Backporting swift 5.8 unnamed async let to 5.7.2

async let has the nice property that it will self-cancel if not awaited-upon on scope exit:

let timeout:Void = id,
    once: started.advanced(by: duration))

return try await withCheckedThrowingContinuation

but this generates a warning

warning: immutable value 'timeout' was never used; consider replacing with '_' or removing it

in swift 5.8 we can use async let with the underscore and fix this warning:

let _:Void = id, once: started.advanced(by: duration))

but this will not compile on 5.7.

is there any way to fix this problem so that the library compiles without warnings on both toolchains?