[invpt]

I appreciate the mention of Box<[T]>. I've never (or at least very rarely?) seen people using it, even in rustc, although I'm sure there's somewhere in the compiler that uses it. I've kind of gotten the feeling that Box<[T]> is frowned upon as a needless optimization because the one-word storage difference is so minimal, but I appreciate having the immutable size semantics as a way of ensuring no extra allocations occur by way of the methods on Vec.

[ironhaven]

Another rust type you should look out for is Box<str>In my rust code I find a lot more uses of Box<str> rather than Box<[T]>. Strings are often fixed length identifiers or usernames that get passed around frequently and stored in data structures. Box<str> can be a drop-in replacement in a surprising amount of code

[zozbot234]

Arc<str> is an atomically ref-counted string, also quite useful in async or multi-threaded code where (unlike w/ Box) you sometimes can't know in advance which task will drop the last reference to some data.

[bobbylarrybobby]

Box<str> is used pervasively in the rust compiler instead of String for exactly this reason. Basically every string of code the compiler looks at is, unsurprisingly, constant.

[steveklabnik]

Yeah, the general concept here is https://en.wikipedia.org/wiki/String_interning, Box<str> is great for it.

[tsnl]

Another downside of Box<T> is that Vec::into_boxed_slice copies all elements from the source vector. Vec::shrink_to_fit does not appear to copy. This is based on the stable docs.

[sapiogram]

It doesn't copy the elements if the source vector is already at max capacity, which you can often arrange for beforehand.

[cmrx64]

For example, by explicitly calling shrink_to_fit. So you either have fragmentation, or an extra copy if you're not careful, but none of the solutions let you forget about the detail entirely.

[invpt]

If you look at the source for into_boxed_slice, it calls shrink_to_fit at the beginning before doing anything else. Hence the documentation is slightly wrong, and no copies occur.

Edit: I submitted a PR to clear up the docs: https://github.com/rust-lang/rust/pull/120110

[cmrx64]

thank you!

[sapiogram]

Or alternatively, the vector was created with `Vec::with_capacity()` or `ExactSizeIterator::collect()`, and had the correct capacity all along.