Trait slimmer_box::slim_pointee::SlimmerPointee

source ·

pub unsafe trait SlimmerPointee<SlimmerMetadata>: Pointeewhere
    <Self as Pointee>::Metadata: Clone,
    SlimmerMetadata: TryFrom<<Self as Pointee>::Metadata> + TryInto<<Self as Pointee>::Metadata>,{ }

Expand description

Trait which can be implemented by any pointer-like (‘Pointee’)types, as long as their metadata might be made smaller.

As such, it is implemented for:

Zero-sized types (no metadata and actually also no pointer)
Statically-sized types (no metadata)
Dynamically-sized types (metadata indicates length which can be made smaller if the length fits in the smaller int)

It is not implemented for trait objects, because their metadata is itself a pointer!

Safety

In a perfect world, we would have hade a SlimmerMetadata: Into<Self as Pointee>::Metadata bound on this trait. However, since pointer sizes differ on different compilation targets, current Rust never implements this conversion.

As such, this trait functions under the assumption that

let slim_meta = meta.try_into();
let recovered_meta = slim_meta.try_into().unwrap();

will never fail.

This assumption is very reasonable and in all likelyhood fulfilled by any pointer metadata type you ever throw at it. But since we cannot trust arbitrary safe code to do ‘the right thing’, this trait needs to be unsafe.