Is there any other example of "length" meaning "byte length", or is it just Rust just being confusing? I've never seen this elsewhere.

Offset is ordinarily just a difference of two indices. In a container I don't recall seeing it implicitly refer to byte offset.

In general in Rust, “length” refers to “count”. If you view strings as being sequences of Unicode scalar values, then it might seem odd that `str::len` counts bytes, but if you view strings as being a subset of byte slices it makes perfect sense that it gives the number of UTF-8 code units (and it is analoguous to, say, how Javascript uses `.length` to return the number of UTF-16 code units). So I think it depends on perspective.

A length could refer to lots of different units - elements, pages, sectors, blocks, N-aligned bytes, kbytes, characters, etc.

Always good to qualify your identifiers with units IMO (or types that reflect units).

It's the usual convention for systems programming languages and has been for decades, e.g. strlen() and std::string.length(). Byte length is also just more useful in many cases.