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When shrinking arrays in the profiling way we currently don't always zero the leftover slop. This means we can't traverse such closures in the heap profiler. The old Note [zeroing slop] and #8402 have some rationale for why this is so but I belive the reasoning doesn't apply to mutable closures. There users already have to ensure multiple threads don't step on each other's toes so zeroing should be safe.
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