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

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新手友好度
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Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
活跃
技术栈
rust
领域
compilers

调研方向

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.

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描述

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

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Rust
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