feat: ADR-0035 Amendment 1 — drop composite UNIQUE; friendlier drop-column + generic-error wording

F1/F2/F3 from the whole-Phase-4 /runda (handoff-42 §3): - F3: drop an anonymous composite UNIQUE via a derived, engine-neutral name `unique_<cols>` — recomputed live, nothing persisted, reusing the existing `DROP CONSTRAINT <name>` grammar (no new syntax/metadata, the §4g anonymity decision intact). A name matching more than one UNIQUE is refused as ambiguous, never guessed. One undo step. `describe` annotates each composite UNIQUE with its name. - F1: dropping a column a composite UNIQUE covers is refused up-front with the derived name + the actionable drop command (was an unhelpful generic engine refusal). - F2: contextless friendly_message() no longer leaks a literal `{table}` in the generic hint (new `error.generic.hint_no_table`, selected when no table is in context). The table-ful path is unchanged. Docs: ADR-0035 Amendment 1 + Status + README index + plan docs/plans/20260526-adr-0035-composite-unique-drop-f1f2f3.md. Tests: +5 (drop-by-name, ambiguous-refused, one-undo-step, F1 guard, F2 no-leak) + a describe-render assertion. 1922 pass / 0 fail / 0 skip; clippy clean.
2026-05-26 16:20:08 +00:00
parent 60d30dd54e
commit cb8ff8a7c2
10 changed files with 475 additions and 6 deletions
@@ -17,7 +17,11 @@ the CREATE-TABLE help/usage refresh — implemented 2026-05-25/26 — plans
 `…-4e.md`, `…-4f.md`, `…-4g.md`,
 `docs/plans/20260526-adr-0035-sql-ddl-4h.md`,
 `docs/plans/20260526-adr-0035-sql-ddl-4i.md`). **Phase 4 is complete**
-(4a–4i all shipped). This is **Phase 4** of the ADR-0030 roadmap (the
+(4a–4i all shipped). **Amendment 1 (2026-05-26)** adds a way to **drop a
+composite UNIQUE** via a derived, engine-neutral name (`unique_<cols>`)
+that reuses the existing `DROP CONSTRAINT <name>` grammar — no new
+syntax, no metadata, the §4g anonymity decision intact (see the amendment
+below). This is **Phase 4** of the ADR-0030 roadmap (the
 advanced-mode SQL surface), the peer of ADR-0031 (expression grammar),
 ADR-0032 (`SELECT`), and ADR-0033 (DML). It **clarifies ADR-0030 §4**
 on how DDL is represented and executed.
@@ -551,6 +555,67 @@ ADR-0033's structure:
    read-side (completion / diagnostics / describe / help), so no undo
    steps are introduced.

+## Amendment 1 — Dropping a composite UNIQUE constraint (2026-05-26)
+
+A whole-Phase-4 `/runda` surfaced that a composite `UNIQUE(a,b)` — kept
+**anonymous** by design (§4a.2, and §4g refused a *named* UNIQUE add) —
+has **no way to be dropped**: `DROP CONSTRAINT <name>` (§4g) resolves
+only a named table-CHECK or a named FK, so recreating the table was the
+only escape. This amendment adds a drop path. Written with explicit user
+approval; the plan is
+`docs/plans/20260526-adr-0035-composite-unique-drop-f1f2f3.md`.
+**Implemented 2026-05-26.**
+
+**It does not reverse the §4g anonymity decision.** Storage stays a bare
+column-list (`unique_constraints: Vec<Vec<String>>`, PRAGMA-detected) and
+a UNIQUE still **cannot be named on `ADD`**. The addition is purely a
+**derived, engine-neutral name** used to *display* and *address* the
+constraint on drop.
+
+### The derived name (user-decided: derived, no storage; `unique_<cols>`)
+
+The name is a deterministic function of the column list —
+`unique_<col1>_<col2>…` — recomputed live wherever it is shown or
+matched. Nothing is persisted: the constraint remains a bare column-list,
+so the name round-trips for free and needs no metadata table and no
+rebuild-arrival migration (the cost §4a.3 deliberately avoided). If a
+column in the UNIQUE is later renamed, the displayed name tracks it —
+arguably more correct than a frozen stored name. Alternatives weighed and
+rejected: naming UNIQUEs with a user-supplied name + new metadata table
+(reverses §4g; heaviest), and a positional `drop … unique (cols)` form
+(needs new grammar). The derived name **reuses the existing `DROP
+CONSTRAINT <name>` grammar — no new syntax**.
+
+### Surfaces
+
+- **`describe` / structure view.** The "Table constraints:" section (4i b)
+  annotates each composite UNIQUE with its name: `unique_b_c: UNIQUE
+  (b, c)`.
+- **`ALTER TABLE <T> DROP CONSTRAINT <name>`** (advanced-SQL only, matching
+  the §4g `ADD` form). `do_drop_constraint_by_name` gains a **third
+  resolution step** after named table-CHECK and named FK: it recomputes
+  the derived name of each composite UNIQUE on `<T>` and matches. On a
+  single match it rebuilds the table without that entry (the
+  `do_alter_add_unique` rebuild in reverse). **A name matching more than
+  one UNIQUE is refused as ambiguous** (e.g. a column literally named
+  `b_c` colliding with `UNIQUE (b, c)`) — it never guesses which to drop.
+  **Resolution order** means a user-named CHECK/FK with the same string
+  shadows a derived UNIQUE name; the distinctive `unique_` prefix makes
+  this unlikely and it is documented, not guarded.
+
+### Dropping a *column* a composite UNIQUE covers (F1)
+
+`do_drop_column` gains an up-front guard (alongside the index-covering
+and CHECK guards): a column participating in any composite UNIQUE is
+refused with the constraint's derived name and the actionable drop
+command — `cannot drop \`T.c\` … part of the UNIQUE constraint
+\`unique_b_c\` (b, c); drop that constraint first (\`alter table T drop
+constraint unique_b_c\`)`. The refusal itself is unchanged (the engine
+already refuses it); the message becomes engine-neutral and actionable.
+Single-column UNIQUE column drops are a **parallel** gap (different
+mechanism — ADR-0029 column-level `drop constraint`) and are **out of
+scope** here.
+
 ## Consequences

 - Advanced mode reaches DDL parity with simple mode and adds
@@ -0,0 +1,110 @@
+# 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.
@@ -4359,6 +4359,26 @@ fn do_drop_column(
        )));
    }

+    // A composite UNIQUE covering this column (ADR-0035 Amendment 1): the
+    // engine refuses to drop a column a UNIQUE constraint spans, so refuse
+    // up-front with the constraint's derived name and the actionable drop
+    // command. Single-column UNIQUEs ride on the column `unique` flag (the
+    // engine drops their auto-index with the column), not
+    // `unique_constraints`, so they do not reach here.
+    if let Some(cols) = schema
+        .unique_constraints
+        .iter()
+        .find(|cols| cols.iter().any(|c| c == column))
+    {
+        let cname = unique_constraint_name(cols);
+        return Err(DbError::Unsupported(format!(
+            "cannot drop `{table}.{column}` — it is part of the UNIQUE \
+             constraint `{cname}` ({}); drop that constraint first \
+             (`alter table {table} drop constraint {cname}`).",
+            cols.join(", "),
+        )));
+    }
+
    // A CHECK (table-level, or a *different* column's column-level CHECK)
    // that references this column (ADR-0035 §4e, the 4a.3 deferral): a
    // deliberate up-front refusal — dropping the column would break that
@@ -6121,6 +6141,16 @@ fn read_unique_constraints(
    Ok((single, composite))
 }

+/// Engine-neutral display/address name for an anonymous composite UNIQUE
+/// constraint (ADR-0035 Amendment 1): `unique_<col1>_<col2>…`. A pure
+/// function of the column list — recomputed wherever the name is shown
+/// (`describe`) or matched (`ALTER TABLE … DROP CONSTRAINT <name>`), so
+/// nothing is persisted; the constraint stays a bare column-list in our
+/// model and the §4g anonymity decision is intact.
+pub(crate) fn unique_constraint_name(cols: &[String]) -> String {
+    format!("unique_{}", cols.join("_"))
+}
+
 /// Generate the CREATE TABLE DDL from a `ReadSchema`. Used during
 /// the rebuild dance.
 fn schema_to_ddl(table: &str, schema: &ReadSchema) -> String {
@@ -7073,7 +7103,46 @@ fn do_drop_constraint_by_name(
        );
    }

-    // 3. Not a known named constraint on this table.
+    // 3. A composite UNIQUE whose derived name (ADR-0035 Amendment 1,
+    //    `unique_<cols>`) matches? The constraint is anonymous in our
+    //    model, so we recompute each composite UNIQUE's name and match.
+    //    Order matters: a named CHECK/FK above shadows a derived UNIQUE
+    //    name (the distinctive `unique_` prefix makes a clash unlikely).
+    let schema = read_schema(conn, table)?;
+    let matched_cols: Vec<Vec<String>> = schema
+        .unique_constraints
+        .iter()
+        .filter(|cols| unique_constraint_name(cols) == name)
+        .cloned()
+        .collect();
+    if matched_cols.len() > 1 {
+        // Two distinct UNIQUEs can derive the same name (e.g. a column
+        // literally named `b_c` vs `UNIQUE (b, c)`). Refuse rather than
+        // guess which to drop.
+        return Err(DbError::Unsupported(format!(
+            "the constraint name `{name}` is ambiguous on `{table}` — it \
+             matches more than one UNIQUE constraint; recreate the table \
+             to change them."
+        )));
+    }
+    if let Some(cols) = matched_cols.first() {
+        let old_schema = schema.clone();
+        let mut new_schema = schema;
+        new_schema.unique_constraints.retain(|c| c != cols);
+        let table_owned = table.to_string();
+        rebuild_table(conn, table, &old_schema, &new_schema, |tx| {
+            let changes = Changes {
+                schema_dirty: true,
+                rewritten_tables: vec![table_owned.clone()],
+                ..Changes::default()
+            };
+            finalize_persistence(tx, persistence, source, &changes)?;
+            Ok(())
+        })?;
+        return Ok(Some(do_describe_table(conn, table)?));
+    }
+
+    // 4. Not a known named constraint on this table.
    Err(DbError::Sqlite {
        message: format!("no such constraint: {name} on {table}"),
        kind: SqliteErrorKind::Other,
@@ -109,6 +109,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
    // ---- Generic engine refusal ----
    ("error.generic.headline", &["operation"]),
    ("error.generic.hint", &["table"]),
+    ("error.generic.hint_no_table", &[]),
    // ---- Invalid-value errors (pre-engine, single-line) ----
    (
        "error.invalid_value.arity.headline",
@@ -142,6 +142,10 @@ error:
  generic:
    headline: "the database refused this `{operation}`."
    hint: "The operation could not be completed against the current state of `{table}`."
+    # Used when no table is in context (e.g. contextless `friendly_message()`
+    # callsites: replay, undo, rebuild, export) so the hint never leaks a
+    # literal `{table}` placeholder.
+    hint_no_table: "The operation could not be completed against the current database state."

  # Errors that are specifically about value validation
  # (DbError::InvalidValue) — wrong arity, wrong literal
@@ -659,10 +659,17 @@ fn translate_generic(message: &str, ctx: &TranslateContext) -> FriendlyError {
    let operation = ctx
        .operation
        .map_or("operation", Operation::keyword);
-    let table = ctx_table(ctx);
+    // F2 (ADR-0035 Amendment 1): when no table is in context, use the
+    // table-less hint so a contextless `friendly_message()` (replay, undo,
+    // rebuild, export) never renders a literal `{table}` placeholder.
+    let hint = if ctx.table.is_some() {
+        t!("error.generic.hint", table = ctx_table(ctx))
+    } else {
+        t!("error.generic.hint_no_table")
+    };
    fe(
        t!("error.generic.headline", operation = operation),
-        verbose_hint(ctx, t!("error.generic.hint", table = table)),
+        verbose_hint(ctx, hint),
    )
 }

@@ -1034,6 +1041,28 @@ mod tests {
        );
    }

+    #[test]
+    fn generic_hint_has_no_unsubstituted_table_without_context() {
+        // F2 (ADR-0035 Amendment 1 plan): `friendly_message()` renders
+        // with a default, table-less context at the default Verbose
+        // verbosity, so a generic-bucket error must not leak a literal
+        // `{table}` in its hint.
+        let err = sqlite("some unclassified engine failure", SqliteErrorKind::Other);
+        let rendered = translate(&err, &TranslateContext::default()).render();
+        assert!(
+            !rendered.contains("{table}"),
+            "no unsubstituted placeholder in the table-less generic hint; got:\n{rendered}"
+        );
+        // The table-ful path is unchanged: a table in context still names it.
+        let mut ctx = TranslateContext::for_op(Operation::Delete);
+        ctx.table = Some("Orders".to_string());
+        let with_table = translate(&err, &ctx).render();
+        assert!(
+            with_table.contains("Orders"),
+            "the table-ful generic hint still names the table; got:\n{with_table}"
+        );
+    }
+
    // ---- passthrough variants ----

    #[test]
@@ -160,7 +160,13 @@ pub fn render_structure(desc: &TableDescription) -> Vec<String> {
    if !desc.unique_constraints.is_empty() || !desc.check_constraints.is_empty() {
        out.push("Table constraints:".to_string());
        for cols in &desc.unique_constraints {
-            out.push(format!("  unique ({})", cols.join(", ")));
+            // Annotate with the derived, addressable name (ADR-0035
+            // Amendment 1) so the user can `drop constraint <name>`.
+            out.push(format!(
+                "  {}: unique ({})",
+                crate::db::unique_constraint_name(cols),
+                cols.join(", ")
+            ));
        }
        for chk in &desc.check_constraints {
            match &chk.name {
@@ -880,6 +886,9 @@ mod tests {
        let out = render_structure(&desc).join("\n");
        assert!(out.contains("Table constraints:"), "got:\n{out}");
        assert!(out.contains("unique (a, b)"), "got:\n{out}");
+        // The composite UNIQUE shows its derived, addressable name
+        // (ADR-0035 Amendment 1) so the user can `drop constraint <name>`.
+        assert!(out.contains("unique_a_b: unique (a, b)"), "got:\n{out}");
        assert!(out.contains("check (a < b)"), "unnamed check; got:\n{out}");
        assert!(out.contains("check a_lt_b (a <> b)"), "named check shows its name; got:\n{out}");
    }
@@ -193,6 +193,56 @@ fn drop_column_referenced_by_a_table_check_is_refused() {
        .expect("dropping an unreferenced column succeeds");
 }

+/// `T (id int pk, a int, b int, c text)` with a composite UNIQUE (a, b)
+/// (ADR-0035 Amendment 1).
+fn make_t_with_composite_unique(db: &Database, r: &tokio::runtime::Runtime) {
+    r.block_on(db.sql_create_table(
+        "T".to_string(),
+        vec![
+            ColumnSpec::new("id", Type::Int),
+            ColumnSpec::new("a", Type::Int),
+            ColumnSpec::new("b", Type::Int),
+            ColumnSpec::new("c", Type::Text),
+        ],
+        vec!["id".to_string()],
+        vec![],
+        vec![],
+        vec![],
+        false,
+        Some("create table T (id int primary key, a int, b int, c text)".to_string()),
+    ))
+    .expect("create T");
+    r.block_on(db.alter_add_unique(
+        "T".to_string(),
+        vec!["a".to_string(), "b".to_string()],
+        Some("alter table T add unique (a, b)".to_string()),
+    ))
+    .expect("add composite UNIQUE (a, b)");
+}
+
+#[test]
+fn drop_column_covered_by_a_composite_unique_is_refused_with_the_derived_name() {
+    let (_p, db, _d) = open();
+    let r = rt();
+    make_t_with_composite_unique(&db, &r);
+    // `a` participates in UNIQUE (a, b) → refused up-front, naming the
+    // derived constraint and the drop command (ADR-0035 Amendment 1, F1).
+    // Without this guard the drop reaches the engine and surfaces an
+    // unhelpful generic refusal.
+    let err = r
+        .block_on(db.drop_column("T".to_string(), "a".to_string(), false, None))
+        .expect_err("dropping a composite-UNIQUE column is refused");
+    let msg = err.friendly_message();
+    assert!(msg.contains("unique_a_b"), "names the derived constraint; got: {msg}");
+    assert!(
+        msg.contains("drop constraint unique_a_b"),
+        "points at the actionable drop command; got: {msg}"
+    );
+    // `c` is in no UNIQUE → the drop succeeds.
+    r.block_on(db.drop_column("T".to_string(), "c".to_string(), false, None))
+        .expect("dropping an uncovered column succeeds");
+}
+
 #[test]
 fn rename_column_referenced_by_a_table_check_is_refused() {
    let (_p, db, _d) = open();
@@ -481,6 +481,138 @@ fn e2e_add_unique_with_duplicate_data_is_refused() {
    );
 }

+#[test]
+fn e2e_drop_composite_unique_by_derived_name() {
+    // ADR-0035 Amendment 1: a composite UNIQUE is anonymous, addressed by
+    // its derived name `unique_<cols>`. DROP CONSTRAINT <derived-name>
+    // removes it via the rebuild primitive and the UNIQUE stops enforcing.
+    let (project, db, _d) = open();
+    let r = rt();
+    std::fs::write(
+        project.path().join("u.commands"),
+        "create table T with pk id(int)\n\
+         add column T: a (int)\n\
+         add column T: b (int)\n\
+         alter table T add unique (a, b)\n",
+    )
+    .expect("write");
+    let events = r.block_on(run_replay(&db, project.path(), "u.commands"));
+    assert!(
+        matches!(events.last(), Some(AppEvent::ReplayCompleted { count, .. }) if *count == 4),
+        "events: {events:?}"
+    );
+    let dup_ok = |id: i64, a: i64, b: i64| {
+        r.block_on(db.insert(
+            "T".to_string(),
+            Some(vec!["id".to_string(), "a".to_string(), "b".to_string()]),
+            vec![
+                Value::Number(id.to_string()),
+                Value::Number(a.to_string()),
+                Value::Number(b.to_string()),
+            ],
+            Some("insert".to_string()),
+        ))
+        .is_ok()
+    };
+    assert!(dup_ok(1, 1, 2), "first (1, 2) accepted");
+    assert!(!dup_ok(2, 1, 2), "duplicate (1, 2) rejected while the UNIQUE stands");
+
+    // Drop the UNIQUE by its derived name through the existing DROP
+    // CONSTRAINT grammar.
+    r.block_on(db.alter_drop_constraint(
+        "T".to_string(),
+        "unique_a_b".to_string(),
+        Some("alter table T drop constraint unique_a_b".to_string()),
+    ))
+    .expect("drop constraint unique_a_b resolves the composite UNIQUE");
+
+    // The UNIQUE no longer enforces: the previously-rejected duplicate is
+    // now accepted.
+    assert!(dup_ok(3, 1, 2), "duplicate (1, 2) accepted after the UNIQUE was dropped");
+
+    // And it stays gone across a rebuild from text.
+    r.block_on(db.rebuild_from_text(project.path().to_path_buf(), Some("rebuild".to_string())))
+        .expect("rebuild");
+    assert!(dup_ok(4, 1, 2), "still no UNIQUE after rebuild");
+}
+
+#[test]
+fn e2e_drop_composite_unique_ambiguous_name_is_refused() {
+    // Two distinct composite UNIQUEs can derive the same name —
+    // `unique (a, b_c)` and `unique (a_b, c)` both → `unique_a_b_c`. The
+    // drop must refuse as ambiguous, never guess which to drop.
+    let (project, db, _d) = open();
+    let r = rt();
+    std::fs::write(
+        project.path().join("u.commands"),
+        "create table T with pk id(int)\n\
+         add column T: a (int)\n\
+         add column T: b_c (int)\n\
+         add column T: a_b (int)\n\
+         add column T: c (int)\n\
+         alter table T add unique (a, b_c)\n\
+         alter table T add unique (a_b, c)\n",
+    )
+    .expect("write");
+    let events = r.block_on(run_replay(&db, project.path(), "u.commands"));
+    assert!(
+        matches!(events.last(), Some(AppEvent::ReplayCompleted { count, .. }) if *count == 7),
+        "setup events: {events:?}"
+    );
+    let err = r
+        .block_on(db.alter_drop_constraint(
+            "T".to_string(),
+            "unique_a_b_c".to_string(),
+            Some("alter table T drop constraint unique_a_b_c".to_string()),
+        ))
+        .expect_err("an ambiguous derived name is refused, not guessed");
+    let msg = err.friendly_message();
+    assert!(
+        msg.to_lowercase().contains("ambiguous") || msg.to_lowercase().contains("more than one"),
+        "refusal explains the ambiguity; got: {msg}"
+    );
+}
+
+#[test]
+fn e2e_drop_composite_unique_is_one_undo_step() {
+    // Dropping a composite UNIQUE rebuilds the table = one undo step; undo
+    // restores the constraint (ADR-0035 Amendment 1). The drop is the last
+    // mutation, so a single undo targets it (checked via describe, so no
+    // extra mutation shifts the undo target).
+    let (project, db, _d) = open_with_undo();
+    let r = rt();
+    std::fs::write(
+        project.path().join("u.commands"),
+        "create table T with pk id(int)\n\
+         add column T: a (int)\n\
+         add column T: b (int)\n\
+         alter table T add unique (a, b)\n",
+    )
+    .expect("write");
+    r.block_on(run_replay(&db, project.path(), "u.commands"));
+    let has_unique = || {
+        !r.block_on(db.describe_table("T".to_string(), None))
+            .expect("describe")
+            .unique_constraints
+            .is_empty()
+    };
+    assert!(has_unique(), "the composite UNIQUE exists before the drop");
+
+    r.block_on(db.alter_drop_constraint(
+        "T".to_string(),
+        "unique_a_b".to_string(),
+        Some("alter table T drop constraint unique_a_b".to_string()),
+    ))
+    .expect("drop the composite UNIQUE");
+    assert!(!has_unique(), "the composite UNIQUE is gone after the drop");
+
+    assert!(
+        r.block_on(db.undo()).expect("undo").is_some(),
+        "the DROP CONSTRAINT was one undo step"
+    );
+    assert!(has_unique(), "one undo restored the composite UNIQUE");
+}
+
 #[test]
 fn e2e_add_foreign_key_creates_an_enforced_relationship() {
    let (project, db, _d) = open();