> Deadline would only be the minimum duration after which the language, when evaluating the future / task, would return the empty set/result.

This appears to be misunderstanding how futures work in Rust. The language doesn't evaluate futures or tasks. A future is just a struct with a poll method, sort of like how a closure in Rust is just a struct with a call method. The await keyword just inserts yield points into the state machine that the language generates for you. If you want to actually run a future, you need an executor. The executor could implement timeouts, but it's not something that the language could possibly have any way to enforce or require.