Suboptimal compilation for rest parameters

[a-bronx]

There is an TS/ES.next idiom for the JS arguments array: (...args: any[]). But using it may produce suboptimal transpilation result, for example:

function debug(...args: any[]) {
    if (!settings.debug) return;
    console.log("DEBUG:", ...args);
}

produces following JavaScript (ES5) code:

function debug() {
    var args = [];
    for (var _i = 0; _i < arguments.length; _i++) {
        args[_i] = arguments[_i];
    }
    if (!settings.debug)
        return;
    console.log.apply(console, ["DEBUG:"].concat(args));
}

Note that the arguments is copied to the args array immediately at the beginning of the function, before its real usage. This causes the code executed every time the debug() function is called, even if it is unnecessary.

Expected behavior would be something more intelligent: either moving the arguments unrolling code closer to the usage place (may be tricky if there are multiple usage points), or using Array.prototype.slice.call(arguments) in-place without copying it into the temporary args.

Compare with Babel's transpilation result:

function debug() {
    var _console;

    if (!settings.debug) return;

    for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
        args[_key] = arguments[_key];
    }

    (_console = console).log.apply(_console, ["DEBUG"].concat(args));
}

Note that here the copying was moved past the conditional expression and happens only when needed.

[RyanCavanaugh]

Accepting PRs for the following optimizations (doesn't need to have all of them):

• Move the array copy emit to the first top-level statement which is before any statement which references it
• Elide the array copy emit if it's never referenced
• Elide the array copy emit if all accesses of the rest arg are of the form x[k] where k is a numeric literal, and replace k with k + n (where n is the correct offset)

[charlespierce]

@Kingwl Are you still working on this issue? I'd like to take a swing at implementing some of the optimizations but don't want to duplicate work.

[Kingwl]

@charlespierce No, I'm not on it, please feel free