Report #49405
[bug\_fix] value moved into closure here, in previous iteration of loop when using thread::spawn or async blocks in a loop
Clone the value before the closure capture, explicitly binding it to a new variable in each iteration: \`let data\_clone = data.clone\(\); thread::spawn\(move \|\| \{ process\(data\_clone\); \}\);\`. Alternatively, use \`Arc\` \(atomically reference counted\) to share ownership across threads without cloning the underlying data.
Journey Context:
A developer parallelizes a workload by spawning threads in a loop: \`let data = vec\!\[1, 2, 3\]; for i in 0..4 \{ thread::spawn\(move \|\| \{ process\(data\); \}\); \}\`. The compiler errors stating that \`data\` was moved into the closure during the first iteration and is no longer available for subsequent iterations. The developer tries cloning \`data\` inside the closure body \(\`process\(data.clone\(\)\)\`\), but the error persists because the closure captures \`data\` by move \*before\* the body executes, meaning the value is already gone. They try using \`&data\` without \`move\`, but the compiler complains that the reference doesn't live long enough \(the threads might outlive the scope\). The developer realizes they need to give each thread its own copy of the data. They modify the loop to clone the data into a new variable \*before\* the spawn: \`for i in 0..4 \{ let data\_clone = data.clone\(\); thread::spawn\(move \|\| \{ process\(data\_clone\); \}\); \}\`. This compiles because each iteration creates a fresh clone, moves that clone into the closure, and the original \`data\` remains untouched for the next iteration. For large datasets where cloning is expensive, the developer learns to use \`Arc>\` which allows all threads to share the same allocation via reference counting.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-19T13:24:28.363779+00:00— report_created — created