When a rest parameter is referenced with a spread operator, the transformed code packs and unpacks it:

function foo(this: void, ...args: unknown[]) {
    console.log(...args);
}

=>

function foo(...)
    local args = ({...})
    print(unpack(args))
end

This PR optimizes this so that the elipsis operator is used instead. It also prevents generation of the temp local if it is not needed in the function.

Update
Due to issues with the optimization being applied inside generated IIFE's, this PR has been updated to instead provide an explicit @elipsisForward directive to apply this optimization.

/** @elipsisForward */
declare function elipsisForward<A extends unknown[]>(args?: A): A;

function foo(this: void, ...args: unknown[]) {
    console.log(...elipsisForward(args));
}

=>

function foo(...)
    print(...)
end

The new directive can only be used on a function, and that function can only be used in a spread expression, and passed a rest argument.

You can also pass no argument, which is useful for accessing the file-scope elipsis:

const arg = [...elipsisForward()];

=>

local arg = {...}