Sub-phase 2b checkpoint 1 — adds the foundation for SQL SELECT
lexical-scope discipline without changing existing walker
semantics.
New types in `dsl::walker::context`:
- `TableBinding` — one FROM-source binding with table name,
optional alias, and schema-resolved columns (§10.1).
- `CteBinding` + `CteColumn` — a CTE definition visible from
inside its body (WITH RECURSIVE self-reference) and from the
outer scope after harvest (§10.3).
- `ScopeFrame` — `from_scope`, `cte_bindings`, and
`projection_aliases` for one lexical scope. Default-empty;
the fields will be populated by later 2b checkpoints.
`WalkContext` gains `from_scope_stack: Vec<ScopeFrame>`,
initialised with one bottom frame in both `new()` and
`with_schema()`. The bottom frame is the implicit top-level
scope DSL paths and top-level SQL statements operate in;
`Node::ScopedSubgrammar` entries push and pop additional frames
on top. `current_table` / `current_table_columns` remain as
direct fields for this checkpoint — converting them to derived
helpers is a later 2b step.
New grammar-tree variant:
- `Node::ScopedSubgrammar(&'static Self)` — like `Subgrammar`,
but pushes a fresh `ScopeFrame` on entry and pops it on exit
(ADR-0032 §10.2). Shares `subgrammar_depth` with the plain
Subgrammar variant so the MAX_SUBGRAMMAR_DEPTH = 64 cap fires
uniformly across both — §9's "no new walker capability for
grammar recursion" claim holds. DSL Expr (ADR-0026) and
sql_expr.rs ladder (ADR-0031) recursion continue to use the
plain Subgrammar variant and never push a scope.
Driver gains a parallel `walk_scoped_subgrammar` arm; the
push/pop is unconditional so a speculatively-walked branch a
later Choice rolls back leaves the stack clean.
Test coverage in `driver.rs`:
- A recursive ScopedSubgrammar test grammar walks correctly
through depths 0-3.
- The depth cap fires the same `expression_too_deep` friendly
validation error as for plain Subgrammar.
- The bottom frame invariant: `WalkContext::new` seeds exactly
one frame, and after a walk the stack is restored.
No grammar tree references the new variant yet — the rewire of
sql_select.rs CTE bodies and the sql_expr.rs additive
extensions for §5/§6 are the next 2b checkpoint. Test totals:
1330 baseline + 3 = 1333 passing, 0 failed, 1 ignored. Clippy
clean.
claude@clouddev1
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