This type explicitly signals if an immediate value is to be returned, if a value is to be returned on the stack, or if no return value is to be generated. Holders of a `ReturnValue` can also use `and_then` to schedule a `StackContinuation` to be executed when and if that value is ready.
`StackContinuations` now yield `ReturnValues` as well, so they have a moderate level of composability. For example, if you need to get a property from an object and push it on the stack, you can return the result of calling `get` directly and the machinery ensures it eventually gets there.
This involved yet another macro, `and_then!`, to avoid a ridiculous amount of duplicate code. It calls a continuation whenever it's value is ready, even if the value resolved on the Rust stack.
`locals_into_form_values` does not currently support this. It skips any property that does not resolve on the Rust stack. Future work is required to resolve this.
This involves the use of a "stack continuation" system. Due to previous lifetime issues with using closures directly (see `8ea6c6234dba925ec5fbc61502627fb62b05916c`), we instead use a macro that constructs a `Collect`able type holding the things the continuation needs to continue working with. The syntax is largely similar to Rust closures but with the addition of an explicit list of bound variables, all of which must be `Collect`.
This pushes an extra `undefined` onto the stack to fix underflow in AS2 interface declarations.
It is currently unknown if this is a miscompilation or if some other value is supposed to be there.
# Conflicts:
# core/src/avm1.rs
# core/src/avm1/object.rs
Gotos now goto the specified frames immediately as opposed to
queuing. Actions on the new frame will still be queued,
and are executed after any current actions are completed.
This has the side effect of letting us remove the `Option` on register_count since setting this to `0` is equivalent now. Furthermore, we can skip an allocation if a function requests no registers.
David Wendt