Phase 7. Query DSL

Field	Value
MEP	MEP-53 §Phases
Status	LANDED
Started	2026-05-28 (GMT+7)
Landed	2026-05-29 07:35 (GMT+7)
Tracking issue	— (umbrella)
Tracking PR	#22499
Commit	cf15291eb7

Gate

TestPhase7Query walks tests/transpiler3/rust/fixtures/phase07-query/ (43 fixtures) and asserts byte-equal stdout. Coverage: single-source query, where clause, select projection (scalar, record, anonymous record), order by ascending / descending, skip / take, multi-source (cross join), group by, count / sum / min / max / avg aggregates, distinct.

Lowering decisions

Mochi's query DSL is desugared by clower (the C-target lower pass shared across backends) into an aotir.QueryExpr with explicit From, Where, Select, OrderBy, Skip, Take, and Aggregate fields. The Rust lower pass renders this as a Rust iterator chain:

let result: Vec<_> = users.iter()
    .filter(|u| u.age >= 18)
    .map(|u| u.name.clone())
    .collect();

order by k desc lowers to .sorted_by_key(|x| std::cmp::Reverse(k(x))) via the itertools crate. The choice to use itertools is a compromise: stable Rust has no Iterator::sorted (it requires Vec collection first). itertools is widely deployed and adds a single thin dep.

Cross-join (multi-source from) is materialised into a Vec then iterated:

let rows: Vec<_> = users.iter().flat_map(|u| {
    orders.iter().map(move |o| (u.clone(), o.clone()))
}).collect();

Group by lowers to BTreeMap<K, Vec<V>> (the BTreeMap gives deterministic iteration order matching vm3's sorted output).

Aggregates (count, sum, min, max, avg) are emitted inline rather than via .fold: .count() as i64, .sum::<i64>(), .min().unwrap_or(0), .max().unwrap_or(0), let total: f64 = ...sum(); total / (n as f64).

Files changed

File	Purpose
transpiler3/rust/lower/query.go	Query DSL → iterator chain
runtime3/rust/mochi-runtime/Cargo.toml	Add itertools dep
transpiler3/rust/build/phase07_test.go	43-fixture gate
tests/transpiler3/rust/fixtures/phase07-query/*.mochi + .out	43 fixtures

Test set

• TestPhase7Query/<fixture> for each .mochi in the fixture directory (43 fixtures).

Closeout notes

Group-by output order was the longest-running bug in phase 7. HashMap's unspecified iteration order produces nondeterministic stdout; switching the group accumulator to BTreeMap fixed every fixture at once. The cost is a log n group-insert vs O(1) for HashMap, which is acceptable for the fixture set (largest group has 100 elements) and matches vm3's behavior.