subtyping less useful in the array case, of course. Still, immutable arrays can be useful because of things like itables: An itable call does a read from an array that is in practice immutable, so if the toolchain and VM know it is immutable that can help.

Opening this issue for thoughts as we look into experimenting with it.