Report #30225
[bug\_fix] cannot borrow \`self\` as mutable more than once at a time / second mutable borrow occurs here
Restructure the code to do one mutable borrow at a time by scoping borrows in blocks \`\{ let val = self.get\_mut\(\); ... \}\`, or split the struct into smaller pieces using the "split borrowing" pattern \(mutably borrowing distinct fields separately\), or use interior mutability \(RefCell\) if runtime checks are acceptable. Root cause: Rust's borrowing rules enforce that to prevent data races and iterator invalidation, there can be only one active mutable reference to any particular data at a time.
Journey Context:
Developer has a \`Cache\` struct with methods \`get\_mut\(&mut self, key: &str\) -> &mut Value\` and \`cleanup\(&mut self\)\`. They try to implement a method \`fn purge\_and\_get\(&mut self, key: &str\)\` that calls \`let val = self.get\_mut\(key\); self.cleanup\(\);\`, intending to clean old entries while returning a reference to the current one. The compiler stops them with "cannot borrow \`\*self\` as mutable more than once". Developer tries to clone the value but it's expensive. They try to use \`std::mem::drop\(val\)\` before cleanup but the borrow persists due to lexical scope. They learn about non-lexical lifetimes \(NLL\) but still hit the issue because the borrows overlap in the same scope. Finally, they refactor by splitting the borrows: \`\{ let val = self.get\_mut\(key\); /\* use val \*/ \} self.cleanup\(\);\` This works because the first mutable borrow ends at the closing brace, releasing the borrow before cleanup runs. Alternatively, they could split the struct into \`CacheData\` and \`CacheStats\` so they can borrow fields independently.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-18T05:07:11.642241+00:00— report_created — created