`break` or `return` inside `for ... in` leaves the enumeration on the stack

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
rust
Domain
compilers

Research direction

Start in crates/rascal/src/internal/as2_codegen/statement.rs, especially gen_for_loop and StatementKind::Return, then inspect how loop cleanup and return values are emitted. Reproduce the nested for ... in cases from the issue and check related code generation tests. Done means both break and return leave enumeration values cleaned up, so the outer loop prints x and y once each.

Written by the indexing model from the issue text.

Description

for ... in keeps the enumerated names on the stack, terminated by null, and only consumes all of them when the loop runs to the end. Leaving the loop early with break or return jumps out without removing the rest. Any enclosing for ... in then picks up those leftover values: depending on what's left, it iterates over stale names, or it stops early on the inner loop's null. In the reproducers below, the outer loop gets b from every inner loop, so it never ends.

break:

var o = {a: 1, b: 2};
for (var k in {x: 1, y: 2}) {
    trace(k);
    for (var j in o) {
        break;
    }
}

return:

function first(o) {
    for (var k in o) {
        return k;
    }
}
for (var k in {x: 1, y: 2}) {
    trace(k);
    first({a: 1, b: 2});
}

Expected for both: x and y, once each.

Actual, in Flash Player (Rascal 0.3.6, SWF 8), for both: x, then b repeated without end.

So the stack isn't reset when a function returns, which is why return is affected too.

gen_for_loop sets the break label to end_label, which is after the loop's cleanup.
https://github.com/Dinnerbone/Rascal/blob/3f1a80c85dcc657cf4a67b72754b9c7869a6d9c5/crates/rascal/src/internal/as2_codegen/statement.rs#L292-L302
StatementKind::Return emits Return directly. Before leaving a for ... in early, the remaining values need popping up to and including the null. Perhaps by emitting a small loop for that, similar to the loop header: Push null, Equals2, Not, If back to the start.

For a return from nested for ... in loops, each enclosing enumeration needs this, and the return value needs to be kept, for example in a register.

Real code hitting this: https://github.com/ruffle-rs/ruffle/blob/c7c2cc871c07d94f246d496e0b41479ba429085a/tests/tests/swfs/avm1/global_instance_decls/test.as#L46-L60

Dominant language
Rust
Stars
22
Forks
2
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