Completes the vocabulary migration started with `BNode`: every child READ in
the cgen files now goes through an accessor that a `.bif` `Cursor` can also
serve, so flipping `newIcBackend` is a matter of implementing the vocabulary
rather than rewriting call sites.
* constant indices -> `firstSon` / `secondSon` / `lastSon` / `son(n, k)`,
including the `namePos`/`paramsPos`/`bodyPos`/... slot reads;
* indexed loops -> `sons` / `sonsFrom` / `sonsButLast` and the index-yielding
`isons` / `isonsButLast`. `for i in 0..<n.len: n[i]` is quadratic once
`BNode` is a `Cursor`, because reaching child `i` costs one `skip` per
preceding SUBTREE;
* `n.len == 0` / `> 0` on a node -> `hasSons`, which does not count.
`astdef` gains `sonsButLast(n, count)` and `isonsButLast` — the
`nkOfBranch`/`nkExceptBranch` shape, whose last child is the branch body, and
with `count = 2` the `nkVarTuple`/`nkIdentDefs` shape.
Three places needed more than a rename:
* the C++/goto/setjmp try generators re-subscripted `t[i]` up to ten times
per iteration of their `while i < t.len` walk; the branch node is now
hoisted once per step;
* `genParams` scans the arguments BACKWARDS to decide which need a temporary,
which a `Cursor` cannot do at all. It materializes them in one forward pass
and indexes that — the same order of work, since `needTmp` already
allocates per call;
* loops that stop at a computed position (`casePos`, `until`, `splitPoint`)
walk forward and break instead of counting up to the bound.
What is left is exactly what a `Cursor` backend will not do: writes that build
a fresh `nkProcDef`, and subscripts of a `PType`, `string`, `seq` or `Table`.
`bnode.nim` records the invariant and the `PType` trap — `ast.sons(t: PType)`
is a proc returning `var TTypeSeq`, not the iterator of the same name — which
the type checker enforces, since the `firstSon`/`secondSon`/`lastSon`/`son`
family exists for `PNode` only.
Pure refactor, verified as one: all 216 generated `.c` files byte-identical to
the parent commit; metamorphic IC 16/16; icSuite 19/19 fragments; categories
gc 78, arc 140, destructor 97, closure 23, iter 71, trmacros 6, cpp 50,
exception 47, casestmt 16 with one pre-existing environmental failure
(tests/cpp/tasync_cpp.nim: `cannot open file: jester`).
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FMyRHByv7hhaQJ4Pa1bHbE
9e076d74c0