Phase 6. Closures (Box<dyn Fn>)

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	ad43642b16

Gate

TestPhase6Closures walks tests/transpiler3/rust/fixtures/phase06-closures/ (34 fixtures) and asserts byte-equal stdout. Coverage: lambda assigned to var, lambda returned from function, lambda passed as arg, lambda capturing by value, lambda capturing recursive bindings, higher-order ops (map, filter, reduce), nested closures.

Lowering decisions

let f = fun(x: int): int => x + n (where n is in scope) lowers to:

let f: Box<dyn Fn(i64) -> i64> = Box::new({
    let n = n.clone();
    move |x: i64| -> i64 { x + n }
});

The capture clause { let n = n.clone(); move |...| { ... } } is computed by transpiler3/rust/lower/closure.go. The clone is mandatory for non-Copy types (String, Vec, structs); the colour pass elides it for Copy types (i64, f64, bool, &str). Closures escape into the heap via Box<dyn Fn> so they can be stored in struct fields, returned, and homogenised in lists.

Higher-order builtins trampoline through .call-equivalent invocation:

let mapped: Vec<i64> = xs.iter().map(|x| f(x.clone())).collect();

The x.clone() is required because Box<dyn Fn(i64) -> i64> takes i64 by value; with f returning a non-Copy result, that result is also cloned on print.

Recursive closures use the Y-combinator trick: a Rc<RefCell<Option<Box<dyn Fn>>>> is allocated, the closure body references it via f.borrow().as_ref().unwrap()(arg), then the body itself is assigned via *f.borrow_mut() = Some(Box::new(...)). This is the only place the runtime touches RefCell at the user-visible level; phase 6 fixtures exercise three recursive closures (factorial, fibonacci, ackermann).

Files changed

File	Purpose
transpiler3/rust/lower/closure.go	Capture analysis + closure conversion
transpiler3/rust/lower/recursive_closure.go	Y-combinator trampoline for recursive closures
transpiler3/rust/colour/colour.go	Wire Copy-vs-clone elision (no-op until phase 6)
transpiler3/rust/build/phase06_test.go	34-fixture gate
tests/transpiler3/rust/fixtures/phase06-closures/*.mochi + .out	34 fixtures

Test set

• TestPhase6Closures/<fixture> for each .mochi in the fixture directory (34 fixtures).

Closeout notes

The colour pass deserves more lines than it has. It is a forward-borrow-flow analysis that decides, at each value use site, whether .clone() is required. The rule: if a value is used after the current site AND the consuming function takes it by value AND the type is not Copy, then clone. The pass is rerun until fixpoint because elision at one site can shift others. Without the colour pass, the emitted code typechecks but clones aggressively (every closure capture clones every captured binding); with it, closures over Copy types are clone-free.