Aliased unique parameters

Meaning

The same argument is passed to two unique parameters in one call. Their no-alias assertions cannot both be true.

unique is an experimental checked intent. Even when both parameters only read today, their signatures promise independent exclusive alias conditions to analysis.

Why it happens

A caller reused one owner for multiple formal parameters, or a function was annotated more strongly than its actual algorithm requires.

Invalid code

struct Buffer { value: Int }
fn combine(unique left: Buffer, unique right: Buffer) -> Int {
    return left.value + right.value
}
fn main() -> Int {
    let buffer = Buffer { value: 1 }
    return combine(buffer, buffer)
}

Correct code

struct Buffer { value: Int }
fn combine(unique left: Buffer, unique right: Buffer) -> Int {
    return left.value + right.value
}
fn main() -> Int {
    let left = Buffer { value: 1 }
    let right = Buffer { value: 2 }
    return combine(left, right)
}

Common fixes

Pass distinct owners or remove unique when exclusive aliasing is not a true requirement. Removing an optimization assertion is always safer than making a false one.

Do not manufacture duplicate raw pointers to bypass the check. If the operation intentionally compares a value with itself, ordinary borrowed parameters express that semantics better.