You don't want the safety of your program to depend on whether the compiler emits a warning or not.

And turning warnings into errors just encourages people to write 'let _ = ...' to get rid of the error.

This has nothing to do with safety, just correctness.

> And turning warnings into errors just encourages people to write 'let _ = ...' to get rid of the error.

No? writing `let _ = make_future()` will clearly not await the future, why would you do it instead of just adding `.await` ?

Using `let _ = ...` is sometimes fine for Result if you really sure you don't care about the potential error you got but it's a no go with futures.

Note that the lint for un-awaited Future doesn't mention the way to silence them by assigning to _:

  warning: unused implementer of `Future` that must be used
   --> src/main.rs:9:5
    |
  9 |     foo();
    |     ^^^^^
    |
    = note: futures do nothing unless you `.await` or poll them
    = note: `#[warn(unused_must_use)]` on by default