Variable assignment in loops in template

What problem does this feature solve?

Sometimes, we need to compute a variable for each iteration of a loop and use this varible for multiple times.
An example:

.SimpleGrid
  .row(v-for="(v, row_idx) in row_count_")
    .cell(v-for="(v, column_idx) in column_count")
      slot(v-if="items[row_idx * column_count + column_idx]" :item="items[row_idx * column_count + column_idx]")

Here, items[row_idx * column_count + column_idx] is repeated.

Note: the total cells may be more than the length of items, so v-if="items[...]" is needed.

So far, I've discovered only one hack, thanks to How to define variable in vue template?:

.SimpleGrid
  .row(v-for="(v, row_idx) in row_count_")
    .cell(v-for="(v, column_idx) in column_count")
      template(v-for="item in [ items[row_idx * column_count + column_idx] ]")
        slot(v-if="item" :item="item")

It works. But we still need a formal way instead of such hack.

What does the proposed API look like?

In most (if not all) cases, variables follow loop iterations. (Otherwise, we should use computed for the whole component.)

I suggest a new directive v-let besides v-for:

.SimpleGrid
  .row(v-for="(v, row_idx) in row_count_")
    .cell(v-for="(v, column_idx) in column_count" v-let="item = items[row_idx * column_count + column_idx]")
      slot(v-if="item" :item="item")

No implementation file, test, or entry point is named. Start by locating the template directive handling and existing v-for behavior, then determine how a formal v-let directive should fit the proposed loop-scoped assignment; done means the example works without the array-based workaround and has coverage for the new behavior.