Chore: prefer unreachable_unchecked in Cow::to_mut method

[BastiDood]

As a small first step in #88606, this PR uses the core::hint::unreachable_unchecked function in place of the unreachable macro in the Cow::to_mut implementation. It should be fairly apparent that this is a safe assertion to make since self has just been assigned to the Owned variant. Thanks!

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

Please see the contribution instructions for more information.

[hellow554]

As already said in the other thread, this actually doesn't change anything, because the compiler is smart enough to detect, that the branch won't be taken. See https://rust.godbolt.org/z/c75xv4df4 for a comparision

[BastiDood]

...this actually doesn't change anything...

Thanks for taking the time to set up the Godbolt example! It's incredible how the compiler even removed the entire panic branch regardless. For the meantime, I'll be looking forward to the discussion in the original thread before resolving this PR. 👍

[dtolnay]

Compilers are way better at this than humans are. Making the human do it, when the compiler is doing the same already, is not an improvement. That puts the maintainers now perpetually on the hook for absence of UB going forward, in the presence of changes to the surrounding code.