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E4004

Error · Borrow/Ownership

E4004 — Cannot borrow (already mutably borrowed)

No example yet

No fixture in the test suite asserts this code, so there is no program here that is known to trigger it. That is a gap in our coverage rather than a statement about the code.

Contributing one is a good first change — add a fixture under tests/ with a CHECK line naming E4004, and this page picks it up.

Related

Ownership and borrowingThe chapter covering the rule this code enforces. All diagnosticsEvery code the compiler can emit, grouped by stage. E4003Cannot borrow as mutable (already immutably borrowed) E4005A returned reference escapes the function borrowing a function-local (dangling return)