`for i in k..<n.len: ... n[i] ...` is the dominant shape for walking a `PNode`'s
children in the code generator: 41 such loops across the cgen files, and 777
indexed node accesses in total. It reads worse than iterating, it bounds-checks
every subscript, and it is quadratic the moment the backend reads children off a
NIF `Cursor` rather than a materialised tree (a child is `firstSon` plus one
`skip` per preceding sibling, and `skip` steps over a whole subtree).
31 of the 41 are converted:
* 20 to `sons`/`sonsFrom` — the index only ever subscripted `n`.
* 9 to `isons`, which now takes a `start` index (defaulting to 0, so its eight
existing call sites are unchanged). These genuinely need `i`: a parallel index
into the routine's `PType` (`typ.n[i]`, `typ[i]`), a `needTmp[i-1]` lookup, an
`i == field.position` test, `$i` in a generated struct name, or the index
passed straight to `genOtherArg`.
* 2 to `sonsFrom` with a variable start (`firstParam`, `offset`).
`sonsFrom` is new, next to `sons`/`isons` in astdef.
The remaining 10 are deliberate. Eight are not `PNode` at all — `varargs[Snippet]`,
`seq[PSym]`, `string`, and `PType`, where `sons` is a `proc ...: var TTypeSeq`
rather than an iterator, so a blind rewrite would compile into something quite
different. Two iterate `0..<it.len-1`, excluding the last child, which no
iterator expresses cleanly.
Pure refactor, and verified as one: all 219 generated `.c` files of a 219-module
program and the linked binary are byte-identical to the parent commit. That is
the bar that matters here, because the index arithmetic (`i-1`, `i == position`,
`$i`) is the easy thing to get wrong. It also caught a real slip on the way:
`genFieldCheck` reassigns its loop variable, which a `for` binding cannot do.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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