r[type.text]
r[type.text.intro] The types char and str hold textual data.
r[type.text.char-value] A value of type char is a Unicode scalar value (i.e. a code point that is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to 0xD7FF or 0xE000 to 0x10FFFF range.
r[type.text.char-precondition] It is immediate undefined behavior to create a char that falls outside this range. A [char] is effectively a UCS-4 / UTF-32 string of length 1.
r[type.text.str-value] A value of type str is represented the same way as [u8], a slice of 8-bit unsigned bytes. However, the Rust standard library makes extra assumptions about str: methods working on str assume and ensure that the data in there is valid UTF-8. Calling a str method with a non-UTF-8 buffer can cause undefined behavior now or in the future.
r[type.text.str-unsized] Since str is a dynamically sized type, it can only be instantiated through a pointer type, such as &str.
r[type.text.layout]
r[type.layout.char-layout] char is guaranteed to have the same size and alignment as u32 on all platforms.
r[type.layout.char-validity] Every byte of a char is guaranteed to be initialized (in other words, transmute::<char, [u8; size_of::<char>()]>(...) is always sound -- but since some bit patterns are invalid chars, the inverse is not always sound).