Stably get length of raw fat pointer?

Are there any ways to get the length encoded in a raw fat pointer on stable Rust? (len exists but is unstable.)

Hmm. Funny that we have this but not the opposite. Another also-unstable way is to_raw_parts.

You could write your own:

fn len<T>(ptr: *const [T]) -> usize {
    let ptr: *const [()] = ptr as _;
    // SAFETY: There is no aliasing as () is zero-sized
    let slice: &[()] = unsafe { &*ptr };
    slice.len()
}
1 Like

Thanks, that works great!

While we're on the subject, do you know of a stable way to emulate size_of_val_raw (assuming we don't know that our raw pointer is specifically a DST - could be a trait object, etc)?

A trait object is a DST.

But you mean, you don't know if it's Sized, a trait object, a slice, or something else? I'm not thinking of a way to pull that off on stable, no.

Yeah, I should have said "custom DST" or something. Just something whose trailing field isn't necessarily a slice.

Oh well :slight_smile:

Posted this for more visibility: Get size of `NonNull<T>` for `T: ?Sized`?

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