Loosen Send/Sync bounds on futures

[Jules-Bertholet]

This ensures that Send/Sync bounds for each future match those of the value produed by the future.

[zeenix]

LGTM otherwise.

[zeenix]

LGTM. @notgull @taiki-e ?

[notgull]

I'm not sure if this is the right approach. I'd prefer to modify the underlying Future types in order to make the trait resolver automatically classify them as Sync. If they contain types that aren't Send/Sync (like raw pointers) then those types should be wrapped to make them Send/Sync.

Instead of this:

struct Foo {
    data: *const (),
    other_data: u32
}

unsafe impl Send for Foo {}

I would do this:

struct Foo {
    data: PtrWrapper,
    other_data: u32
}

struct PtrWrapper(*const ());
unsafe impl Send for PtrWrapper {}

[notgull]

Thanks!