DRY await destructure and each destructure code

[tanhauhau]

Reference: #4548 (comment)

[Conduitry]

To answer your question on that PR, yeah I'd definitely be in favor of switching each block destructuring over to your new code that can handle default values.

[tanhauhau]

@Conduitry yea i understand about the code part. i will use the each block destructuring code, but how about the behavior? should we have a default value in the destructuring?

[Conduitry]

Oh I may have misunderstood your other comment. I thought your spread implementation for awaits already supported default values. If it doesn't, I don't think we need to get that in as part of this issue, because it's apparently not a highly demanded feature as it's never come up as far as I know.

[tanhauhau]

oh i think i misunderstood you. the await destructure supports default value, but the each destructure does not. so, i was asking of whether to support default value on each destructure too...

...and i noticed then in the generated code, each destructure will "transpile" to assignment statements:

function get_each_context(ctx) {
   const child_ctx = ctx.slice();
   child_ctx[2] = ctx[1].foo
   child_ctx[3] = ctx[1].bar
   child_ctx[4] = rest(ctx[1], ['foo', 'bar'])
   return child_ctx
}

whereas the await destructure, i was using object destructure:

function get_each_context(ctx) {
   const child_ctx = ctx.slice();
   ({ foo: child_ctx[2], bar: child_ctx[3], ...child_ctx[4] } = ctx[1]);
   return child_ctx
}

is there any concern or preference for either?

according to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment,

array destructuring rest is still not supported for safari, is that why?