Swift ownership: borrowing, consuming, inout

Jan 12, 2026 |

swift swift 5.9 structs enums

Swift 5.9 introduced ownership modifiers for non-copyable types (~Copyable). Here’s the difference:

struct DBConnection: ~Copyable {
    mutating func open() { /* ... */ }
    mutating func close() { /* ... */ }
    func query(_ sql: String) { /* ... */ }
}

borrowing - read-only access, caller keeps ownership:

func inspect(_ connection: borrowing DBConnection) {
    connection.query("SELECT 1")  // OK - read only
    // connection.open()          // Error - can't mutate
}

consuming - takes ownership, caller loses the value:

func runAndClose(_ connection: consuming DBConnection) {
    connection.open()
    connection.close()
    // connection is destroyed here
}

runAndClose(db)
// db.query("...")  // Error - db was consumed

inout - mutable borrow, caller keeps ownership:

func reopen(_ connection: inout DBConnection) {
    connection.open()  // OK - can mutate
}
// db still usable after call

Useful for modeling resources like DB connections, file handles, or locks where you want compile-time guarantees against use-after-close bugs.

Source: Proposal: SE-0390

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