if the language default was anything other than this, THAT WOULD BE WRONG and unexpected. I would prefer the default to be the dumb, fast thing. then if I want the slow, fancy thing, I can import some first or third party package.
That feels a bit "pit of despair" design[2], the default thing is unhelpful and doing more than that requires the programmer to climb up out of it.
[1] https://go.dev/blog/strings
[2] https://blog.codinghorror.com/falling-into-the-pit-of-succes...
If you want something else than utf-8 you can use another data type, like a vector of bytes.
Rust gives you the freedom to specify what you mean.
I don't remember, but I think the size hint is set on the Chars iterator, so it can see it has 17 bytes of data, it knows that can't encode more than 17 Unicode scalar values, nor can it encode fewer than five. But since we ask for an exact count that hint is unused, the actual decoding will take place.
Rust doesn't take sides here. It exposes all the different ways you might want to calculate the "length" of a string, and lets you pick which one you mean. The non-zero-cost choices involve a multi-step specification (like `.chars().count()`), which states explicitly the calculation involved.
In contrast chars().count() creates an iterator and runs the iterator to completion counting steps, that's O(N) for a string of length N, and is in practice very expensive, you should definitely cache this value if you will need it repeatedly. It is possible the compiler can see what you're doing and cache it, but I am very far from certain so you should do so explicitly.
This is important in contrast to say, C, where strlen(str) is O(N) because it doesn't have fat pointers and so it has no idea how long the string is in any sense.
(and it's not expected that a character's length is>1 unless you've been conditioned to excpect it)
And bytes is the only thing that matter for a specific type of string, conveniently named, sequence of bytes