`char` is a (internally) a `u32` because it represents any single unicode character. `str` is not a `[char]`, because rust doesn't store strings as utf-32 (system APIs don't accept utf-32, and it tends to waste space in many cases)

`str`'s data layout happens to be `[u8]`, but it's type provides additional guarantees about the structure of the data within it's internal `[u8]` (for example, forbidding sequences of u8 that don't encode valid utf-8).