rust: implement PointerWrapper for unit type.

[wedsonaf]

This is useful for cases when we don't need to store any information as
context. Implementing PointerWrapper for () allows us to directly
use it as the context type.

Signed-off-by: Wedson Almeida Filho [email protected]

[bjorn3]

as opposed to, for example, Box<()>, which would require an allocation.

Box and Vec don't allocate for ZST's. Instead they cast an integer to a pointer. (the alignment of the type to be precise. this gives a non-null aligned pointer)

[wedsonaf]

as opposed to, for example, Box<()>, which would require an allocation.

Box and Vec don't allocate for ZST's. Instead they cast an integer to a pointer. (the alignment of the type to be precise. this gives a non-null aligned pointer)

This is interesting. So if we have two instances of Box where T is a ZST, we can't tell them apart?

[bjorn3]

Correct.

[wedsonaf]

PTAL.

• Updated commit message to remove claim that Box<()> allocates memory.
• Return 1 as the pointer value now.
• Fixed clippy errors (omitting () as the return value).

[wedsonaf]

Correct.

I need to review the LockedBy code to make sure all pointer comparisons there are expected to work then.