Outdated claims

[Conaclos]

Hi!

The following claim is wrong:

For the most common pairs of values - &str and String, or &[u8] and Vec - this means that the entire enum is 4 words wide

Indeed, Rust performs niche optimizations for Vec and String.
This means that std::borow::Cow<str> has the same size as String (3 words).

[ginnyTheCat]

Yea, this is a optimization that exists between 1.65 and 1.67 and again starting from 1.76

[overlookmotel]

Looks like compiler also lays out the fields of std::borrow::Cow so that Cow::Borrowed and Cow::Owned both store length and pointer in same positions. So dereferencing Cow<str> to &str is branchless:

https://godbolt.org/z/8h933vrbM

A hacky demonstration the layout of Cow<str>: https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=702c30aaa9dccf4a199d1c7b3c06f75c

Of course, this layout is not guaranteed, but I assume that if it changed, it would cause a regression in benchmarks which would be caught by Rust's maintainers.