# Two-phase refactor — slice 3 (designators → AST)

Branch `ast-stage-c`.  Suite **178/178**; self-hosting fixpoint **OK**
(**2,970,007 bytes**; slice 2 was 2,966,245).

## What landed

`Design` now builds a designator node in addition to emitting, so real
expressions (identifiers, indexing, fields, derefs, qualified names)
have a meaningful AST, not just literals.

| Part | AST |
| --- | --- |
| base identifier | `NkDesignator` with child 0 = `NkIdent(name)` |
| `[ expr ]` / `, expr` | one `NkSelector` (`SelIndex`) per index, child 0 = the index expression |
| `. field` | `NkSelector` (`SelField`), child 0 = `NkIdent(field)` |
| `^` | `NkSelector` (`SelDeref`), no child |

`Design` keeps a per-invocation `astDes` local; each selector is appended
with `AST.SetChild(astDes, AST.NChild(astDes), …)`, and the production
ends with `astCur := astDes`.  `Fact`'s designator alternative captures
that node right after `Design` returns (`astD := astCur`) and, at the end
of the alternative, does

```modula2
IF astCall THEN astCur := AST.NoNode ELSE astCur := astD END;
astIsLit := TRUE
```

so a bare designator survives the `Fact` reset, while a brace literal or
a call (both marked `astCall := TRUE`) still yields `NoNode` for now.

## Verified (temporary `AST.Dump`, then removed)

- `a[1]` → `NkDesignator(Ident a, SelIndex(1))`
- `r.f` → `NkDesignator(Ident r, SelField(Ident f))`
- `p^` → `NkDesignator(Ident p, SelDeref)`
- `r.f + a[2]` → `Add(NkDesignator(r, SelField f), NkDesignator(a, SelIndex 2))`

Two bugs were caught this way:

1. `AST.AddChild` returns `BOOLEAN`; GNU Modula-2 refuses to discard a
   function result.  Switched to `AST.SetChild(n, AST.NChild(n), …)`.
2. The field name was first stored as a raw `TxtIndex` in
   `NkSelector.child[0]`.  Any tree walk (e.g. `AST.Dump`) then reads
   that integer as a *node index* and can recurse forever — `r.f` hung
   the dump.  Fixed by storing a real `NkIdent` node as the child.

## Deliberately deferred

Still `NoNode` in `Fact`: calls (`ArgList`), brace/set literals,
`ResultComp` suffixes, builds `CHR`/`ORD`/`ABS`/`HIGH`/`LEN`/…, `NOT`.
`Lower` + `.ssa` byte-compare still not started.

## Next

Slice 4: statements (`Block`/`Stat`) build `NkAssign`/`NkIf`/`NkWhile`/…,
then calls/braces; or first finish `Fact` (calls, builtins).  Statements
are the missing piece before a body can be lowered from the AST.