Phase 11. Try / catch / panic (panic::catch_unwind)

Field	Value
MEP	MEP-53 §Phases
Status	LANDED
Started	2026-05-28 (GMT+7)
Landed	2026-05-29 07:35 (GMT+7)
Tracking issue	#22584
Tracking PR	#22499
Commit	00d3ee0f28

Gate

TestPhase11Errors walks tests/transpiler3/rust/fixtures/phase11-errors/ (36 fixtures) and asserts byte-equal stdout. Coverage: explicit panic(code), try / catch with code binding, panic from division by zero (code 5), panic from out-of-bounds index (code 4), nested try / catch, try without catch (re-panic), catch with mixed-type bodies.

Lowering decisions

try { risky() } catch e { print(e) }

lowers to:

match mochi_runtime::panic::catch(|| {
    risky();
}) {
    Some(__code) => {
        let e: i64 = __code;
        mochi_runtime::io::print_i64(e);
    }
    None => {}
}

mochi_runtime::panic::catch wraps std::panic::catch_unwind(panic::AssertUnwindSafe(f)), downcasts the payload to extract an i64, and falls back to a message-string map for stdlib panics:

• "out of bounds" / "index out of" → code 4
• "divide by zero" / "attempt to divide" / "remainder" → code 5
• everything else → code 1

mochi_runtime::panic::raise(code) calls panic::panic_any(code). silence_hook() installs a panic::set_hook(Box::new(|_| {})) exactly once via Once, so panic messages don't leak to stderr — important because tests diff stdout but also need stderr clean (a non-empty stderr would cause downstream tools to flag the binary as failing even with stdout matching).

The choice to use panic::catch_unwind rather than Result<T, E> is deliberate: Mochi's try block can wrap arbitrary statements, not just expression-level operations. Routing through Result would require either a full effect-system colouring pass or wrapping every fallible op site (every division, every index) in ?. The catch_unwind approach is single-site, decouples the panic-raising code from the catching code, and matches the semantic shape of Mochi's source-level try exactly.

Files changed

File	Purpose
transpiler3/rust/lower/try.go	try / catch / panic lowering
runtime3/rust/mochi-runtime/src/lib.rs	Add panic module
transpiler3/rust/build/phase11_test.go	36-fixture gate
tests/transpiler3/rust/fixtures/phase11-errors/*.mochi + .out	36 fixtures

Test set

• TestPhase11Errors/<fixture> for each .mochi in the fixture directory (36 fixtures).

Closeout notes

AssertUnwindSafe is required because closures that capture &mut state are not UnwindSafe by default. This is a Rust quirk: a panic can leave a &mut in an unspecified state, so the compiler refuses unless the user asserts safety. For Mochi semantics, this is always safe because a panic always returns the caught code and never continues with a torn state. The assertion is wrapped inside mochi_runtime::panic::catch, so user code never sees it.