# Plan — ADR-0035 Phase-4 `/runda` follow-ups F1 / F2 / F3 (2026-05-26)

Bundle of the three error-message / capability follow-ups surfaced by the
whole-Phase-4 `/runda` (handoff-42 §3). All three live on the **safe**
composite-UNIQUE edge (dropping a UNIQUE-covered column is correctly
*refused* today — no corruption); the work improves messaging and adds a
way to drop the constraint itself.

## Phase 1 — requirements

- **F1 — friendly refusal for dropping a composite-UNIQUE column.**
  `do_drop_column`'s covering-index guard reads `read_table_indexes`,
  which filters to `origin='c'` (explicit `CREATE INDEX`) and excludes
  the UNIQUE-constraint auto-index (`origin='u'`). So `drop column c`
  when `unique (b, c)` spans `c` skips the guard, reaches the engine, and
  is refused with an unhelpful generic message. Add an up-front guard
  detecting the column in `schema.unique_constraints` (composite only —
  `read_unique_constraints` routes single-column UNIQUEs to the column
  flag, multi-column to `unique_constraints`), refusing with the
  constraint's **derived name** (F3) + the drop command. Behaviour stays
  "refused"; only the message improves. **Message-only — no `--cascade`
  extension** (the SQL drop-column has no `--cascade` spelling; dropping
  a constraint via cascade is a larger semantic change, out of scope
  unless the user asks).

- **F2 — literal `{table}` leak in contextless `friendly_message()`.**
  `Verbosity` defaults to `Verbose`, so `friendly_message()` (which uses
  `TranslateContext::default()`, no table) renders the generic hint
  `"…current state of `{table}`."` with the literal placeholder via
  `ctx_table()`'s `"{table}"` fallback. Hits every contextless
  `friendly_message()` callsite whose error lands in the **generic
  bucket**: replay, undo, rebuild-from-text, export. Fix: a tableless
  generic-hint variant selected when `ctx.table` is `None`. **Broader
  finding** (DA): the same `{name}`-marker fallbacks leak in *other*
  templates (e.g. a replayed UNIQUE violation → `error.unique.*`) when
  reached contextless. The documented F2 is the generic case; the broader
  leak is surfaced for the user to scope, not silently expanded/narrowed.

- **F3 — a way to drop an anonymous composite UNIQUE (user-raised).**
  By design (§4a.2/§4g) a composite `UNIQUE(a,b)` is anonymous —
  PRAGMA-detected, a bare column-list, no name — so `DROP CONSTRAINT
  <name>` can't target it and recreating the table is the only escape.
  Add a way to drop it. **(Amends ADR-0035 — see Amendment 1.)**

Baseline: `cargo test` → 1917 pass / 0 fail / 0 skip / 1 ignored doctest;
`cargo clippy --all-targets -- -D warnings` clean.

## Phase 2/3 — F3 design (the genuine fork; user-decided)

Composite UNIQUE has no name. Options considered:

- **A — name composite UNIQUEs (user-supplied):** reverse the §4g
  anonymity decision; needs a new `__rdbms_*` table + YAML round-trip +
  rebuild-arrival migration (the cost §4a.3 deliberately avoided). Most
  SQL-standard, largest.
- **B — positional drop by column-list** (`drop … unique (cols)`):
  preserves anonymity, no metadata, but needs a *new* grammar form.
- **C — auto-assigned, engine-neutral *derived* name (chosen).** The
  name is a deterministic function of the columns (`unique_<cols>`),
  recomputed live wherever shown or matched. Storage stays a bare
  column-list (anonymity preserved); the name is purely a
  presentation/addressing label. **Reuses the existing `DROP CONSTRAINT
  <name>` grammar — no new syntax at all.** Zero metadata, zero
  migration, round-trips for free. Tracks column renames.

**User decisions (2026-05-26):** approach **C / derived (no storage)**;
name format **`unique_<cols>`**; doc vehicle **amend ADR-0035**; scope
**advanced-SQL only** (matching the 4g `ADD` form — no simple-mode verb).

### DA critique (written down)

1. **Ambiguous derived names** (e.g. a column literally named `b_c` vs
   `UNIQUE (b, c)`): drop-by-name must **detect ambiguity and refuse**,
   never guess. *In scope.*
2. **Collision with a user-named CHECK/FK** of the same string: the
   `do_drop_constraint_by_name` order is CHECK → FK → UNIQUE, so a
   CHECK/FK shadows a derived UNIQUE name. Acceptable given the
   distinctive `unique_` prefix; **document the order**.
3. **F1 `--cascade`**: not extended to drop a covering UNIQUE
   (constraint, not index). Refuse-only. *Flagged.*
4. **F2 breadth**: the leak is broader than `error.generic.hint`. Fix the
   documented generic case; **surface** the broader leak. *Flagged.*
5. **Single-column UNIQUE column drop**: a parallel gap (a single-column
   UNIQUE column drop also reaches the engine with a poor message) exists
   but is **outside the documented F1 scope** (different mechanism —
   ADR-0029 column-level `drop constraint`). Noted, not fixed here.

## Phase 4 — execution (order: F3 → F1 → F2)

1. **F3.** `unique_constraint_name(cols) -> "unique_<cols>"` helper
   (`db.rs`, `pub(crate)`). Extend `do_drop_constraint_by_name` with a
   third step: match each composite UNIQUE's derived name; >1 match →
   refuse (ambiguous); 1 match → `rebuild_table` with that entry removed
   from `unique_constraints` (mirrors `do_alter_add_unique` in reverse) +
   `do_describe_table`. Annotate the describe "Table constraints:"
   section: `unique_b_c: UNIQUE (b, c)`.
2. **F1.** Up-front guard in `do_drop_column` after the index-covering
   guard: column in any `schema.unique_constraints` entry → refuse with
   the derived name + `alter table T drop constraint <name>`.
3. **F2.** `error.generic.hint_no_table` catalog entry; in
   `translate_generic`, pick it when `ctx.table` is `None`.

Test-first for each (reproduce → fail → fix → pass), across the worker
API (Tier-1/3) and the friendly-layer unit tests + insta snapshots.

## Phase 5 — verification

Full `cargo test` + clippy; compare to baseline; every checklist item
addressed; engine-neutral vocab held (no SQLite/STRICT/PRAGMA in new
user-facing strings); ADR + README + this plan lockstep.