These were originally a SpiderMonkey-ism and have been unused ever since. It was introduced for GC integration, where the runtime could do something to make Cranelift code hit a trap and pause for a GC and then resume execution once GC completed. But it is unclear that, as implemented, this is actually a useful mechanism for doing that (compared to, say, loading from some Well Known page and the GC protecting that page and catching signals to interrupt the mutator, or simply branching and doing a libcall). And if someone has that particular use case in the future (Wasmtime and its GC integration doesn't need exactly this) then we can design something for what is actually needed at that time, instead of carrying this cruft forward forever.
These were originally a SpiderMonkey-ism and have been unused ever since. It was introduced for GC integration, where the runtime could do something to make Cranelift code hit a trap and pause for a GC and then resume execution once GC completed. But it is unclear that, as implemented, this is actually a useful mechanism for doing that (compared to, say, loading from some Well Known page and the GC protecting that page and catching signals to interrupt the mutator, or simply branching and doing a libcall). And if someone has that particular use case in the future (Wasmtime and its GC integration doesn't need exactly this) then we can design something for what is actually needed at that time, instead of carrying this cruft forward forever.