This commit updates the drop glue generated for closures to simply
ignore null pointers. The drop glue can be called in erroneous
situations such as when a closure is invoked after it's been destroyed.
In these cases we don't want to segfault and/or corrupt memory but
instead let the normal error message from the invoke glue continue to
get propagated.
Closes#1526
This is work towards #1399, although it's just for one-argument closures
where the first argument is a reference. No other closures with
references in argument position are supported yet
This commit improves the codegen for `Closure<T>`, primarily for ZST
where the closure doesn't actually capture anything. Previously
`wasm-bindgen` would unconditionally allocate an `Rc` for a fat pointer,
meaning that it would always hit the allocator even when the `Box<T>`
didn't actually contain an allocation. Now the reference count for the
closure is stored on the JS object rather than in Rust.
Some more advanced tests were added along the way to ensure that
functionality didn't regress, and otherwise the calling convention for
`Closure` changed a good deal but should still be the same user-facing.
The primary change was that the reference count reaching zero may cause
JS to need to run the destructor. It simply returns this information in
`Drop for Closure` and otherwise when calling it now also retains a
function pointer that runs the destructor.
Closes#874
