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Single quotes construct a char, while double quotes construct text. A char is one Unicode scalar value, not necessarily one byte. The crab emoji in the example fits in one char but occupies several UTF-8 bytes. &str is a borrowed view of UTF-8 text; its len() reports bytes. If you ask for chars().count(), you count scalar values instead. A user-perceived character can contain more than one scalar, such as a letter plus a combining mark, so even that count is not a universal measure of what a person sees.
Rust does not allow s[0] on a string because byte position zero is not a safe general-purpose character index. Instead, choose the operation that matches the question: bytes() when inspecting the encoding, chars() when processing scalars, and a grapheme-aware library when the interface needs visible clusters. This distinction prevents slicing through a multibyte UTF-8 sequence.
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